[PATCHv8 1/3] tun: export underlying socket

[PATCHv8 1/3] tun: export underlying socket

[Michael S. Tsirkin]

Tun device looks similar to a packet socket
in that both pass complete frames from/to userspace.

This patch fills in enough fields in the socket underlying tun driver
to support sendmsg/recvmsg operations, and message flags
MSG_TRUNC and MSG_DONTWAIT, and exports access to this socket
to modules.  Regular read/write behaviour is unchanged.

This way, code using raw sockets to inject packets
into a physical device, can support injecting
packets into host network stack almost without modification.

First user of this interface will be vhost virtualization
accelerator.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: David S. Miller <davem@davemloft.net>
---

DaveM agreed that this will go in through Rusty's tree together
with vhost patch, because vhost is the first user of the new
interface.

 drivers/net/tun.c      |  101 +++++++++++++++++++++++++++++++++++++++---------
 include/linux/if_tun.h |   14 +++++++
 2 files changed, 96 insertions(+), 19 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 4fdfa2a..18f8876 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -144,6 +144,7 @@ static int tun_attach(struct tun_struct *tun, struct file *file)
 	err = 0;
 	tfile->tun = tun;
 	tun->tfile = tfile;
+	tun->socket.file = file;
 	dev_hold(tun->dev);
 	sock_hold(tun->socket.sk);
 	atomic_inc(&tfile->count);
@@ -158,6 +159,7 @@ static void __tun_detach(struct tun_struct *tun)
 	/* Detach from net device */
 	netif_tx_lock_bh(tun->dev);
 	tun->tfile = NULL;
+	tun->socket.file = NULL;
 	netif_tx_unlock_bh(tun->dev);
 
 	/* Drop read queue */
@@ -387,7 +389,8 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
 	/* Notify and wake up reader process */
 	if (tun->flags & TUN_FASYNC)
 		kill_fasync(&tun->fasync, SIGIO, POLL_IN);
-	wake_up_interruptible(&tun->socket.wait);
+	wake_up_interruptible_poll(&tun->socket.wait, POLLIN |
+				   POLLRDNORM | POLLRDBAND);
 	return NETDEV_TX_OK;
 
 drop:
@@ -743,7 +746,7 @@ static __inline__ ssize_t tun_put_user(struct tun_struct *tun,
 	len = min_t(int, skb->len, len);
 
 	skb_copy_datagram_const_iovec(skb, 0, iv, total, len);
-	total += len;
+	total += skb->len;
 
 	tun->dev->stats.tx_packets++;
 	tun->dev->stats.tx_bytes += len;
@@ -751,34 +754,23 @@ static __inline__ ssize_t tun_put_user(struct tun_struct *tun,
 	return total;
 }
 
-static ssize_t tun_chr_aio_read(struct kiocb *iocb, const struct iovec *iv,
-			    unsigned long count, loff_t pos)
+static ssize_t tun_do_read(struct tun_struct *tun,
+			   struct kiocb *iocb, const struct iovec *iv,
+			   ssize_t len, int noblock)
 {
-	struct file *file = iocb->ki_filp;
-	struct tun_file *tfile = file->private_data;
-	struct tun_struct *tun = __tun_get(tfile);
 	DECLARE_WAITQUEUE(wait, current);
 	struct sk_buff *skb;
-	ssize_t len, ret = 0;
-
-	if (!tun)
-		return -EBADFD;
+	ssize_t ret = 0;
 
 	DBG(KERN_INFO "%s: tun_chr_read\n", tun->dev->name);
 
-	len = iov_length(iv, count);
-	if (len < 0) {
-		ret = -EINVAL;
-		goto out;
-	}
-
 	add_wait_queue(&tun->socket.wait, &wait);
 	while (len) {
 		current->state = TASK_INTERRUPTIBLE;
 
 		/* Read frames from the queue */
 		if (!(skb=skb_dequeue(&tun->socket.sk->sk_receive_queue))) {
-			if (file->f_flags & O_NONBLOCK) {
+			if (noblock) {
 				ret = -EAGAIN;
 				break;
 			}
@@ -805,6 +797,27 @@ static ssize_t tun_chr_aio_read(struct kiocb *iocb, const struct iovec *iv,
 	current->state = TASK_RUNNING;
 	remove_wait_queue(&tun->socket.wait, &wait);
 
+	return ret;
+}
+
+static ssize_t tun_chr_aio_read(struct kiocb *iocb, const struct iovec *iv,
+			    unsigned long count, loff_t pos)
+{
+	struct file *file = iocb->ki_filp;
+	struct tun_file *tfile = file->private_data;
+	struct tun_struct *tun = __tun_get(tfile);
+	ssize_t len, ret;
+
+	if (!tun)
+		return -EBADFD;
+	len = iov_length(iv, count);
+	if (len < 0) {
+		ret = -EINVAL;
+		goto out;
+	}
+
+	ret = tun_do_read(tun, iocb, iv, len, file->f_flags & O_NONBLOCK);
+	ret = min_t(ssize_t, ret, len);
 out:
 	tun_put(tun);
 	return ret;
@@ -847,7 +860,8 @@ static void tun_sock_write_space(struct sock *sk)
 		return;
 
 	if (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
-		wake_up_interruptible_sync(sk->sk_sleep);
+		wake_up_interruptible_sync_poll(sk->sk_sleep, POLLOUT |
+						POLLWRNORM | POLLWRBAND);
 
 	tun = container_of(sk, struct tun_sock, sk)->tun;
 	kill_fasync(&tun->fasync, SIGIO, POLL_OUT);
@@ -858,6 +872,37 @@ static void tun_sock_destruct(struct sock *sk)
 	free_netdev(container_of(sk, struct tun_sock, sk)->tun->dev);
 }
 
+static int tun_sendmsg(struct kiocb *iocb, struct socket *sock,
+		       struct msghdr *m, size_t total_len)
+{
+	struct tun_struct *tun = container_of(sock, struct tun_struct, socket);
+	return tun_get_user(tun, m->msg_iov, total_len,
+			    m->msg_flags & MSG_DONTWAIT);
+}
+
+static int tun_recvmsg(struct kiocb *iocb, struct socket *sock,
+		       struct msghdr *m, size_t total_len,
+		       int flags)
+{
+	struct tun_struct *tun = container_of(sock, struct tun_struct, socket);
+	int ret;
+	if (flags & ~(MSG_DONTWAIT|MSG_TRUNC))
+		return -EINVAL;
+	ret = tun_do_read(tun, iocb, m->msg_iov, total_len,
+			  flags & MSG_DONTWAIT);
+	if (ret > total_len) {
+		m->msg_flags |= MSG_TRUNC;
+		ret = flags & MSG_TRUNC ? ret : total_len;
+	}
+	return ret;
+}
+
+/* Ops structure to mimic raw sockets with tun */
+static const struct proto_ops tun_socket_ops = {
+	.sendmsg = tun_sendmsg,
+	.recvmsg = tun_recvmsg,
+};
+
 static struct proto tun_proto = {
 	.name		= "tun",
 	.owner		= THIS_MODULE,
@@ -986,6 +1031,7 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
 			goto err_free_dev;
 
 		init_waitqueue_head(&tun->socket.wait);
+		tun->socket.ops = &tun_socket_ops;
 		sock_init_data(&tun->socket, sk);
 		sk->sk_write_space = tun_sock_write_space;
 		sk->sk_sndbuf = INT_MAX;
@@ -1489,6 +1535,23 @@ static void tun_cleanup(void)
 	rtnl_link_unregister(&tun_link_ops);
 }
 
+/* Get an underlying socket object from tun file.  Returns error unless file is
+ * attached to a device.  The returned object works like a packet socket, it
+ * can be used for sock_sendmsg/sock_recvmsg.  The caller is responsible for
+ * holding a reference to the file for as long as the socket is in use. */
+struct socket *tun_get_socket(struct file *file)
+{
+	struct tun_struct *tun;
+	if (file->f_op != &tun_fops)
+		return ERR_PTR(-EINVAL);
+	tun = tun_get(file);
+	if (!tun)
+		return ERR_PTR(-EBADFD);
+	tun_put(tun);
+	return &tun->socket;
+}
+EXPORT_SYMBOL_GPL(tun_get_socket);
+
 module_init(tun_init);
 module_exit(tun_cleanup);
 MODULE_DESCRIPTION(DRV_DESCRIPTION);
diff --git a/include/linux/if_tun.h b/include/linux/if_tun.h
index 3f5fd52..404abe0 100644
--- a/include/linux/if_tun.h
+++ b/include/linux/if_tun.h
@@ -86,4 +86,18 @@ struct tun_filter {
 	__u8   addr[0][ETH_ALEN];
 };
 
+#ifdef __KERNEL__
+#if defined(CONFIG_TUN) || defined(CONFIG_TUN_MODULE)
+struct socket *tun_get_socket(struct file *);
+#else
+#include <linux/err.h>
+#include <linux/errno.h>
+struct file;
+struct socket;
+static inline struct socket *tun_get_socket(struct file *f)
+{
+	return ERR_PTR(-EINVAL);
+}
+#endif /* CONFIG_TUN */
+#endif /* __KERNEL__ */
 #endif /* __IF_TUN_H */
-- 
1.6.5.2.143.g8cc62

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[PATCHv8 2/3] mm: export use_mm/unuse_mm to modules

[Michael S. Tsirkin]

vhost net module wants to do copy to/from user from a kernel thread,
which needs use_mm. Export it to modules.

Acked-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 mm/mmu_context.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/mm/mmu_context.c b/mm/mmu_context.c
index ded9081..0777654 100644
--- a/mm/mmu_context.c
+++ b/mm/mmu_context.c
@@ -5,6 +5,7 @@
 
 #include <linux/mm.h>
 #include <linux/mmu_context.h>
+#include <linux/module.h>
 #include <linux/sched.h>
 
 #include <asm/mmu_context.h>
@@ -37,6 +38,7 @@ void use_mm(struct mm_struct *mm)
 	if (active_mm != mm)
 		mmdrop(active_mm);
 }
+EXPORT_SYMBOL_GPL(use_mm);
 
 /*
  * unuse_mm
@@ -56,3 +58,4 @@ void unuse_mm(struct mm_struct *mm)
 	enter_lazy_tlb(mm, tsk);
 	task_unlock(tsk);
 }
+EXPORT_SYMBOL_GPL(unuse_mm);
-- 
1.6.5.2.143.g8cc62

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[PATCHv8 3/3] vhost_net: a kernel-level virtio server

[Michael S. Tsirkin]

What it is: vhost net is a character device that can be used to reduce
the number of system calls involved in virtio networking.
Existing virtio net code is used in the guest without modification.

There's similarity with vringfd, with some differences and reduced scope
- uses eventfd for signalling
- structures can be moved around in memory at any time (good for
  migration, bug work-arounds in userspace)
- write logging is supported (good for migration)
- support memory table and not just an offset (needed for kvm)

common virtio related code has been put in a separate file vhost.c and
can be made into a separate module if/when more backends appear.  I used
Rusty's lguest.c as the source for developing this part : this supplied
me with witty comments I wouldn't be able to write myself.

What it is not: vhost net is not a bus, and not a generic new system
call. No assumptions are made on how guest performs hypercalls.
Userspace hypervisors are supported as well as kvm.

How it works: Basically, we connect virtio frontend (configured by
userspace) to a backend. The backend could be a network device, or a tap
device.  Backend is also configured by userspace, including vlan/mac
etc.

Status: This works for me, and I haven't see any crashes.
Compared to userspace, people reported improved latency (as I save up to
4 system calls per packet), as well as better bandwidth and CPU
utilization.

Features that I plan to look at in the future:
- mergeable buffers
- zero copy
- scalability tuning: figure out the best threading model to use

Note on RCU usage (this is also documented in vhost.h, near
private_pointer which is the value protected by this variant of RCU):
what is happening is that the rcu_dereference() is being used
in a workqueue item.  The role of rcu_read_lock() is taken on by the
start of execution of the workqueue item, of rcu_read_unlock() by the
end of execution of the workqueue item, and of synchronize_rcu() by
flush_workqueue()/flush_work().

Acked-by: Arnd Bergmann <arnd@arndb.de>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---

Paul, I'm Cc you so that you can verify that nothing significant changed
in vhost/net.c since you approved it the previous time.  Could you
please Ack use of rcu_dereference/rcu_assign_pointer there?  Eric
Dumazet proposed that I open-code these, he suggested that this is an
abuse of these macros and not enough like RCU. What is your opinion?

 MAINTAINERS                |    9 +
 arch/x86/kvm/Kconfig       |    1 +
 drivers/Makefile           |    1 +
 drivers/vhost/Kconfig      |   11 +
 drivers/vhost/Makefile     |    2 +
 drivers/vhost/net.c        |  633 +++++++++++++++++++++++++++++
 drivers/vhost/vhost.c      |  970 ++++++++++++++++++++++++++++++++++++++++++++
 drivers/vhost/vhost.h      |  158 +++++++
 include/linux/Kbuild       |    1 +
 include/linux/miscdevice.h |    1 +
 include/linux/vhost.h      |  126 ++++++
 11 files changed, 1913 insertions(+), 0 deletions(-)
 create mode 100644 drivers/vhost/Kconfig
 create mode 100644 drivers/vhost/Makefile
 create mode 100644 drivers/vhost/net.c
 create mode 100644 drivers/vhost/vhost.c
 create mode 100644 drivers/vhost/vhost.h
 create mode 100644 include/linux/vhost.h

diff --git a/MAINTAINERS b/MAINTAINERS
index 8824115..980a69b 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -5619,6 +5619,15 @@ S:	Maintained
 F:	Documentation/filesystems/vfat.txt
 F:	fs/fat/
 
+VIRTIO HOST (VHOST)
+M:	"Michael S. Tsirkin" <mst@redhat.com>
+L:	kvm@vger.kernel.org
+L:	virtualization@lists.osdl.org
+L:	netdev@vger.kernel.org
+S:	Maintained
+F:	drivers/vhost/
+F:	include/linux/vhost.h
+
 VIA RHINE NETWORK DRIVER
 M:	Roger Luethi <rl@hellgate.ch>
 S:	Maintained
diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
index b84e571..94f44d9 100644
--- a/arch/x86/kvm/Kconfig
+++ b/arch/x86/kvm/Kconfig
@@ -64,6 +64,7 @@ config KVM_AMD
 
 # OK, it's a little counter-intuitive to do this, but it puts it neatly under
 # the virtualization menu.
+source drivers/vhost/Kconfig
 source drivers/lguest/Kconfig
 source drivers/virtio/Kconfig
 
diff --git a/drivers/Makefile b/drivers/Makefile
index 6ee53c7..81e3659 100644
--- a/drivers/Makefile
+++ b/drivers/Makefile
@@ -106,6 +106,7 @@ obj-$(CONFIG_HID)		+= hid/
 obj-$(CONFIG_PPC_PS3)		+= ps3/
 obj-$(CONFIG_OF)		+= of/
 obj-$(CONFIG_SSB)		+= ssb/
+obj-$(CONFIG_VHOST_NET)		+= vhost/
 obj-$(CONFIG_VIRTIO)		+= virtio/
 obj-$(CONFIG_VLYNQ)		+= vlynq/
 obj-$(CONFIG_STAGING)		+= staging/
diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
new file mode 100644
index 0000000..9f409f4
--- /dev/null
+++ b/drivers/vhost/Kconfig
@@ -0,0 +1,11 @@
+config VHOST_NET
+	tristate "Host kernel accelerator for virtio net (EXPERIMENTAL)"
+	depends on NET && EVENTFD && EXPERIMENTAL
+	---help---
+	  This kernel module can be loaded in host kernel to accelerate
+	  guest networking with virtio_net. Not to be confused with virtio_net
+	  module itself which needs to be loaded in guest kernel.
+
+	  To compile this driver as a module, choose M here: the module will
+	  be called vhost_net.
+
diff --git a/drivers/vhost/Makefile b/drivers/vhost/Makefile
new file mode 100644
index 0000000..72dd020
--- /dev/null
+++ b/drivers/vhost/Makefile
@@ -0,0 +1,2 @@
+obj-$(CONFIG_VHOST_NET) += vhost_net.o
+vhost_net-y := vhost.o net.o
diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
new file mode 100644
index 0000000..aeafaf3
--- /dev/null
+++ b/drivers/vhost/net.c
@@ -0,0 +1,633 @@
+/* Copyright (C) 2009 Red Hat, Inc.
+ * Author: Michael S. Tsirkin <mst@redhat.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2.
+ *
+ * virtio-net server in host kernel.
+ */
+
+#include <linux/compat.h>
+#include <linux/eventfd.h>
+#include <linux/vhost.h>
+#include <linux/virtio_net.h>
+#include <linux/mmu_context.h>
+#include <linux/miscdevice.h>
+#include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/workqueue.h>
+#include <linux/rcupdate.h>
+#include <linux/file.h>
+
+#include <linux/net.h>
+#include <linux/if_packet.h>
+#include <linux/if_arp.h>
+#include <linux/if_tun.h>
+
+#include <net/sock.h>
+
+#include "vhost.h"
+
+/* Max number of bytes transferred before requeueing the job.
+ * Using this limit prevents one virtqueue from starving others. */
+#define VHOST_NET_WEIGHT 0x80000
+
+enum {
+	VHOST_NET_VQ_RX = 0,
+	VHOST_NET_VQ_TX = 1,
+	VHOST_NET_VQ_MAX = 2,
+};
+
+enum vhost_net_poll_state {
+	VHOST_NET_POLL_DISABLED = 0,
+	VHOST_NET_POLL_STARTED = 1,
+	VHOST_NET_POLL_STOPPED = 2,
+};
+
+struct vhost_net {
+	struct vhost_dev dev;
+	struct vhost_virtqueue vqs[VHOST_NET_VQ_MAX];
+	struct vhost_poll poll[VHOST_NET_VQ_MAX];
+	/* Tells us whether we are polling a socket for TX.
+	 * We only do this when socket buffer fills up.
+	 * Protected by tx vq lock. */
+	enum vhost_net_poll_state tx_poll_state;
+};
+
+/* Pop first len bytes from iovec. Return number of segments used. */
+static int move_iovec_hdr(struct iovec *from, struct iovec *to,
+			  size_t len, int iov_count)
+{
+	int seg = 0;
+	size_t size;
+	while (len && seg < iov_count) {
+		size = min(from->iov_len, len);
+		to->iov_base = from->iov_base;
+		to->iov_len = size;
+		from->iov_len -= size;
+		from->iov_base += size;
+		len -= size;
+		++from;
+		++to;
+		++seg;
+	}
+	return seg;
+}
+
+/* Caller must have TX VQ lock */
+static void tx_poll_stop(struct vhost_net *net)
+{
+	if (likely(net->tx_poll_state != VHOST_NET_POLL_STARTED))
+		return;
+	vhost_poll_stop(net->poll + VHOST_NET_VQ_TX);
+	net->tx_poll_state = VHOST_NET_POLL_STOPPED;
+}
+
+/* Caller must have TX VQ lock */
+static void tx_poll_start(struct vhost_net *net, struct socket *sock)
+{
+	if (unlikely(net->tx_poll_state != VHOST_NET_POLL_STOPPED))
+		return;
+	vhost_poll_start(net->poll + VHOST_NET_VQ_TX, sock->file);
+	net->tx_poll_state = VHOST_NET_POLL_STARTED;
+}
+
+/* Expects to be always run from workqueue - which acts as
+ * read-size critical section for our kind of RCU. */
+static void handle_tx(struct vhost_net *net)
+{
+	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_TX];
+	unsigned head, out, in, s;
+	struct msghdr msg = {
+		.msg_name = NULL,
+		.msg_namelen = 0,
+		.msg_control = NULL,
+		.msg_controllen = 0,
+		.msg_iov = vq->iov,
+		.msg_flags = MSG_DONTWAIT,
+	};
+	size_t len, total_len = 0;
+	int err, wmem;
+	size_t hdr_size;
+	struct socket *sock = rcu_dereference(vq->private_data);
+	if (!sock)
+		return;
+
+	wmem = atomic_read(&sock->sk->sk_wmem_alloc);
+	if (wmem >= sock->sk->sk_sndbuf)
+		return;
+
+	use_mm(net->dev.mm);
+	mutex_lock(&vq->mutex);
+	vhost_no_notify(vq);
+
+	if (wmem < sock->sk->sk_sndbuf * 2)
+		tx_poll_stop(net);
+	hdr_size = vq->hdr_size;
+
+	for (;;) {
+		head = vhost_get_vq_desc(&net->dev, vq, vq->iov, &out, &in,
+					 NULL, NULL);
+		/* Nothing new?  Wait for eventfd to tell us they refilled. */
+		if (head == vq->num) {
+			wmem = atomic_read(&sock->sk->sk_wmem_alloc);
+			if (wmem >= sock->sk->sk_sndbuf * 3 / 4) {
+				tx_poll_start(net, sock);
+				set_bit(SOCK_ASYNC_NOSPACE, &sock->flags);
+				break;
+			}
+			if (vhost_notify(vq))
+				continue;
+			break;
+		}
+		if (in) {
+			vq_err(vq, "Unexpected descriptor format for TX: "
+			       "out %d, int %d\n", out, in);
+			break;
+		}
+		/* Skip header. TODO: support TSO. */
+		s = move_iovec_hdr(vq->iov, vq->hdr, hdr_size, out);
+		msg.msg_iovlen = out;
+		len = iov_length(vq->iov, out);
+		/* Sanity check */
+		if (!len) {
+			vq_err(vq, "Unexpected header len for TX: "
+			       "%zd expected %zd\n",
+			       iov_length(vq->hdr, s), hdr_size);
+			break;
+		}
+		/* TODO: Check specific error and bomb out unless ENOBUFS? */
+		err = sock->ops->sendmsg(NULL, sock, &msg, len);
+		if (unlikely(err < 0)) {
+			vhost_discard_vq_desc(vq);
+			tx_poll_start(net, sock);
+			break;
+		}
+		if (err != len)
+			pr_err("Truncated TX packet: "
+			       " len %d != %zd\n", err, len);
+		vhost_add_used_and_trigger(&net->dev, vq, head, 0);
+		total_len += len;
+		if (unlikely(total_len >= VHOST_NET_WEIGHT)) {
+			vhost_poll_queue(&vq->poll);
+			break;
+		}
+	}
+
+	mutex_unlock(&vq->mutex);
+	unuse_mm(net->dev.mm);
+}
+
+/* Expects to be always run from workqueue - which acts as
+ * read-size critical section for our kind of RCU. */
+static void handle_rx(struct vhost_net *net)
+{
+	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_RX];
+	unsigned head, out, in, log, s;
+	struct vhost_log *vq_log;
+	struct msghdr msg = {
+		.msg_name = NULL,
+		.msg_namelen = 0,
+		.msg_control = NULL, /* FIXME: get and handle RX aux data. */
+		.msg_controllen = 0,
+		.msg_iov = vq->iov,
+		.msg_flags = MSG_DONTWAIT,
+	};
+
+	struct virtio_net_hdr hdr = {
+		.flags = 0,
+		.gso_type = VIRTIO_NET_HDR_GSO_NONE
+	};
+
+	size_t len, total_len = 0;
+	int err;
+	size_t hdr_size;
+	struct socket *sock = rcu_dereference(vq->private_data);
+	if (!sock || skb_queue_empty(&sock->sk->sk_receive_queue))
+		return;
+
+	use_mm(net->dev.mm);
+	mutex_lock(&vq->mutex);
+	vhost_no_notify(vq);
+	hdr_size = vq->hdr_size;
+
+	vq_log = unlikely(vhost_has_feature(&net->dev, VHOST_F_LOG_ALL)) ?
+		vq->log : NULL;
+
+	for (;;) {
+		head = vhost_get_vq_desc(&net->dev, vq, vq->iov, &out, &in,
+					 vq_log, &log);
+		/* OK, now we need to know about added descriptors. */
+		if (head == vq->num && vhost_notify(vq))
+			/* They could have slipped one in as we were doing that:
+			 * check again. */
+			continue;
+		/* Nothing new?  Wait for eventfd to tell us they refilled. */
+		if (head == vq->num)
+			break;
+		/* We don't need to be notified again. */
+		vhost_no_notify(vq);
+		if (out) {
+			vq_err(vq, "Unexpected descriptor format for RX: "
+			       "out %d, int %d\n",
+			       out, in);
+			break;
+		}
+		/* Skip header. TODO: support TSO/mergeable rx buffers. */
+		s = move_iovec_hdr(vq->iov, vq->hdr, hdr_size, in);
+		msg.msg_iovlen = in;
+		len = iov_length(vq->iov, in);
+		/* Sanity check */
+		if (!len) {
+			vq_err(vq, "Unexpected header len for RX: "
+			       "%zd expected %zd\n",
+			       iov_length(vq->hdr, s), hdr_size);
+			break;
+		}
+		err = sock->ops->recvmsg(NULL, sock, &msg,
+					 len, MSG_DONTWAIT | MSG_TRUNC);
+		/* TODO: Check specific error and bomb out unless EAGAIN? */
+		if (err < 0) {
+			vhost_discard_vq_desc(vq);
+			break;
+		}
+		/* TODO: Should check and handle checksum. */
+		if (err > len) {
+			pr_err("Discarded truncated rx packet: "
+			       " len %d > %zd\n", err, len);
+			vhost_discard_vq_desc(vq);
+			continue;
+		}
+		len = err;
+		err = memcpy_toiovec(vq->hdr, (unsigned char *)&hdr, hdr_size);
+		if (err) {
+			vq_err(vq, "Unable to write vnet_hdr at addr %p: %d\n",
+			       vq->iov->iov_base, err);
+			break;
+		}
+		len += hdr_size;
+		vhost_add_used_and_trigger(&net->dev, vq, head, len);
+		if (unlikely(vq_log))
+			vhost_log_write(vq, vq_log, log, len);
+		total_len += len;
+		if (unlikely(total_len >= VHOST_NET_WEIGHT)) {
+			vhost_poll_queue(&vq->poll);
+			break;
+		}
+	}
+
+	mutex_unlock(&vq->mutex);
+	unuse_mm(net->dev.mm);
+}
+
+static void handle_tx_kick(struct work_struct *work)
+{
+	struct vhost_virtqueue *vq;
+	struct vhost_net *net;
+	vq = container_of(work, struct vhost_virtqueue, poll.work);
+	net = container_of(vq->dev, struct vhost_net, dev);
+	handle_tx(net);
+}
+
+static void handle_rx_kick(struct work_struct *work)
+{
+	struct vhost_virtqueue *vq;
+	struct vhost_net *net;
+	vq = container_of(work, struct vhost_virtqueue, poll.work);
+	net = container_of(vq->dev, struct vhost_net, dev);
+	handle_rx(net);
+}
+
+static void handle_tx_net(struct work_struct *work)
+{
+	struct vhost_net *net;
+	net = container_of(work, struct vhost_net, poll[VHOST_NET_VQ_TX].work);
+	handle_tx(net);
+}
+
+static void handle_rx_net(struct work_struct *work)
+{
+	struct vhost_net *net;
+	net = container_of(work, struct vhost_net, poll[VHOST_NET_VQ_RX].work);
+	handle_rx(net);
+}
+
+static int vhost_net_open(struct inode *inode, struct file *f)
+{
+	struct vhost_net *n = kzalloc(sizeof *n, GFP_KERNEL);
+	int r;
+	if (!n)
+		return -ENOMEM;
+	f->private_data = n;
+	n->vqs[VHOST_NET_VQ_TX].handle_kick = handle_tx_kick;
+	n->vqs[VHOST_NET_VQ_RX].handle_kick = handle_rx_kick;
+	r = vhost_dev_init(&n->dev, n->vqs, VHOST_NET_VQ_MAX);
+	if (r < 0) {
+		kfree(n);
+		return r;
+	}
+
+	vhost_poll_init(n->poll + VHOST_NET_VQ_TX, handle_tx_net, POLLOUT);
+	vhost_poll_init(n->poll + VHOST_NET_VQ_RX, handle_rx_net, POLLIN);
+	n->tx_poll_state = VHOST_NET_POLL_DISABLED;
+	return 0;
+}
+
+static void vhost_net_disable_vq(struct vhost_net *n, int index)
+{
+	if (!n->vqs[index].private_data)
+		return;
+	if (index == VHOST_NET_VQ_TX) {
+		tx_poll_stop(n);
+		n->tx_poll_state = VHOST_NET_POLL_DISABLED;
+	} else
+		vhost_poll_stop(n->poll + VHOST_NET_VQ_RX);
+}
+
+static void vhost_net_enable_vq(struct vhost_net *n, int index)
+{
+	struct socket *sock = n->vqs[index].private_data;
+	if (!sock)
+		return;
+	if (index == VHOST_NET_VQ_TX) {
+		n->tx_poll_state = VHOST_NET_POLL_STOPPED;
+		tx_poll_start(n, sock);
+	} else
+		vhost_poll_start(n->poll + VHOST_NET_VQ_RX, sock->file);
+}
+
+static struct socket *vhost_net_stop_vq(struct vhost_net *n, int index)
+{
+	struct socket *sock;
+
+	mutex_lock(&n->vqs[index].mutex);
+	sock = n->vqs[index].private_data;
+	vhost_net_disable_vq(n, index);
+	rcu_assign_pointer(n->vqs[index].private_data, NULL);
+	mutex_unlock(&n->vqs[index].mutex);
+	return sock;
+}
+
+static void vhost_net_stop(struct vhost_net *n, struct socket **tx_sock,
+			   struct socket **rx_sock)
+{
+	*tx_sock = vhost_net_stop_vq(n, VHOST_NET_VQ_TX);
+	*rx_sock = vhost_net_stop_vq(n, VHOST_NET_VQ_RX);
+}
+
+static void vhost_net_flush_vq(struct vhost_net *n, int index)
+{
+	vhost_poll_flush(n->poll + index);
+	vhost_poll_flush(&n->dev.vqs[index].poll);
+}
+
+static void vhost_net_flush(struct vhost_net *n)
+{
+	vhost_net_flush_vq(n, VHOST_NET_VQ_TX);
+	vhost_net_flush_vq(n, VHOST_NET_VQ_RX);
+}
+
+static int vhost_net_release(struct inode *inode, struct file *f)
+{
+	struct vhost_net *n = f->private_data;
+	struct socket *tx_sock;
+	struct socket *rx_sock;
+
+	vhost_net_stop(n, &tx_sock, &rx_sock);
+	vhost_net_flush(n);
+	vhost_dev_cleanup(&n->dev);
+	if (tx_sock)
+		fput(tx_sock->file);
+	if (rx_sock)
+		fput(rx_sock->file);
+	/* We do an extra flush before freeing memory,
+	 * since jobs can re-queue themselves. */
+	vhost_net_flush(n);
+	kfree(n);
+	return 0;
+}
+
+static struct socket *get_raw_socket(int fd)
+{
+	struct {
+		struct sockaddr_ll sa;
+		char  buf[MAX_ADDR_LEN];
+	} uaddr;
+	int uaddr_len = sizeof uaddr, r;
+	struct socket *sock = sockfd_lookup(fd, &r);
+	if (!sock)
+		return ERR_PTR(-ENOTSOCK);
+
+	/* Parameter checking */
+	if (sock->sk->sk_type != SOCK_RAW) {
+		r = -ESOCKTNOSUPPORT;
+		goto err;
+	}
+
+	r = sock->ops->getname(sock, (struct sockaddr *)&uaddr.sa,
+			       &uaddr_len, 0);
+	if (r)
+		goto err;
+
+	if (uaddr.sa.sll_family != AF_PACKET) {
+		r = -EPFNOSUPPORT;
+		goto err;
+	}
+	return sock;
+err:
+	fput(sock->file);
+	return ERR_PTR(r);
+}
+
+static struct socket *get_tun_socket(int fd)
+{
+	struct file *file = fget(fd);
+	struct socket *sock;
+	if (!file)
+		return ERR_PTR(-EBADF);
+	sock = tun_get_socket(file);
+	if (IS_ERR(sock))
+		fput(file);
+	return sock;
+}
+
+static struct socket *get_socket(int fd)
+{
+	struct socket *sock;
+	if (fd == -1)
+		return NULL;
+	sock = get_raw_socket(fd);
+	if (!IS_ERR(sock))
+		return sock;
+	sock = get_tun_socket(fd);
+	if (!IS_ERR(sock))
+		return sock;
+	return ERR_PTR(-ENOTSOCK);
+}
+
+static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
+{
+	struct socket *sock, *oldsock = NULL;
+	struct vhost_virtqueue *vq;
+	int r;
+
+	mutex_lock(&n->dev.mutex);
+	r = vhost_dev_check_owner(&n->dev);
+	if (r)
+		goto done;
+
+	if (index >= VHOST_NET_VQ_MAX) {
+		r = -ENOBUFS;
+		goto done;
+	}
+	vq = n->vqs + index;
+	mutex_lock(&vq->mutex);
+	sock = get_socket(fd);
+	if (IS_ERR(sock)) {
+		r = PTR_ERR(sock);
+		goto done;
+	}
+
+	/* start polling new socket */
+	oldsock = vq->private_data;
+	if (sock == oldsock)
+		goto done;
+
+	vhost_net_disable_vq(n, index);
+	rcu_assign_pointer(vq->private_data, sock);
+	vhost_net_enable_vq(n, index);
+	mutex_unlock(&vq->mutex);
+done:
+	mutex_unlock(&n->dev.mutex);
+	if (oldsock) {
+		vhost_net_flush_vq(n, index);
+		fput(oldsock->file);
+	}
+	return r;
+}
+
+static long vhost_net_reset_owner(struct vhost_net *n)
+{
+	struct socket *tx_sock = NULL;
+	struct socket *rx_sock = NULL;
+	long r;
+	mutex_lock(&n->dev.mutex);
+	r = vhost_dev_check_owner(&n->dev);
+	if (r)
+		goto done;
+	vhost_net_stop(n, &tx_sock, &rx_sock);
+	vhost_net_flush(n);
+	r = vhost_dev_reset_owner(&n->dev);
+done:
+	mutex_unlock(&n->dev.mutex);
+	if (tx_sock)
+		fput(tx_sock->file);
+	if (rx_sock)
+		fput(rx_sock->file);
+	return r;
+}
+
+static void vhost_net_set_features(struct vhost_net *n, u64 features)
+{
+	size_t hdr_size = features & (1 << VHOST_NET_F_VIRTIO_NET_HDR) ?
+		sizeof(struct virtio_net_hdr) : 0;
+	int i;
+	mutex_lock(&n->dev.mutex);
+	n->dev.acked_features = features;
+	smp_wmb();
+	for (i = 0; i < VHOST_NET_VQ_MAX; ++i) {
+		mutex_lock(&n->vqs[i].mutex);
+		n->vqs[i].hdr_size = hdr_size;
+		mutex_unlock(&n->vqs[i].mutex);
+	}
+	mutex_unlock(&n->dev.mutex);
+	vhost_net_flush(n);
+}
+
+static long vhost_net_ioctl(struct file *f, unsigned int ioctl,
+			    unsigned long arg)
+{
+	struct vhost_net *n = f->private_data;
+	void __user *argp = (void __user *)arg;
+	u32 __user *featurep = argp;
+	struct vhost_vring_file backend;
+	u64 features;
+	int r;
+	switch (ioctl) {
+	case VHOST_NET_SET_BACKEND:
+		r = copy_from_user(&backend, argp, sizeof backend);
+		if (r < 0)
+			return r;
+		return vhost_net_set_backend(n, backend.index, backend.fd);
+	case VHOST_GET_FEATURES:
+		features = VHOST_FEATURES;
+		return put_user(features, featurep);
+	case VHOST_ACK_FEATURES:
+		r = get_user(features, featurep);
+		/* No features for now */
+		if (r < 0)
+			return r;
+		if (features & ~VHOST_FEATURES)
+			return -EOPNOTSUPP;
+		vhost_net_set_features(n, features);
+		return 0;
+	case VHOST_RESET_OWNER:
+		return vhost_net_reset_owner(n);
+	default:
+		r = vhost_dev_ioctl(&n->dev, ioctl, arg);
+		vhost_net_flush(n);
+		return r;
+	}
+}
+
+#ifdef CONFIG_COMPAT
+static long vhost_net_compat_ioctl(struct file *f, unsigned int ioctl,
+				   unsigned long arg)
+{
+	return vhost_net_ioctl(f, ioctl, (unsigned long)compat_ptr(arg));
+}
+#endif
+
+const static struct file_operations vhost_net_fops = {
+	.owner          = THIS_MODULE,
+	.release        = vhost_net_release,
+	.unlocked_ioctl = vhost_net_ioctl,
+#ifdef CONFIG_COMPAT
+	.compat_ioctl   = vhost_net_compat_ioctl,
+#endif
+	.open           = vhost_net_open,
+};
+
+static struct miscdevice vhost_net_misc = {
+	VHOST_NET_MINOR,
+	"vhost-net",
+	&vhost_net_fops,
+};
+
+int vhost_net_init(void)
+{
+	int r = vhost_init();
+	if (r)
+		goto err_init;
+	r = misc_register(&vhost_net_misc);
+	if (r)
+		goto err_reg;
+	return 0;
+err_reg:
+	vhost_cleanup();
+err_init:
+	return r;
+
+}
+module_init(vhost_net_init);
+
+void vhost_net_exit(void)
+{
+	misc_deregister(&vhost_net_misc);
+	vhost_cleanup();
+}
+module_exit(vhost_net_exit);
+
+MODULE_VERSION("0.0.1");
+MODULE_LICENSE("GPL v2");
+MODULE_AUTHOR("Michael S. Tsirkin");
+MODULE_DESCRIPTION("Host kernel accelerator for virtio net");
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
new file mode 100644
index 0000000..985f7f8
--- /dev/null
+++ b/drivers/vhost/vhost.c
@@ -0,0 +1,970 @@
+/* Copyright (C) 2009 Red Hat, Inc.
+ * Copyright (C) 2006 Rusty Russell IBM Corporation
+ *
+ * Author: Michael S. Tsirkin <mst@redhat.com>
+ *
+ * Inspiration, some code, and most witty comments come from
+ * Documentation/lguest/lguest.c, by Rusty Russell
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2.
+ *
+ * Generic code for virtio server in host kernel.
+ */
+
+#include <linux/eventfd.h>
+#include <linux/vhost.h>
+#include <linux/virtio_net.h>
+#include <linux/mm.h>
+#include <linux/miscdevice.h>
+#include <linux/mutex.h>
+#include <linux/workqueue.h>
+#include <linux/rcupdate.h>
+#include <linux/poll.h>
+#include <linux/file.h>
+#include <linux/highmem.h>
+
+#include <linux/net.h>
+#include <linux/if_packet.h>
+#include <linux/if_arp.h>
+
+#include <net/sock.h>
+
+#include "vhost.h"
+
+enum {
+	VHOST_MEMORY_MAX_NREGIONS = 64,
+	VHOST_MEMORY_F_LOG = 0x1,
+};
+
+static struct workqueue_struct *vhost_workqueue;
+
+static void vhost_poll_func(struct file *file, wait_queue_head_t *wqh,
+			    poll_table *pt)
+{
+	struct vhost_poll *poll;
+	poll = container_of(pt, struct vhost_poll, table);
+
+	poll->wqh = wqh;
+	add_wait_queue(wqh, &poll->wait);
+}
+
+static int vhost_poll_wakeup(wait_queue_t *wait, unsigned mode, int sync,
+			     void *key)
+{
+	struct vhost_poll *poll;
+	poll = container_of(wait, struct vhost_poll, wait);
+	if (!((unsigned long)key & poll->mask))
+		return 0;
+
+	queue_work(vhost_workqueue, &poll->work);
+	return 0;
+}
+
+/* Init poll structure */
+void vhost_poll_init(struct vhost_poll *poll, work_func_t func,
+		     unsigned long mask)
+{
+	INIT_WORK(&poll->work, func);
+	init_waitqueue_func_entry(&poll->wait, vhost_poll_wakeup);
+	init_poll_funcptr(&poll->table, vhost_poll_func);
+	poll->mask = mask;
+}
+
+/* Start polling a file. We add ourselves to file's wait queue. The caller must
+ * keep a reference to a file until after vhost_poll_stop is called. */
+void vhost_poll_start(struct vhost_poll *poll, struct file *file)
+{
+	unsigned long mask;
+	mask = file->f_op->poll(file, &poll->table);
+	if (mask)
+		vhost_poll_wakeup(&poll->wait, 0, 0, (void *)mask);
+}
+
+/* Stop polling a file. After this function returns, it becomes safe to drop the
+ * file reference. You must also flush afterwards. */
+void vhost_poll_stop(struct vhost_poll *poll)
+{
+	remove_wait_queue(poll->wqh, &poll->wait);
+}
+
+/* Flush any work that has been scheduled. When calling this, don't hold any
+ * locks that are also used by the callback. */
+void vhost_poll_flush(struct vhost_poll *poll)
+{
+	flush_work(&poll->work);
+}
+
+void vhost_poll_queue(struct vhost_poll *poll)
+{
+	queue_work(vhost_workqueue, &poll->work);
+}
+
+long vhost_dev_init(struct vhost_dev *dev,
+		    struct vhost_virtqueue *vqs, int nvqs)
+{
+	int i;
+	dev->vqs = vqs;
+	dev->nvqs = nvqs;
+	mutex_init(&dev->mutex);
+
+	for (i = 0; i < dev->nvqs; ++i) {
+		dev->vqs[i].dev = dev;
+		mutex_init(&dev->vqs[i].mutex);
+		if (dev->vqs[i].handle_kick)
+			vhost_poll_init(&dev->vqs[i].poll,
+					dev->vqs[i].handle_kick,
+					POLLIN);
+	}
+	return 0;
+}
+
+/* Caller should have device mutex */
+long vhost_dev_check_owner(struct vhost_dev *dev)
+{
+	/* Are you the owner? If not, I don't think you mean to do that */
+	return dev->mm == current->mm ? 0 : -EPERM;
+}
+
+/* Caller should have device mutex */
+static long vhost_dev_set_owner(struct vhost_dev *dev)
+{
+	/* Is there an owner already? */
+	if (dev->mm)
+		return -EBUSY;
+	/* No owner, become one */
+	dev->mm = get_task_mm(current);
+	return 0;
+}
+
+/* Caller should have device mutex */
+long vhost_dev_reset_owner(struct vhost_dev *dev)
+{
+	struct vhost_memory *memory;
+
+	/* Restore memory to default 1:1 mapping. */
+	memory = kzalloc(offsetof(struct vhost_memory, regions) +
+			 2 * sizeof *memory->regions, GFP_KERNEL);
+	if (!memory)
+		return -ENOMEM;
+
+	vhost_dev_cleanup(dev);
+
+	memory->nregions = 2;
+	memory->regions[0].guest_phys_addr = 1;
+	memory->regions[0].userspace_addr = 1;
+	memory->regions[0].memory_size = ~0ULL;
+	memory->regions[1].guest_phys_addr = 0;
+	memory->regions[1].userspace_addr = 0;
+	memory->regions[1].memory_size = 1;
+	dev->memory = memory;
+	return 0;
+}
+
+/* Caller should have device mutex */
+void vhost_dev_cleanup(struct vhost_dev *dev)
+{
+	int i;
+	for (i = 0; i < dev->nvqs; ++i) {
+		if (dev->vqs[i].kick && dev->vqs[i].handle_kick) {
+			vhost_poll_stop(&dev->vqs[i].poll);
+			vhost_poll_flush(&dev->vqs[i].poll);
+		}
+		if (dev->vqs[i].error_ctx)
+			eventfd_ctx_put(dev->vqs[i].error_ctx);
+		if (dev->vqs[i].error)
+			fput(dev->vqs[i].error);
+		if (dev->vqs[i].kick)
+			fput(dev->vqs[i].kick);
+		if (dev->vqs[i].call_ctx)
+			eventfd_ctx_put(dev->vqs[i].call_ctx);
+		if (dev->vqs[i].call)
+			fput(dev->vqs[i].call);
+		dev->vqs[i].error_ctx = NULL;
+		dev->vqs[i].error = NULL;
+		dev->vqs[i].kick = NULL;
+		dev->vqs[i].call_ctx = NULL;
+		dev->vqs[i].call = NULL;
+	}
+	if (dev->log_ctx)
+		eventfd_ctx_put(dev->log_ctx);
+	dev->log_ctx = NULL;
+	if (dev->log_file)
+		fput(dev->log_file);
+	dev->log_file = NULL;
+	/* No one will access memory at this point */
+	kfree(dev->memory);
+	dev->memory = NULL;
+	if (dev->mm)
+		mmput(dev->mm);
+	dev->mm = NULL;
+}
+
+static long vhost_set_memory(struct vhost_dev *d, struct vhost_memory __user *m)
+{
+	struct vhost_memory mem, *newmem, *oldmem;
+	unsigned long size = offsetof(struct vhost_memory, regions);
+	long r;
+	r = copy_from_user(&mem, m, size);
+	if (r)
+		return r;
+	if (mem.padding)
+		return -EOPNOTSUPP;
+	if (mem.nregions > VHOST_MEMORY_MAX_NREGIONS)
+		return -E2BIG;
+	newmem = kmalloc(size + mem.nregions * sizeof *m->regions, GFP_KERNEL);
+	if (!newmem)
+		return -ENOMEM;
+
+	memcpy(newmem, &mem, size);
+	r = copy_from_user(newmem->regions, m->regions,
+			   mem.nregions * sizeof *m->regions);
+	if (r) {
+		kfree(newmem);
+		return r;
+	}
+	oldmem = d->memory;
+	rcu_assign_pointer(d->memory, newmem);
+	synchronize_rcu();
+	kfree(oldmem);
+	return 0;
+}
+
+static int init_used(struct vhost_virtqueue *vq)
+{
+	int r = put_user(vq->used_flags, &vq->used->flags);
+	if (r)
+		return r;
+	return get_user(vq->last_used_idx, &vq->used->idx);
+}
+
+static long vhost_set_vring(struct vhost_dev *d, int ioctl, void __user *argp)
+{
+	struct file *eventfp, *filep = NULL,
+		    *pollstart = NULL, *pollstop = NULL;
+	struct eventfd_ctx *ctx = NULL;
+	u32 __user *idxp = argp;
+	struct vhost_virtqueue *vq;
+	struct vhost_vring_state s;
+	struct vhost_vring_file f;
+	struct vhost_vring_addr a;
+	u32 idx;
+	long r;
+
+	r = get_user(idx, idxp);
+	if (r < 0)
+		return r;
+	if (idx > d->nvqs)
+		return -ENOBUFS;
+
+	vq = d->vqs + idx;
+
+	mutex_lock(&vq->mutex);
+
+	switch (ioctl) {
+	case VHOST_SET_VRING_NUM:
+		r = copy_from_user(&s, argp, sizeof s);
+		if (r < 0)
+			break;
+		if (s.num > 0xffff) {
+			r = -EINVAL;
+			break;
+		}
+		vq->num = s.num;
+		break;
+	case VHOST_SET_VRING_BASE:
+		r = copy_from_user(&s, argp, sizeof s);
+		if (r < 0)
+			break;
+		if (s.num > 0xffff) {
+			r = -EINVAL;
+			break;
+		}
+		vq->avail_idx = vq->last_avail_idx = s.num;
+		break;
+	case VHOST_GET_VRING_BASE:
+		s.index = idx;
+		s.num = vq->last_avail_idx;
+		r = copy_to_user(argp, &s, sizeof s);
+		break;
+	case VHOST_SET_VRING_DESC:
+		r = copy_from_user(&a, argp, sizeof a);
+		if (r < 0)
+			break;
+		if (a.padding) {
+			r = -EOPNOTSUPP;
+			break;
+		}
+		if ((u64)(unsigned long)a.user_addr != a.user_addr) {
+			r = -EFAULT;
+			break;
+		}
+		vq->desc = (void __user *)(unsigned long)a.user_addr;
+		break;
+	case VHOST_SET_VRING_AVAIL:
+		r = copy_from_user(&a, argp, sizeof a);
+		if (r < 0)
+			break;
+		if (a.padding) {
+			r = -EOPNOTSUPP;
+			break;
+		}
+		if ((u64)(unsigned long)a.user_addr != a.user_addr) {
+			r = -EFAULT;
+			break;
+		}
+		if (a.user_addr & (sizeof *vq->avail->ring - 1)) {
+			r = -EINVAL;
+			break;
+		}
+		vq->avail = (void __user *)(unsigned long)a.user_addr;
+		/* Forget the cached index value. */
+		vq->avail_idx = vq->last_avail_idx;
+		break;
+	case VHOST_SET_VRING_USED:
+		r = copy_from_user(&a, argp, sizeof a);
+		if (r < 0)
+			break;
+		if (a.padding) {
+			r = -EOPNOTSUPP;
+			break;
+		}
+		if ((u64)(unsigned long)a.user_addr != a.user_addr) {
+			r = -EFAULT;
+			break;
+		}
+		if (a.user_addr & (sizeof *vq->used->ring - 1)) {
+			r = -EINVAL;
+			break;
+		}
+		vq->used = (void __user *)(unsigned long)a.user_addr;
+		r = init_used(vq);
+		if (r)
+			break;
+		break;
+	case VHOST_SET_VRING_LOG:
+		r = copy_from_user(&a, argp, sizeof a);
+		if (r < 0)
+			break;
+		if (a.padding) {
+			r = -EOPNOTSUPP;
+			break;
+		}
+		if (a.user_addr == VHOST_VRING_LOG_DISABLE) {
+			vq->log_used = false;
+			break;
+		}
+		if (a.user_addr & (sizeof *vq->used->ring - 1)) {
+			r = -EINVAL;
+			break;
+		}
+		vq->log_used = true;
+		vq->log_addr = a.user_addr;
+		break;
+	case VHOST_SET_VRING_KICK:
+		r = copy_from_user(&f, argp, sizeof f);
+		if (r < 0)
+			break;
+		eventfp = f.fd == -1 ? NULL : eventfd_fget(f.fd);
+		if (IS_ERR(eventfp))
+			return PTR_ERR(eventfp);
+		if (eventfp != vq->kick) {
+			pollstop = filep = vq->kick;
+			pollstart = vq->kick = eventfp;
+		} else
+			filep = eventfp;
+		break;
+	case VHOST_SET_VRING_CALL:
+		r = copy_from_user(&f, argp, sizeof f);
+		if (r < 0)
+			break;
+		eventfp = f.fd == -1 ? NULL : eventfd_fget(f.fd);
+		if (IS_ERR(eventfp))
+			return PTR_ERR(eventfp);
+		if (eventfp != vq->call) {
+			filep = vq->call;
+			ctx = vq->call_ctx;
+			vq->call = eventfp;
+			vq->call_ctx = eventfp ?
+				eventfd_ctx_fileget(eventfp) : NULL;
+		} else
+			filep = eventfp;
+		break;
+	case VHOST_SET_VRING_ERR:
+		r = copy_from_user(&f, argp, sizeof f);
+		if (r < 0)
+			break;
+		eventfp = f.fd == -1 ? NULL : eventfd_fget(f.fd);
+		if (IS_ERR(eventfp))
+			return PTR_ERR(eventfp);
+		if (eventfp != vq->error) {
+			filep = vq->error;
+			vq->error = eventfp;
+			ctx = vq->error_ctx;
+			vq->error_ctx = eventfp ?
+				eventfd_ctx_fileget(eventfp) : NULL;
+		} else
+			filep = eventfp;
+		break;
+	default:
+		r = -ENOIOCTLCMD;
+	}
+
+	if (pollstop && vq->handle_kick)
+		vhost_poll_stop(&vq->poll);
+
+	if (ctx)
+		eventfd_ctx_put(ctx);
+	if (filep)
+		fput(filep);
+
+	if (pollstart && vq->handle_kick)
+		vhost_poll_start(&vq->poll, vq->kick);
+
+	mutex_unlock(&vq->mutex);
+
+	if (pollstop && vq->handle_kick)
+		vhost_poll_flush(&vq->poll);
+	return r;
+}
+
+long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, unsigned long arg)
+{
+	void __user *argp = (void __user *)arg;
+	struct file *eventfp, *filep = NULL;
+	struct eventfd_ctx *ctx = NULL;
+	u64 p;
+	long r;
+	int i, fd;
+
+	mutex_lock(&d->mutex);
+	/* If you are not the owner, you can become one */
+	if (ioctl == VHOST_SET_OWNER) {
+		r = vhost_dev_set_owner(d);
+		goto done;
+	}
+
+	/* You must be the owner to do anything else */
+	r = vhost_dev_check_owner(d);
+	if (r)
+		goto done;
+
+	switch (ioctl) {
+	case VHOST_SET_MEM_TABLE:
+		r = vhost_set_memory(d, argp);
+		break;
+	case VHOST_SET_LOG_BASE:
+		r = copy_from_user(&p, argp, sizeof p);
+		if (r < 0)
+			break;
+		if ((u64)(unsigned long)p != p) {
+			r = -EFAULT;
+			break;
+		}
+		for (i = 0; i < d->nvqs; ++i) {
+			mutex_lock(&d->vqs[i].mutex);
+			d->vqs[i].log_base = (void __user *)(unsigned long)p;
+			mutex_unlock(&d->vqs[i].mutex);
+		}
+		break;
+	case VHOST_SET_LOG_FD:
+		r = get_user(fd, (int __user *)argp);
+		if (r < 0)
+			break;
+		eventfp = fd == -1 ? NULL : eventfd_fget(fd);
+		if (IS_ERR(eventfp)) {
+			r = PTR_ERR(eventfp);
+			break;
+		}
+		if (eventfp != d->log_file) {
+			filep = d->log_file;
+			ctx = d->log_ctx;
+			d->log_ctx = eventfp ?
+				eventfd_ctx_fileget(eventfp) : NULL;
+		} else
+			filep = eventfp;
+		for (i = 0; i < d->nvqs; ++i) {
+			mutex_lock(&d->vqs[i].mutex);
+			d->vqs[i].log_ctx = d->log_ctx;
+			mutex_unlock(&d->vqs[i].mutex);
+		}
+		if (ctx)
+			eventfd_ctx_put(ctx);
+		if (filep)
+			fput(filep);
+		break;
+	default:
+		r = vhost_set_vring(d, ioctl, argp);
+		break;
+	}
+done:
+	mutex_unlock(&d->mutex);
+	return r;
+}
+
+static const struct vhost_memory_region *find_region(struct vhost_memory *mem,
+						     __u64 addr, __u32 len)
+{
+	struct vhost_memory_region *reg;
+	int i;
+	/* linear search is not brilliant, but we really have on the order of 6
+	 * regions in practice */
+	for (i = 0; i < mem->nregions; ++i) {
+		reg = mem->regions + i;
+		if (reg->guest_phys_addr <= addr &&
+		    reg->guest_phys_addr + reg->memory_size - 1 >= addr)
+			return reg;
+	}
+	return NULL;
+}
+
+/* TODO: This is really inefficient.  We need something like get_user()
+ * (instruction directly accesses the data, with an exception table entry
+ * returning -EFAULT). See Documentation/x86/exception-tables.txt.
+ */
+static int set_bit_to_user(int nr, void __user *addr)
+{
+	unsigned long log = (unsigned long)addr;
+	struct page *page;
+	void *base;
+	int bit = nr + (log % PAGE_SIZE) * 8;
+	int r;
+	r = get_user_pages_fast(log, 1, 1, &page);
+	if (r)
+		return r;
+	base = kmap_atomic(page, KM_USER0);
+	set_bit(bit, base);
+	kunmap_atomic(base, KM_USER0);
+	set_page_dirty_lock(page);
+	put_page(page);
+	return 0;
+}
+
+static int log_write(void __user *log_base,
+		     u64 write_address, u64 write_length)
+{
+	int r;
+	if (!write_length)
+		return 0;
+	write_address /= VHOST_PAGE_SIZE;
+	for (;;) {
+		u64 base = (u64)(unsigned long)log_base;
+		u64 log = base + write_address / 8;
+		int bit = write_address % 8;
+		if ((u64)(unsigned long)log != log)
+			return -EFAULT;
+		r = set_bit_to_user(bit, (void __user *)(unsigned long)log);
+		if (r < 0)
+			return r;
+		if (write_length <= VHOST_PAGE_SIZE)
+			break;
+		write_length -= VHOST_PAGE_SIZE;
+		write_address += VHOST_PAGE_SIZE;
+	}
+	return r;
+}
+
+int vhost_log_write(struct vhost_virtqueue *vq, struct vhost_log *log,
+		    unsigned int log_num, u64 len)
+{
+	int i, r;
+
+	/* Make sure data written is seen before log. */
+	wmb();
+	for (i = 0; i < log_num; ++i) {
+		u64 l = min(log[i].len, len);
+		r = log_write(vq->log_base, log[i].addr, l);
+		if (r < 0)
+			return r;
+		len -= l;
+		if (!len)
+			return 0;
+	}
+	if (vq->log_ctx)
+		eventfd_signal(vq->log_ctx, 1);
+	/* Length written exceeds what we have stored. This is a bug. */
+	BUG();
+	return 0;
+}
+
+int translate_desc(struct vhost_dev *dev, u64 addr, u32 len,
+		   struct iovec iov[], int iov_size)
+{
+	const struct vhost_memory_region *reg;
+	struct vhost_memory *mem;
+	struct iovec *_iov;
+	u64 s = 0;
+	int ret = 0;
+
+	rcu_read_lock();
+
+	mem = rcu_dereference(dev->memory);
+	while ((u64)len > s) {
+		u64 size;
+		if (ret >= iov_size) {
+			ret = -ENOBUFS;
+			break;
+		}
+		reg = find_region(mem, addr, len);
+		if (!reg) {
+			ret = -EFAULT;
+			break;
+		}
+		_iov = iov + ret;
+		size = reg->memory_size - addr + reg->guest_phys_addr;
+		_iov->iov_len = min((u64)len, size);
+		_iov->iov_base = (void *)(unsigned long)
+			(reg->userspace_addr + addr - reg->guest_phys_addr);
+		s += size;
+		addr += size;
+		++ret;
+	}
+
+	rcu_read_unlock();
+	return ret;
+}
+
+/* Each buffer in the virtqueues is actually a chain of descriptors.  This
+ * function returns the next descriptor in the chain,
+ * or -1 if we're at the end. */
+static unsigned next_desc(struct vring_desc *desc)
+{
+	unsigned int next;
+
+	/* If this descriptor says it doesn't chain, we're done. */
+	if (!(desc->flags & VRING_DESC_F_NEXT))
+		return -1;
+
+	/* Check they're not leading us off end of descriptors. */
+	next = desc->next;
+	/* Make sure compiler knows to grab that: we don't want it changing! */
+	/* We will use the result as an index in an array, so most
+	 * architectures only need a compiler barrier here. */
+	read_barrier_depends();
+
+	return next;
+}
+
+static unsigned get_indirect(struct vhost_dev *dev, struct vhost_virtqueue *vq,
+			     struct iovec iov[],
+			     unsigned int *out_num, unsigned int *in_num,
+			     struct vhost_log *log, unsigned int *log_num,
+			     struct vring_desc *indirect)
+{
+	struct vring_desc desc;
+	unsigned int i = 0, count, found = 0;
+	int ret;
+
+	/* Sanity check */
+	if (indirect->len % sizeof desc) {
+		vq_err(vq, "Invalid length in indirect descriptor: "
+		       "len 0x%llx not multiple of 0x%zx\n",
+		       (unsigned long long)indirect->len,
+		       sizeof desc);
+		return -EINVAL;
+	}
+
+	ret = translate_desc(dev, indirect->addr, indirect->len, vq->indirect,
+			     ARRAY_SIZE(vq->indirect));
+	if (ret < 0) {
+		vq_err(vq, "Translation failure %d in indirect.\n", ret);
+		return ret;
+	}
+
+	/* We will use the result as an address to read from, so most
+	 * architectures only need a compiler barrier here. */
+	read_barrier_depends();
+
+	count = indirect->len / sizeof desc;
+	/* Buffers are chained via a 16 bit next field, so
+	 * we can have at most 2^16 of these. */
+	if (count > USHORT_MAX + 1) {
+		vq_err(vq, "Indirect buffer length too big: %d\n",
+		       indirect->len);
+		return -E2BIG;
+	}
+
+	do {
+		unsigned iov_count = *in_num + *out_num;
+		if (++found > count) {
+			vq_err(vq, "Loop detected: last one at %u "
+			       "indirect size %u\n",
+			       i, count);
+			return -EINVAL;
+		}
+		if (memcpy_fromiovec((unsigned char *)&desc, vq->indirect,
+				     sizeof desc)) {
+			vq_err(vq, "Failed indirect descriptor: idx %d, %zx\n",
+			       i, (size_t)indirect->addr + i * sizeof desc);
+			return -EINVAL;
+		}
+		if (desc.flags & VRING_DESC_F_INDIRECT) {
+			vq_err(vq, "Nested indirect descriptor: idx %d, %zx\n",
+			       i, (size_t)indirect->addr + i * sizeof desc);
+			return -EINVAL;
+		}
+
+		ret = translate_desc(dev, desc.addr, desc.len, iov + iov_count,
+				     VHOST_NET_MAX_SG - iov_count);
+		if (ret < 0) {
+			vq_err(vq, "Translation failure %d indirect idx %d\n",
+			       ret, i);
+			return ret;
+		}
+		/* If this is an input descriptor, increment that count. */
+		if (desc.flags & VRING_DESC_F_WRITE) {
+			*in_num += ret;
+			if (unlikely(log)) {
+				log[*log_num].addr = desc.addr;
+				log[*log_num].len = desc.len;
+				++*log_num;
+			}
+		} else {
+			/* If it's an output descriptor, they're all supposed
+			 * to come before any input descriptors. */
+			if (*in_num) {
+				vq_err(vq, "Indirect descriptor "
+				       "has out after in: idx %d\n", i);
+				return -EINVAL;
+			}
+			*out_num += ret;
+		}
+	} while ((i = next_desc(&desc)) != -1);
+	return 0;
+}
+
+/* This looks in the virtqueue and for the first available buffer, and converts
+ * it to an iovec for convenient access.  Since descriptors consist of some
+ * number of output then some number of input descriptors, it's actually two
+ * iovecs, but we pack them into one and note how many of each there were.
+ *
+ * This function returns the descriptor number found, or vq->num (which
+ * is never a valid descriptor number) if none was found. */
+unsigned vhost_get_vq_desc(struct vhost_dev *dev, struct vhost_virtqueue *vq,
+			   struct iovec iov[],
+			   unsigned int *out_num, unsigned int *in_num,
+			   struct vhost_log *log, unsigned int *log_num)
+{
+	struct vring_desc desc;
+	unsigned int i, head, found = 0;
+	u16 last_avail_idx;
+	int ret;
+
+	/* Check it isn't doing very strange things with descriptor numbers. */
+	last_avail_idx = vq->last_avail_idx;
+	if (get_user(vq->avail_idx, &vq->avail->idx)) {
+		vq_err(vq, "Failed to access avail idx at %p\n",
+		       &vq->avail->idx);
+		return vq->num;
+	}
+
+	if ((u16)(vq->avail_idx - last_avail_idx) > vq->num) {
+		vq_err(vq, "Guest moved used index from %u to %u",
+		       last_avail_idx, vq->avail_idx);
+		return vq->num;
+	}
+
+	/* If there's nothing new since last we looked, return invalid. */
+	if (vq->avail_idx == last_avail_idx)
+		return vq->num;
+
+	/* Only get avail ring entries after they have been exposed by guest. */
+	rmb();
+
+	/* Grab the next descriptor number they're advertising, and increment
+	 * the index we've seen. */
+	if (get_user(head, &vq->avail->ring[last_avail_idx % vq->num])) {
+		vq_err(vq, "Failed to read head: idx %d address %p\n",
+		       last_avail_idx,
+		       &vq->avail->ring[last_avail_idx % vq->num]);
+		return vq->num;
+	}
+
+	/* If their number is silly, that's an error. */
+	if (head >= vq->num) {
+		vq_err(vq, "Guest says index %u > %u is available",
+		       head, vq->num);
+		return vq->num;
+	}
+
+	/* When we start there are none of either input nor output. */
+	*out_num = *in_num = 0;
+	if (unlikely(log))
+		*log_num = 0;
+
+	i = head;
+	do {
+		unsigned iov_count = *in_num + *out_num;
+		if (i >= vq->num) {
+			vq_err(vq, "Desc index is %u > %u, head = %u",
+			       i, vq->num, head);
+			return vq->num;
+		}
+		if (++found > vq->num) {
+			vq_err(vq, "Loop detected: last one at %u "
+			       "vq size %u head %u\n",
+			       i, vq->num, head);
+			return vq->num;
+		}
+		ret = copy_from_user(&desc, vq->desc + i, sizeof desc);
+		if (ret) {
+			vq_err(vq, "Failed to get descriptor: idx %d addr %p\n",
+			       i, vq->desc + i);
+			return vq->num;
+		}
+		if (desc.flags & VRING_DESC_F_INDIRECT) {
+			ret = get_indirect(dev, vq, iov, out_num, in_num,
+					   log, log_num, &desc);
+			if (ret < 0) {
+				vq_err(vq, "Failure detected "
+				       "in indirect descriptor at idx %d\n", i);
+				return vq->num;
+			}
+			continue;
+		}
+
+		ret = translate_desc(dev, desc.addr, desc.len, iov + iov_count,
+				     VHOST_NET_MAX_SG - iov_count);
+		if (ret < 0) {
+			vq_err(vq, "Translation failure %d descriptor idx %d\n",
+			       ret, i);
+			return vq->num;
+		}
+		if (desc.flags & VRING_DESC_F_WRITE) {
+			/* If this is an input descriptor,
+			 * increment that count. */
+			*in_num += ret;
+			if (unlikely(log)) {
+				log[*log_num].addr = desc.addr;
+				log[*log_num].len = desc.len;
+				++*log_num;
+			}
+		} else {
+			/* If it's an output descriptor, they're all supposed
+			 * to come before any input descriptors. */
+			if (*in_num) {
+				vq_err(vq, "Descriptor has out after in: "
+				       "idx %d\n", i);
+				return vq->num;
+			}
+			*out_num += ret;
+		}
+	} while ((i = next_desc(&desc)) != -1);
+
+	/* On success, increment avail index. */
+	vq->last_avail_idx++;
+	return head;
+}
+
+/* Reverse the effect of vhost_get_vq_desc. Useful for error handling. */
+void vhost_discard_vq_desc(struct vhost_virtqueue *vq)
+{
+	vq->last_avail_idx--;
+}
+
+/* After we've used one of their buffers, we tell them about it.  We'll then
+ * want to send them an interrupt, using vq->call. */
+int vhost_add_used(struct vhost_virtqueue *vq,
+			  unsigned int head, int len)
+{
+	struct vring_used_elem *used;
+
+	/* The virtqueue contains a ring of used buffers.  Get a pointer to the
+	 * next entry in that used ring. */
+	used = &vq->used->ring[vq->last_used_idx % vq->num];
+	if (put_user(head, &used->id)) {
+		vq_err(vq, "Failed to write used id");
+		return -EFAULT;
+	}
+	if (put_user(len, &used->len)) {
+		vq_err(vq, "Failed to write used len");
+		return -EFAULT;
+	}
+	/* Make sure buffer is written before we update index. */
+	wmb();
+	if (put_user(vq->last_used_idx + 1, &vq->used->idx)) {
+		vq_err(vq, "Failed to increment used idx");
+		return -EFAULT;
+	}
+	if (unlikely(vq->log_used)) {
+		/* Make sure data is seen before log. */
+		wmb();
+		log_write(vq->log_base, vq->log_addr + sizeof *vq->used->ring *
+			  (vq->last_used_idx % vq->num),
+			  sizeof *vq->used->ring);
+		log_write(vq->log_base, vq->log_addr, sizeof *vq->used->ring);
+		if (vq->log_ctx)
+			eventfd_signal(vq->log_ctx, 1);
+	}
+	vq->last_used_idx++;
+	return 0;
+}
+
+/* This actually sends the interrupt for this virtqueue */
+void vhost_trigger_irq(struct vhost_dev *dev, struct vhost_virtqueue *vq)
+{
+	__u16 flags = 0;
+	if (get_user(flags, &vq->avail->flags)) {
+		vq_err(vq, "Failed to get flags");
+		return;
+	}
+
+	/* If they don't want an interrupt, don't send one, unless empty. */
+	if ((flags & VRING_AVAIL_F_NO_INTERRUPT) &&
+	    (vq->avail_idx != vq->last_avail_idx ||
+	     !vhost_has_feature(dev, VIRTIO_F_NOTIFY_ON_EMPTY)))
+		return;
+
+	/* Send the Guest an interrupt tell them we used something up. */
+	if (vq->call_ctx)
+		eventfd_signal(vq->call_ctx, 1);
+}
+
+/* And here's the combo meal deal.  Supersize me! */
+void vhost_add_used_and_trigger(struct vhost_dev *dev,
+				struct vhost_virtqueue *vq,
+				unsigned int head, int len)
+{
+	vhost_add_used(vq, head, len);
+	vhost_trigger_irq(dev, vq);
+}
+
+/* OK, now we need to know about added descriptors. */
+bool vhost_notify(struct vhost_virtqueue *vq)
+{
+	int r;
+	if (!(vq->used_flags & VRING_USED_F_NO_NOTIFY))
+		return false;
+	vq->used_flags &= ~VRING_USED_F_NO_NOTIFY;
+	r = put_user(vq->used_flags, &vq->used->flags);
+	if (r)
+		vq_err(vq, "Failed to disable notification: %d\n", r);
+	/* They could have slipped one in as we were doing that: make
+	 * sure it's written, tell caller it needs to check again. */
+	mb();
+	return true;
+}
+
+/* We don't need to be notified again. */
+void vhost_no_notify(struct vhost_virtqueue *vq)
+{
+	int r;
+	if (vq->used_flags & VRING_USED_F_NO_NOTIFY)
+		return;
+	vq->used_flags |= VRING_USED_F_NO_NOTIFY;
+	r = put_user(vq->used_flags, &vq->used->flags);
+	if (r)
+		vq_err(vq, "Failed to enable notification: %d\n", r);
+}
+
+int vhost_init(void)
+{
+	vhost_workqueue = create_singlethread_workqueue("vhost");
+	if (!vhost_workqueue)
+		return -ENOMEM;
+	return 0;
+}
+
+void vhost_cleanup(void)
+{
+	destroy_workqueue(vhost_workqueue);
+}
diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
new file mode 100644
index 0000000..d3900de
--- /dev/null
+++ b/drivers/vhost/vhost.h
@@ -0,0 +1,158 @@
+#ifndef _VHOST_H
+#define _VHOST_H
+
+#include <linux/eventfd.h>
+#include <linux/vhost.h>
+#include <linux/mm.h>
+#include <linux/mutex.h>
+#include <linux/workqueue.h>
+#include <linux/poll.h>
+#include <linux/file.h>
+#include <linux/skbuff.h>
+#include <linux/uio.h>
+#include <linux/virtio_config.h>
+#include <linux/virtio_ring.h>
+
+struct vhost_device;
+
+enum {
+	VHOST_NET_MAX_SG = MAX_SKB_FRAGS + 2,
+};
+
+/* Poll a file (eventfd or socket) */
+/* Note: there's nothing vhost specific about this structure. */
+struct vhost_poll {
+	poll_table                table;
+	wait_queue_head_t        *wqh;
+	wait_queue_t              wait;
+	/* struct which will handle all actual work. */
+	struct work_struct        work;
+	unsigned long		  mask;
+};
+
+void vhost_poll_init(struct vhost_poll *poll, work_func_t func,
+		     unsigned long mask);
+void vhost_poll_start(struct vhost_poll *poll, struct file *file);
+void vhost_poll_stop(struct vhost_poll *poll);
+void vhost_poll_flush(struct vhost_poll *poll);
+void vhost_poll_queue(struct vhost_poll *poll);
+
+struct vhost_log {
+	u64 addr;
+	u64 len;
+};
+
+/* The virtqueue structure describes a queue attached to a device. */
+struct vhost_virtqueue {
+	struct vhost_dev *dev;
+
+	/* The actual ring of buffers. */
+	struct mutex mutex;
+	unsigned int num;
+	struct vring_desc __user *desc;
+	struct vring_avail __user *avail;
+	struct vring_used __user *used;
+	struct file *kick;
+	struct file *call;
+	struct file *error;
+	struct eventfd_ctx *call_ctx;
+	struct eventfd_ctx *error_ctx;
+	struct eventfd_ctx *log_ctx;
+
+	struct vhost_poll poll;
+
+	/* The routine to call when the Guest pings us, or timeout. */
+	work_func_t handle_kick;
+
+	/* Last available index we saw. */
+	u16 last_avail_idx;
+
+	/* Caches available index value from user. */
+	u16 avail_idx;
+
+	/* Last index we used. */
+	u16 last_used_idx;
+
+	/* Used flags */
+	u16 used_flags;
+
+	/* Log writes to used structure. */
+	bool log_used;
+	u64 log_addr;
+
+	struct iovec indirect[VHOST_NET_MAX_SG];
+	struct iovec iov[VHOST_NET_MAX_SG];
+	struct iovec hdr[VHOST_NET_MAX_SG];
+	size_t hdr_size;
+	/* We use a kind of RCU to access private pointer.
+	 * All readers access it from workqueue, which makes it possible to
+	 * flush the workqueue instead of synchronize_rcu. Therefore readers do
+	 * not need to call rcu_read_lock/rcu_read_unlock: the beginning of
+	 * work item execution acts instead of rcu_read_lock() and the end of
+	 * work item execution acts instead of rcu_read_lock().
+	 * Writers use virtqueue mutex. */
+	void *private_data;
+	/* Log write descriptors */
+	void __user *log_base;
+	struct vhost_log log[VHOST_NET_MAX_SG];
+};
+
+struct vhost_dev {
+	/* Readers use RCU to access memory table pointer
+	 * log base pointer and features.
+	 * Writers use mutex below.*/
+	struct vhost_memory *memory;
+	struct mm_struct *mm;
+	struct mutex mutex;
+	unsigned acked_features;
+	struct vhost_virtqueue *vqs;
+	int nvqs;
+	struct file *log_file;
+	struct eventfd_ctx *log_ctx;
+};
+
+long vhost_dev_init(struct vhost_dev *, struct vhost_virtqueue *vqs, int nvqs);
+long vhost_dev_check_owner(struct vhost_dev *);
+long vhost_dev_reset_owner(struct vhost_dev *);
+void vhost_dev_cleanup(struct vhost_dev *);
+long vhost_dev_ioctl(struct vhost_dev *, unsigned int ioctl, unsigned long arg);
+
+unsigned vhost_get_vq_desc(struct vhost_dev *, struct vhost_virtqueue *,
+			   struct iovec iov[],
+			   unsigned int *out_num, unsigned int *in_num,
+			   struct vhost_log *log, unsigned int *log_num);
+void vhost_discard_vq_desc(struct vhost_virtqueue *);
+
+int vhost_add_used(struct vhost_virtqueue *, unsigned int head, int len);
+void vhost_trigger_irq(struct vhost_dev *, struct vhost_virtqueue *);
+void vhost_add_used_and_trigger(struct vhost_dev *, struct vhost_virtqueue *,
+				unsigned int head, int len);
+void vhost_no_notify(struct vhost_virtqueue *);
+bool vhost_notify(struct vhost_virtqueue *);
+
+int vhost_log_write(struct vhost_virtqueue *vq, struct vhost_log *log,
+		    unsigned int log_num, u64 len);
+
+int vhost_init(void);
+void vhost_cleanup(void);
+
+#define vq_err(vq, fmt, ...) do {                                  \
+		pr_debug(pr_fmt(fmt), ##__VA_ARGS__);       \
+		if ((vq)->error_ctx)                               \
+				eventfd_signal((vq)->error_ctx, 1);\
+	} while (0)
+
+enum {
+	VHOST_FEATURES = (1 << VIRTIO_F_NOTIFY_ON_EMPTY) |
+			 (1 << VIRTIO_RING_F_INDIRECT_DESC) |
+			 (1 << VHOST_F_LOG_ALL) |
+			 (1 << VHOST_NET_F_VIRTIO_NET_HDR),
+};
+
+static inline int vhost_has_feature(struct vhost_dev *dev, int bit)
+{
+	unsigned acked_features = rcu_dereference(dev->acked_features);
+	return acked_features & (1 << bit);
+}
+
+#endif
diff --git a/include/linux/Kbuild b/include/linux/Kbuild
index 3f384d4..b6335c6 100644
--- a/include/linux/Kbuild
+++ b/include/linux/Kbuild
@@ -361,6 +361,7 @@ unifdef-y += uio.h
 unifdef-y += unistd.h
 unifdef-y += usbdevice_fs.h
 unifdef-y += utsname.h
+unifdef-y += vhost.h
 unifdef-y += videodev2.h
 unifdef-y += videodev.h
 unifdef-y += virtio_config.h
diff --git a/include/linux/miscdevice.h b/include/linux/miscdevice.h
index adaf3c1..8b5f7cc 100644
--- a/include/linux/miscdevice.h
+++ b/include/linux/miscdevice.h
@@ -30,6 +30,7 @@
 #define HPET_MINOR		228
 #define FUSE_MINOR		229
 #define KVM_MINOR		232
+#define VHOST_NET_MINOR		233
 #define MISC_DYNAMIC_MINOR	255
 
 struct device;
diff --git a/include/linux/vhost.h b/include/linux/vhost.h
new file mode 100644
index 0000000..f66142f
--- /dev/null
+++ b/include/linux/vhost.h
@@ -0,0 +1,126 @@
+#ifndef _LINUX_VHOST_H
+#define _LINUX_VHOST_H
+/* Userspace interface for in-kernel virtio accelerators. */
+
+/* vhost is used to reduce the number of system calls involved in virtio.
+ *
+ * Existing virtio net code is used in the guest without modification.
+ *
+ * This header includes interface used by userspace hypervisor for
+ * device configuration.
+ */
+
+#include <linux/types.h>
+#include <linux/compiler.h>
+#include <linux/ioctl.h>
+#include <linux/virtio_config.h>
+#include <linux/virtio_ring.h>
+
+struct vhost_vring_state {
+	unsigned int index;
+	unsigned int num;
+};
+
+struct vhost_vring_file {
+	unsigned int index;
+	int fd; /* Pass -1 to unbind from file. */
+
+};
+
+struct vhost_vring_addr {
+	unsigned int index;
+	unsigned int padding;
+	__u64 user_addr;
+};
+
+struct vhost_memory_region {
+	__u64 guest_phys_addr;
+	__u64 memory_size; /* bytes */
+	__u64 userspace_addr;
+	__u64 flags_padding; /* No flags are currently specified. */
+};
+
+/* All region addresses and sizes must be 4K aligned. */
+#define VHOST_PAGE_SIZE 0x1000
+
+struct vhost_memory {
+	__u32 nregions;
+	__u32 padding;
+	struct vhost_memory_region regions[0];
+};
+
+/* ioctls */
+
+#define VHOST_VIRTIO 0xAF
+
+/* Features bitmask for forward compatibility.  Transport bits are used for
+ * vhost specific features. */
+#define VHOST_GET_FEATURES	_IOR(VHOST_VIRTIO, 0x00, __u64)
+#define VHOST_ACK_FEATURES	_IOW(VHOST_VIRTIO, 0x00, __u64)
+
+/* Set current process as the (exclusive) owner of this file descriptor.  This
+ * must be called before any other vhost command.  Further calls to
+ * VHOST_OWNER_SET fail until VHOST_OWNER_RESET is called. */
+#define VHOST_SET_OWNER _IO(VHOST_VIRTIO, 0x01)
+/* Give up ownership, and reset the device to default values.
+ * Allows subsequent call to VHOST_OWNER_SET to succeed. */
+#define VHOST_RESET_OWNER _IO(VHOST_VIRTIO, 0x02)
+
+/* Set up/modify memory layout */
+#define VHOST_SET_MEM_TABLE	_IOW(VHOST_VIRTIO, 0x03, struct vhost_memory)
+
+/* Write logging setup. */
+/* Memory writes can optionally be logged by setting bit at an offset
+ * (calculated from the physical address) from specified log base.
+ * The bit is set using an atomic 32 bit operation. */
+/* Set base address for logging. */
+#define VHOST_SET_LOG_BASE _IOW(VHOST_VIRTIO, 0x04, __u64)
+/* Specify an eventfd file descriptor to signal on log write. */
+#define VHOST_SET_LOG_FD _IOW(VHOST_VIRTIO, 0x07, int)
+
+/* Ring setup. These parameters can not be modified while ring is running
+ * (bound to a device). */
+/* Set number of descriptors in ring */
+#define VHOST_SET_VRING_NUM _IOW(VHOST_VIRTIO, 0x10, struct vhost_vring_state)
+/* Start of array of descriptors (virtually contiguous) */
+#define VHOST_SET_VRING_DESC _IOW(VHOST_VIRTIO, 0x11, struct vhost_vring_addr)
+/* Used structure address. Must be 32 bit aligned */
+#define VHOST_SET_VRING_USED _IOW(VHOST_VIRTIO, 0x12, struct vhost_vring_addr)
+/* Available structure address. Must be 16 bit aligned */
+#define VHOST_SET_VRING_AVAIL _IOW(VHOST_VIRTIO, 0x13, struct vhost_vring_addr)
+/* Base value where queue looks for available descriptors */
+#define VHOST_SET_VRING_BASE _IOW(VHOST_VIRTIO, 0x14, struct vhost_vring_state)
+/* Get accessor: reads index, writes value in num */
+#define VHOST_GET_VRING_BASE _IOWR(VHOST_VIRTIO, 0x14, struct vhost_vring_state)
+
+/* Logging support. Can be modified while ring is running. */
+/* Log writes to used structure, at offset calculated from specified address.
+ * Address must be 32 bit aligned. Pass 0x1 to disable logging. */
+#define VHOST_SET_VRING_LOG _IOW(VHOST_VIRTIO, 0x18, struct vhost_vring_addr)
+#define VHOST_VRING_LOG_DISABLE (0x1)
+
+/* The following ioctls use eventfd file descriptors to signal and poll
+ * for events. */
+
+/* Set eventfd to poll for added buffers */
+#define VHOST_SET_VRING_KICK _IOW(VHOST_VIRTIO, 0x20, struct vhost_vring_file)
+/* Set eventfd to signal when buffers have beed used */
+#define VHOST_SET_VRING_CALL _IOW(VHOST_VIRTIO, 0x21, struct vhost_vring_file)
+/* Set eventfd to signal an error */
+#define VHOST_SET_VRING_ERR _IOW(VHOST_VIRTIO, 0x22, struct vhost_vring_file)
+
+/* VHOST_NET specific defines */
+
+/* Attach virtio net ring to a raw socket, or tap device.
+ * The socket must be already bound to an ethernet device, this device will be
+ * used for transmit.  Pass fd -1 to unbind from the socket and the transmit
+ * device.  This can be used to stop the ring (e.g. for migration). */
+#define VHOST_NET_SET_BACKEND _IOW(VHOST_VIRTIO, 0x30, struct vhost_vring_file)
+
+/* Feature bits */
+/* Log all write descriptors. Can be changed while device is active. */
+#define VHOST_F_LOG_ALL 26
+/* vhost-net should add virtio_net_hdr for RX, and strip for TX packets. */
+#define VHOST_NET_F_VIRTIO_NET_HDR 27
+
+#endif
-- 
1.6.5.2.143.g8cc62

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Re: [PATCHv8 3/3] vhost_net: a kernel-level virtio server

[Rusty Russell]

On Thu, 5 Nov 2009 02:27:24 am Michael S. Tsirkin wrote:
> What it is: vhost net is a character device that can be used to reduce
> the number of system calls involved in virtio networking.

Hi Michael,

   Now everyone else has finally kicked all the tires and it seems to pass,
I've done a fairly complete review.  Generally, it's really nice; just one
bug and a few minor suggestions for polishing.

> +/* Caller must have TX VQ lock */
> +static void tx_poll_stop(struct vhost_net *net)
> +{
> +	if (likely(net->tx_poll_state != VHOST_NET_POLL_STARTED))
> +		return;

likely?  Really?

> +	for (;;) {
> +		head = vhost_get_vq_desc(&net->dev, vq, vq->iov, &out, &in,
> +					 NULL, NULL);

Danger!  You need an arg to vhost_get_vq_desc to tell it the max desc size
you can handle.  Otherwise, it's only limited by ring size, and a malicious
guest can overflow you here, and below:

> +		/* Skip header. TODO: support TSO. */
> +		s = move_iovec_hdr(vq->iov, vq->hdr, hdr_size, out);
...
> +
> +	use_mm(net->dev.mm);
> +	mutex_lock(&vq->mutex);
> +	vhost_no_notify(vq);

I prefer a name like "vhost_disable_notify()".

> +		/* OK, now we need to know about added descriptors. */
> +		if (head == vq->num && vhost_notify(vq))
> +			/* They could have slipped one in as we were doing that:
> +			 * check again. */
> +			continue;
> +		/* Nothing new?  Wait for eventfd to tell us they refilled. */
> +		if (head == vq->num)
> +			break;
> +		/* We don't need to be notified again. */
> +		vhost_no_notify(vq);

Similarly, vhost_enable_notify.  This one is particularly misleading since
it doesn't actually notify anything!

In particular, this code would be neater as:

	if (head == vq->num) {
		if (vhost_enable_notify(vq)) {
			/* Try again, they could have slipped one in. */
			continue;
		}
		/* Nothing more to do. */
		break;
	}
	vhost_disable_notify(vq);

Now, AFAICT vhost_notify()/enable_notify() would be better rewritten to
return true only when there's more pending.  Saves a loop around here most
of the time.  Also, the vhost_no_notify/vhost_disable_notify() can be moved
out of the loop entirely.  (It could be under an if (unlikely(enabled)), not
sure if it's worth it).

> +		len = err;
> +		err = memcpy_toiovec(vq->hdr, (unsigned char *)&hdr, hdr_size);

That unsigned char * arg to memcpy_toiovec is annoying.  A patch might be
nice, separate from this effort.

> +static int vhost_net_open(struct inode *inode, struct file *f)
> +{
> +	struct vhost_net *n = kzalloc(sizeof *n, GFP_KERNEL);
> +	int r;
> +	if (!n)
> +		return -ENOMEM;
> +	f->private_data = n;
> +	n->vqs[VHOST_NET_VQ_TX].handle_kick = handle_tx_kick;
> +	n->vqs[VHOST_NET_VQ_RX].handle_kick = handle_rx_kick;

I have a personal dislike of calloc for structures.  In userspace, it's
because valgrind can't spot uninitialized fields.  These days a similar
argument applies in the kernel, because we have KMEMCHECK now.  If someone
adds a field to the struct and forgets to initialize it, we can spot it.

> +static void vhost_net_enable_vq(struct vhost_net *n, int index)
> +{
> +	struct socket *sock = n->vqs[index].private_data;

OK, I can't help but this that presenting the vqs as an array doesn't buy
us very much.  Esp. if you change vhost_dev_init to take a NULL-terminated
varargs.  I think readability would improve.  It means passing a vq around
rather than an index.

Not completely sure it'll be a win tho.

> +static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
> +{
> +	struct socket *sock, *oldsock = NULL;
...
> +	sock = get_socket(fd);
> +	if (IS_ERR(sock)) {
> +		r = PTR_ERR(sock);
> +		goto done;
> +	}
> +
> +	/* start polling new socket */
> +	oldsock = vq->private_data;
...
> +done:
> +	mutex_unlock(&n->dev.mutex);
> +	if (oldsock) {
> +		vhost_net_flush_vq(n, index);
> +		fput(oldsock->file);

I dislike this style; I prefer multiple different goto points, one for when
oldsock is set, and one for when it's not.

That way, gcc warns us about uninitialized variables if we get it wrong.

> +static long vhost_net_reset_owner(struct vhost_net *n)
> +{
> +	struct socket *tx_sock = NULL;
> +	struct socket *rx_sock = NULL;
> +	long r;

This should be called "err", since that's what it is.

> +static void vhost_net_set_features(struct vhost_net *n, u64 features)
> +{
> +	size_t hdr_size = features & (1 << VHOST_NET_F_VIRTIO_NET_HDR) ?
> +		sizeof(struct virtio_net_hdr) : 0;
> +	int i;
> +	mutex_lock(&n->dev.mutex);
> +	n->dev.acked_features = features;

Why is this called "acked_features"?  Not just "features"?  I expected
to see code which exposed these back to userspace, and didn't.

> +	case VHOST_GET_FEATURES:
> +		features = VHOST_FEATURES;
> +		return put_user(features, featurep);
> +	case VHOST_ACK_FEATURES:
> +		r = get_user(features, featurep);
> +		/* No features for now */
> +		if (r < 0)
> +			return r;
> +		if (features & ~VHOST_FEATURES)
> +			return -EOPNOTSUPP;
> +		vhost_net_set_features(n, features);

OK, from the userspace POV it's "get features" then "ack features".  But
I think "VHOST_SET_FEATURES" is more consistent, despite this usage.

> +	switch (ioctl) {
> +	case VHOST_SET_VRING_NUM:

I haven't looked at your userspace implementation, but does a generic
VHOST_SET_VRING_STATE & VHOST_GET_VRING_STATE with a struct make more
sense?  It'd be simpler here, but not sure if it'd be simpler to use?

(Not the fd-setting ioctls of course)

> +	case VHOST_SET_VRING_LOG:
> +		r = copy_from_user(&a, argp, sizeof a);
> +		if (r < 0)
> +			break;
> +		if (a.padding) {
> +			r = -EOPNOTSUPP;
> +			break;
> +		}
> +		if (a.user_addr == VHOST_VRING_LOG_DISABLE) {
> +			vq->log_used = false;
> +			break;
> +		}
> +		if (a.user_addr & (sizeof *vq->used->ring - 1)) {
> +			r = -EINVAL;
> +			break;
> +		}
> +		vq->log_used = true;
> +		vq->log_addr = a.user_addr;
> +		break;

For future reference, this is *exactly* the kind of thing which would have
been nice as a followup patch.  Easy to separate, easy to review, not critical
to the core.

> +/* TODO: This is really inefficient.  We need something like get_user()
> + * (instruction directly accesses the data, with an exception table entry
> + * returning -EFAULT). See Documentation/x86/exception-tables.txt.
> + */
> +static int set_bit_to_user(int nr, void __user *addr)
> +{

I guess we won't be dealing with many contiguous pages, otherwise we could
get a cheap speedup making this set_bits_to_user(int nr, int num_bits...).

> +/* Each buffer in the virtqueues is actually a chain of descriptors.  This
> + * function returns the next descriptor in the chain,
> + * or -1 if we're at the end. */
> +static unsigned next_desc(struct vring_desc *desc)
> +{
> +	unsigned int next;
> +
> +	/* If this descriptor says it doesn't chain, we're done. */
> +	if (!(desc->flags & VRING_DESC_F_NEXT))
> +		return -1;

Hmm, prefer s/-1/-1U/ in comment, here, and below.  Clarifies a bit.

> +/* After we've used one of their buffers, we tell them about it.  We'll then
> + * want to send them an interrupt, using vq->call. */

This comment has too much cut & paste:
	... want to notify the guest, using the eventfd */

> +/* This actually sends the interrupt for this virtqueue */
> +void vhost_trigger_irq(struct vhost_dev *dev, struct vhost_virtqueue *vq)
> +{

Rename vhost_notify_eventfd() or something, and fix comments?

> +enum {
> +	VHOST_NET_MAX_SG = MAX_SKB_FRAGS + 2,

+2?  Believable, but is it correct?

> +/* Poll a file (eventfd or socket) */
> +/* Note: there's nothing vhost specific about this structure. */
> +struct vhost_poll {

This comment really helped while reading the code.  Kudos!

Thanks!
Rusty.

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Re: [PATCHv8 3/3] vhost_net: a kernel-level virtio server

[Michael S. Tsirkin]

On Fri, Nov 06, 2009 at 03:29:17PM +1030, Rusty Russell wrote:
> On Thu, 5 Nov 2009 02:27:24 am Michael S. Tsirkin wrote:
> > What it is: vhost net is a character device that can be used to reduce
> > the number of system calls involved in virtio networking.
> 
> Hi Michael,
> 
>    Now everyone else has finally kicked all the tires and it seems to pass,
> I've done a fairly complete review.  Generally, it's really nice; just one
> bug and a few minor suggestions for polishing.

Thanks for the review! I'll add more polishing and repost.
Answers to some questions below.

> > +/* Caller must have TX VQ lock */
> > +static void tx_poll_stop(struct vhost_net *net)
> > +{
> > +	if (likely(net->tx_poll_state != VHOST_NET_POLL_STARTED))
> > +		return;
> 
> likely?  Really?

Hmm ... yes. tx poll stop is called on each packet (as long as we do not
fill up 1/2 backend queue), the first call will stop polling
the rest checks state and does nothing.

This is because we normally do not care when the message has left the
queue in backend device: we tell backend to send it and forget. We only
start polling when backend tx queue fills up.

Makes sense?

> > +	for (;;) {
> > +		head = vhost_get_vq_desc(&net->dev, vq, vq->iov, &out, &in,
> > +					 NULL, NULL);
> 
> Danger!  You need an arg to vhost_get_vq_desc to tell it the max desc size
> you can handle.  Otherwise, it's only limited by ring size, and a malicious
> guest can overflow you here, and below:

In fact, I think this is not a bug.  This happens to work correctly
(even with malicious guests) because vhost_get_vq_desc is hard-coded to
check VHOST_NET_MAX_SG, so in fact no overflow is possible.  I agree
that it's mich nicer to pass iovec size to vhost_get_vq_desc.

> 
> > +		/* Skip header. TODO: support TSO. */
> > +		s = move_iovec_hdr(vq->iov, vq->hdr, hdr_size, out);
> ...
> > +
> > +	use_mm(net->dev.mm);
> > +	mutex_lock(&vq->mutex);
> > +	vhost_no_notify(vq);
> 
> I prefer a name like "vhost_disable_notify()".

Good idea.

> > +		/* OK, now we need to know about added descriptors. */
> > +		if (head == vq->num && vhost_notify(vq))
> > +			/* They could have slipped one in as we were doing that:
> > +			 * check again. */
> > +			continue;
> > +		/* Nothing new?  Wait for eventfd to tell us they refilled. */
> > +		if (head == vq->num)
> > +			break;
> > +		/* We don't need to be notified again. */
> > +		vhost_no_notify(vq);
> 
> Similarly, vhost_enable_notify.  This one is particularly misleading since
> it doesn't actually notify anything!

Good idea.


> 
> In particular, this code would be neater as:
> 
> 	if (head == vq->num) {
> 		if (vhost_enable_notify(vq)) {
> 			/* Try again, they could have slipped one in. */
> 			continue;
> 		}
> 		/* Nothing more to do. */
> 		break;
> 	}
> 	vhost_disable_notify(vq);
> 
> Now, AFAICT vhost_notify()/enable_notify() would be better rewritten to
> return true only when there's more pending.  Saves a loop around here most
> of the time.

OKay, I'll look into this. It kind of annoys me that we would do
get_user for the same value twice: once in vhost_enable_notify and once
in vhost_get_vq_desc.  OTOH, all the loop does is call vhost_get_vq_desc
again.

>  Also, the vhost_no_notify/vhost_disable_notify() can be moved
> out of the loop entirely.

I don't think it can, if we enabled notification and then see more
descriptors in queue, we want to disable notification again. But it can
be
>  (It could be under an if (unlikely(enabled)), not
> sure if it's worth it).

 		if (unlikely(vhost_enable_notify(vq))) {
 			/* Try again, they have slipped one in. */
 			vhost_disable_notify(vq);
 			continue;
 		}

> 
> > +		len = err;
> > +		err = memcpy_toiovec(vq->hdr, (unsigned char *)&hdr, hdr_size);
> 
> That unsigned char * arg to memcpy_toiovec is annoying.  A patch might be
> nice, separate from this effort.

Sounds good.

> > +static int vhost_net_open(struct inode *inode, struct file *f)
> > +{
> > +	struct vhost_net *n = kzalloc(sizeof *n, GFP_KERNEL);
> > +	int r;
> > +	if (!n)
> > +		return -ENOMEM;
> > +	f->private_data = n;
> > +	n->vqs[VHOST_NET_VQ_TX].handle_kick = handle_tx_kick;
> > +	n->vqs[VHOST_NET_VQ_RX].handle_kick = handle_rx_kick;
> 
> I have a personal dislike of calloc for structures.

You mean zalloc?

> In userspace, it's because valgrind can't spot uninitialized fields.
> These days a similar argument applies in the kernel, because we have
> KMEMCHECK now.  If someone adds a field to the struct and forgets to
> initialize it, we can spot it.

OK.

> > +static void vhost_net_enable_vq(struct vhost_net *n, int index)
> > +{
> > +	struct socket *sock = n->vqs[index].private_data;
> 
> OK, I can't help but this that presenting the vqs as an array doesn't buy
> us very much.  Esp. if you change vhost_dev_init to take a NULL-terminated
> varargs.  I think readability would improve.  It means passing a vq around
> rather than an index.
> 
> Not completely sure it'll be a win tho.

Hmm, varargs sounds a bit complex. But I agree readability for
vhost_net_enable_vq and friends would benefit from passing a vq around
rather than an index.  I'll try it out and do it if it's a win.

> > +static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
> > +{
> > +	struct socket *sock, *oldsock = NULL;
> ...
> > +	sock = get_socket(fd);
> > +	if (IS_ERR(sock)) {
> > +		r = PTR_ERR(sock);
> > +		goto done;
> > +	}
> > +
> > +	/* start polling new socket */
> > +	oldsock = vq->private_data;
> ...
> > +done:
> > +	mutex_unlock(&n->dev.mutex);
> > +	if (oldsock) {
> > +		vhost_net_flush_vq(n, index);
> > +		fput(oldsock->file);
> 
> I dislike this style; I prefer multiple different goto points, one for when
> oldsock is set, and one for when it's not.
> 
> That way, gcc warns us about uninitialized variables if we get it wrong.

OK.

> > +static long vhost_net_reset_owner(struct vhost_net *n)
> > +{
> > +	struct socket *tx_sock = NULL;
> > +	struct socket *rx_sock = NULL;
> > +	long r;
> 
> This should be called "err", since that's what it is.

OK.

> > +static void vhost_net_set_features(struct vhost_net *n, u64 features)
> > +{
> > +	size_t hdr_size = features & (1 << VHOST_NET_F_VIRTIO_NET_HDR) ?
> > +		sizeof(struct virtio_net_hdr) : 0;
> > +	int i;
> > +	mutex_lock(&n->dev.mutex);
> > +	n->dev.acked_features = features;
> 
> Why is this called "acked_features"?  Not just "features"?  I expected
> to see code which exposed these back to userspace, and didn't.

Not sure how do you mean. Userspace sets them, why
does it want to get them exposed back?

> > +	case VHOST_GET_FEATURES:
> > +		features = VHOST_FEATURES;
> > +		return put_user(features, featurep);
> > +	case VHOST_ACK_FEATURES:
> > +		r = get_user(features, featurep);
> > +		/* No features for now */
> > +		if (r < 0)
> > +			return r;
> > +		if (features & ~VHOST_FEATURES)
> > +			return -EOPNOTSUPP;
> > +		vhost_net_set_features(n, features);
> 
> OK, from the userspace POV it's "get features" then "ack features".  But
> I think "VHOST_SET_FEATURES" is more consistent, despite this usage.

OK.

> > +	switch (ioctl) {
> > +	case VHOST_SET_VRING_NUM:
> 
> I haven't looked at your userspace implementation, but does a generic
> VHOST_SET_VRING_STATE & VHOST_GET_VRING_STATE with a struct make more
> sense?  It'd be simpler here,

Not by much though, right?

> but not sure if it'd be simpler to use?

The problem is with VHOST_SET_VRING_BASE as well. I want it to be
separate because I want to make it possible to relocate e.g. used ring
to another address while ring is running. This would be a good debugging
tool (you look at kernel's used ring, check descriptor, then update
guest's used ring) and also possibly an extra way to do migration.  And
it's nicer to have vring size separate as well, because it is
initialized by host and never changed, right?

We could merge DESC, AVAIL, USED, and it will reduce the amount of code
in userspace. With both base, size and fds separate, it seemed a bit
more symmetrical to have desc/avail/used separate as well.
What's your opinion?


> (Not the fd-setting ioctls of course)
> 
> > +	case VHOST_SET_VRING_LOG:
> > +		r = copy_from_user(&a, argp, sizeof a);
> > +		if (r < 0)
> > +			break;
> > +		if (a.padding) {
> > +			r = -EOPNOTSUPP;
> > +			break;
> > +		}
> > +		if (a.user_addr == VHOST_VRING_LOG_DISABLE) {
> > +			vq->log_used = false;
> > +			break;
> > +		}
> > +		if (a.user_addr & (sizeof *vq->used->ring - 1)) {
> > +			r = -EINVAL;
> > +			break;
> > +		}
> > +		vq->log_used = true;
> > +		vq->log_addr = a.user_addr;
> > +		break;
> 
> For future reference, this is *exactly* the kind of thing which would have
> been nice as a followup patch.  Easy to separate, easy to review, not critical
> to the core.

Yes. It's not too late to split it out though: should I do it yet?

> > +/* TODO: This is really inefficient.  We need something like get_user()
> > + * (instruction directly accesses the data, with an exception table entry
> > + * returning -EFAULT). See Documentation/x86/exception-tables.txt.
> > + */
> > +static int set_bit_to_user(int nr, void __user *addr)
> > +{
> 
> I guess we won't be dealing with many contiguous pages, otherwise we could
> get a cheap speedup making this set_bits_to_user(int nr, int num_bits...).

No idea. Let's keep it as simple as possible for now?

> > +/* Each buffer in the virtqueues is actually a chain of descriptors.  This
> > + * function returns the next descriptor in the chain,
> > + * or -1 if we're at the end. */
> > +static unsigned next_desc(struct vring_desc *desc)
> > +{
> > +	unsigned int next;
> > +
> > +	/* If this descriptor says it doesn't chain, we're done. */
> > +	if (!(desc->flags & VRING_DESC_F_NEXT))
> > +		return -1;
> 
> Hmm, prefer s/-1/-1U/ in comment, here, and below.  Clarifies a bit.

Good idea.

> > +/* After we've used one of their buffers, we tell them about it.  We'll then
> > + * want to send them an interrupt, using vq->call. */
> 
> This comment has too much cut & paste:

I tried to cut and paste as many comments as possible, this
made it easy to audit the code by comparing it with lguest,
and made them much more witty. But yes, this is definitely going
overboard as we do not have vq->call at all :)

> 	... want to notify the guest, using the eventfd */
> 
> > +/* This actually sends the interrupt for this virtqueue */
> > +void vhost_trigger_irq(struct vhost_dev *dev, struct vhost_virtqueue *vq)
> > +{
> 
> Rename vhost_notify_eventfd() or something, and fix comments?

Sounds good. Since I'm renaming vhost_notify to vhost_enable_notify,
this one can just become vhost_notify.

> > +enum {
> > +	VHOST_NET_MAX_SG = MAX_SKB_FRAGS + 2,
> 
> +2?  Believable, but is it correct?

+ 1 is for skb head, + 1 is for virtio net header.
I'll add a comment.

> > +/* Poll a file (eventfd or socket) */
> > +/* Note: there's nothing vhost specific about this structure. */
> > +struct vhost_poll {
> 
> This comment really helped while reading the code.  Kudos!
> 
> Thanks!
> Rusty.

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Re: [PATCHv8 3/3] vhost_net: a kernel-level virtio server

[Rusty Russell]

On Sun, 8 Nov 2009 10:05:16 pm Michael S. Tsirkin wrote:
> On Fri, Nov 06, 2009 at 03:29:17PM +1030, Rusty Russell wrote:
> > > +/* Caller must have TX VQ lock */
> > > +static void tx_poll_stop(struct vhost_net *net)
> > > +{
> > > +	if (likely(net->tx_poll_state != VHOST_NET_POLL_STARTED))
> > > +		return;
> > 
> > likely?  Really?
> 
> Hmm ... yes. tx poll stop is called on each packet (as long as we do not
> fill up 1/2 backend queue), the first call will stop polling
> the rest checks state and does nothing.
> 
> This is because we normally do not care when the message has left the
> queue in backend device: we tell backend to send it and forget. We only
> start polling when backend tx queue fills up.

OK, good.

> > > +static void vhost_net_set_features(struct vhost_net *n, u64 features)
> > > +{
> > > +	size_t hdr_size = features & (1 << VHOST_NET_F_VIRTIO_NET_HDR) ?
> > > +		sizeof(struct virtio_net_hdr) : 0;
> > > +	int i;
> > > +	mutex_lock(&n->dev.mutex);
> > > +	n->dev.acked_features = features;
> > 
> > Why is this called "acked_features"?  Not just "features"?  I expected
> > to see code which exposed these back to userspace, and didn't.
> 
> Not sure how do you mean. Userspace sets them, why
> does it want to get them exposed back?

There's something about the 'acked' which rubs me the wrong way.
"enabled_features" is perhaps a better term than "acked_features"; "acked"
seems more a user point-of-view, "enabled" seems more driver POV?

set_features matches your ioctl names, but it sounds like a fn name :(

It's marginal.  And 'features' is shorter than both.

> > > +	switch (ioctl) {
> > > +	case VHOST_SET_VRING_NUM:
> > 
> > I haven't looked at your userspace implementation, but does a generic
> > VHOST_SET_VRING_STATE & VHOST_GET_VRING_STATE with a struct make more
> > sense?  It'd be simpler here,
> 
> Not by much though, right?
> 
> > but not sure if it'd be simpler to use?
> 
> The problem is with VHOST_SET_VRING_BASE as well. I want it to be
> separate because I want to make it possible to relocate e.g. used ring
> to another address while ring is running. This would be a good debugging
> tool (you look at kernel's used ring, check descriptor, then update
> guest's used ring) and also possibly an extra way to do migration.  And
> it's nicer to have vring size separate as well, because it is
> initialized by host and never changed, right?

Actually, this looks wrong to me:

+	case VHOST_SET_VRING_BASE:
...
+		vq->avail_idx = vq->last_avail_idx = s.num;

The last_avail_idx is part of the state of the driver.  It needs to be saved
and restored over susp/resume.  The only reason it's not in the ring itself
is because I figured the other side doesn't need to see it (which is true, but
missed debugging opportunities as well as man-in-the-middle issues like this
one).  I had a patch which put this field at the end of the ring, I might
resurrect it to avoid this problem.  This is backwards compatible with all
implementations.  See patch at end.

I would drop avail_idx altogether: get_user is basically free, and simplifies
a lot.  As most state is in the ring, all you need is an ioctl to save/restore
the last_avail_idx.

> We could merge DESC, AVAIL, USED, and it will reduce the amount of code
> in userspace. With both base, size and fds separate, it seemed a bit
> more symmetrical to have desc/avail/used separate as well.
> What's your opinion?

Well, DESC, AVAIL, and USED could easily be turned into SET/GET_LAYOUT.

> > For future reference, this is *exactly* the kind of thing which would have
> > been nice as a followup patch.  Easy to separate, easy to review, not critical
> > to the core.
> 
> Yes. It's not too late to split it out though: should I do it yet?

Only if you're feeling enthused.  It's lightly reviewed now.

Cheers,
Rusty.

virtio: put last_used and last_avail index into ring itself.

Generally, the other end of the virtio ring doesn't need to see where
you're up to in consuming the ring.  However, to completely understand
what's going on from the outside, this information must be exposed.
For example, if you want to save and restore a virtio_ring, but you're
not the consumer because the kernel is using it directly.

Fortunately, we have room to expand: the ring is always a whole number
of pages and there's hundreds of bytes of padding after the avail ring
and the used ring, whatever the number of descriptors (which must be a
power of 2).

We add a feature bit so the guest can tell the host that it's writing
out the current value there, if it wants to use that.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
---
 drivers/virtio/virtio_ring.c |   23 +++++++++++++++--------
 include/linux/virtio_ring.h  |   12 +++++++++++-
 2 files changed, 26 insertions(+), 9 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -71,9 +71,6 @@ struct vring_virtqueue
 	/* Number we've added since last sync. */
 	unsigned int num_added;
 
-	/* Last used index we've seen. */
-	u16 last_used_idx;
-
 	/* How to notify other side. FIXME: commonalize hcalls! */
 	void (*notify)(struct virtqueue *vq);
 
@@ -278,12 +275,13 @@ static void detach_buf(struct vring_virt
 
 static inline bool more_used(const struct vring_virtqueue *vq)
 {
-	return vq->last_used_idx != vq->vring.used->idx;
+	return vring_last_used(&vq->vring) != vq->vring.used->idx;
 }
 
 static void *vring_get_buf(struct virtqueue *_vq, unsigned int *len)
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
+	struct vring_used_elem *u;
 	void *ret;
 	unsigned int i;
 
@@ -300,8 +298,11 @@ static void *vring_get_buf(struct virtqu
 		return NULL;
 	}
 
-	i = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].id;
-	*len = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].len;
+	u = &vq->vring.used->ring[vring_last_used(&vq->vring) % vq->vring.num];
+	i = u->id;
+	*len = u->len;
+	/* Make sure we don't reload i after doing checks. */
+	rmb();
 
 	if (unlikely(i >= vq->vring.num)) {
 		BAD_RING(vq, "id %u out of range\n", i);
@@ -315,7 +316,8 @@ static void *vring_get_buf(struct virtqu
 	/* detach_buf clears data, so grab it now. */
 	ret = vq->data[i];
 	detach_buf(vq, i);
-	vq->last_used_idx++;
+	vring_last_used(&vq->vring)++;
+
 	END_USE(vq);
 	return ret;
 }
@@ -402,7 +404,6 @@ struct virtqueue *vring_new_virtqueue(un
 	vq->vq.name = name;
 	vq->notify = notify;
 	vq->broken = false;
-	vq->last_used_idx = 0;
 	vq->num_added = 0;
 	list_add_tail(&vq->vq.list, &vdev->vqs);
 #ifdef DEBUG
@@ -413,6 +414,10 @@ struct virtqueue *vring_new_virtqueue(un
 
 	vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC);
 
+	/* We publish indices whether they offer it or not: if not, it's junk
+	 * space anyway.  But calling this acknowledges the feature. */
+	virtio_has_feature(vdev, VIRTIO_RING_F_PUBLISH_INDICES);
+
 	/* No callback?  Tell other side not to bother us. */
 	if (!callback)
 		vq->vring.avail->flags |= VRING_AVAIL_F_NO_INTERRUPT;
@@ -443,6 +448,8 @@ void vring_transport_features(struct vir
 		switch (i) {
 		case VIRTIO_RING_F_INDIRECT_DESC:
 			break;
+		case VIRTIO_RING_F_PUBLISH_INDICES:
+			break;
 		default:
 			/* We don't understand this bit. */
 			clear_bit(i, vdev->features);
diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
--- a/include/linux/virtio_ring.h
+++ b/include/linux/virtio_ring.h
@@ -29,6 +29,9 @@
 /* We support indirect buffer descriptors */
 #define VIRTIO_RING_F_INDIRECT_DESC	28
 
+/* We publish our last-seen used index at the end of the avail ring. */
+#define VIRTIO_RING_F_PUBLISH_INDICES	29
+
 /* Virtio ring descriptors: 16 bytes.  These can chain together via "next". */
 struct vring_desc
 {
@@ -87,6 +90,7 @@ struct vring {
  *	__u16 avail_flags;
  *	__u16 avail_idx;
  *	__u16 available[num];
+ *	__u16 last_used_idx;
  *
  *	// Padding to the next align boundary.
  *	char pad[];
@@ -95,6 +99,7 @@ struct vring {
  *	__u16 used_flags;
  *	__u16 used_idx;
  *	struct vring_used_elem used[num];
+ *	__u16 last_avail_idx;
  * };
  */
 static inline void vring_init(struct vring *vr, unsigned int num, void *p,
@@ -111,9 +116,14 @@ static inline unsigned vring_size(unsign
 {
 	return ((sizeof(struct vring_desc) * num + sizeof(__u16) * (2 + num)
 		 + align - 1) & ~(align - 1))
-		+ sizeof(__u16) * 2 + sizeof(struct vring_used_elem) * num;
+		+ sizeof(__u16) * 2 + sizeof(struct vring_used_elem) * num + 2;
 }
 
+/* We publish the last-seen used index at the end of the available ring, and
+ * vice-versa.  These are at the end for backwards compatibility. */
+#define vring_last_used(vr) ((vr)->avail->ring[(vr)->num])
+#define vring_last_avail(vr) (*(__u16 *)&(vr)->used->ring[(vr)->num])
+
 #ifdef __KERNEL__
 #include <linux/irqreturn.h>
 struct virtio_device;

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Re: [PATCHv8 3/3] vhost_net: a kernel-level virtio server

[Michael S. Tsirkin]

On Mon, Nov 9, 2009 at 8:17 AM, Rusty Russell <rusty@rustcorp.com.au> wrote:
>> > > +static void vhost_net_set_features(struct vhost_net *n, u64 features)
>> > > +{
>> > > + size_t hdr_size = features & (1 << VHOST_NET_F_VIRTIO_NET_HDR) ?
>> > > +         sizeof(struct virtio_net_hdr) : 0;
>> > > + int i;
>> > > + mutex_lock(&n->dev.mutex);
>> > > + n->dev.acked_features = features;
>> >
>> > Why is this called "acked_features"?  Not just "features"?  I expected
>> > to see code which exposed these back to userspace, and didn't.
>>
>> Not sure how do you mean. Userspace sets them, why
>> does it want to get them exposed back?
>
> There's something about the 'acked' which rubs me the wrong way.
> "enabled_features" is perhaps a better term than "acked_features"; "acked"
> seems more a user point-of-view, "enabled" seems more driver POV?
>

Hmm. Are you happy with the ioctl name? If yes I think being consistent
with that is important.


> set_features matches your ioctl names, but it sounds like a fn name :(
>
> It's marginal.  And 'features' is shorter than both.

I started with this but I was always getting confused whether this
includes all features or just acked features.  I'll go with
enabled_features.


>
>> > > + switch (ioctl) {
>> > > + case VHOST_SET_VRING_NUM:
>> >
>> > I haven't looked at your userspace implementation, but does a generic
>> > VHOST_SET_VRING_STATE & VHOST_GET_VRING_STATE with a struct make more
>> > sense?  It'd be simpler here,
>>
>> Not by much though, right?
>>
>> > but not sure if it'd be simpler to use?
>>
>> The problem is with VHOST_SET_VRING_BASE as well. I want it to be
>> separate because I want to make it possible to relocate e.g. used ring
>> to another address while ring is running. This would be a good debugging
>> tool (you look at kernel's used ring, check descriptor, then update
>> guest's used ring) and also possibly an extra way to do migration.  And
>> it's nicer to have vring size separate as well, because it is
>> initialized by host and never changed, right?
>
> Actually, this looks wrong to me:
>
> +       case VHOST_SET_VRING_BASE:
> ...
> +               vq->avail_idx = vq->last_avail_idx = s.num;
>
> The last_avail_idx is part of the state of the driver.  It needs to be saved
> and restored over susp/resume.


Exactly. That's what VHOST_GET/SET_VRING_BASE does.  avail_idx is just a
cached value for notify on empty, so what this does is clear the value.
What exactly do you refer to when you say "this looks wrong"?
This could trigger an extra notification if I ever called
trigger_irq without get first. As I don't, it in fact has no effect.

>  The only reason it's not in the ring itself
> is because I figured the other side doesn't need to see it (which is true, but
> missed debugging opportunities as well as man-in-the-middle issues like this
> one).  I had a patch which put this field at the end of the ring, I might
> resurrect it to avoid this problem.  This is backwards compatible with all
> implementations.  See patch at end.

Yes, I remember that patch. There seems to be little point though, at
this stage.


>
> I would drop avail_idx altogether: get_user is basically free, and simplifies
> a lot.  As most state is in the ring, all you need is an ioctl to save/restore
> the last_avail_idx.


avail_idx is there for notify on empty: I had this thought that it's
better to leave the avail cache line alone when we are triggering
interrupt to avoid bouncing it around if guest is updating it meanwhile
on another CPU, and I think my testing showed that it helped
performance, but could be a mistake.  You don't believe this can help?



>
>> We could merge DESC, AVAIL, USED, and it will reduce the amount of code
>> in userspace. With both base, size and fds separate, it seemed a bit
>> more symmetrical to have desc/avail/used separate as well.
>> What's your opinion?
>
> Well, DESC, AVAIL, and USED could easily be turned into SET/GET_LAYOUT.


Will do.

>
>> > For future reference, this is *exactly* the kind of thing which would have
>> > been nice as a followup patch.  Easy to separate, easy to review, not critical
>> > to the core.
>>
>> Yes. It's not too late to split it out though: should I do it yet?
>
> Only if you're feeling enthused.  It's lightly reviewed now.


Not really :) I'll keep this in mind for the future.
Thanks!



>
> Cheers,
> Rusty.

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Re: [PATCHv8 3/3] vhost_net: a kernel-level virtio server

[Rusty Russell]

On Mon, 9 Nov 2009 05:40:32 pm Michael S. Tsirkin wrote:
> On Mon, Nov 9, 2009 at 8:17 AM, Rusty Russell <rusty@rustcorp.com.au> wrote:
> > There's something about the 'acked' which rubs me the wrong way.
> > "enabled_features" is perhaps a better term than "acked_features"; "acked"
> > seems more a user point-of-view, "enabled" seems more driver POV?
> 
> Hmm. Are you happy with the ioctl name? If yes I think being consistent
> with that is important.

I think in my original comments I noted that I preferred GET / SET, rather
than GET/ACK.

> > Actually, this looks wrong to me:
> >
> > +       case VHOST_SET_VRING_BASE:
> > ...
> > +               vq->avail_idx = vq->last_avail_idx = s.num;
> >
> > The last_avail_idx is part of the state of the driver.  It needs to be saved
> > and restored over susp/resume.
> 
> 
> Exactly. That's what VHOST_GET/SET_VRING_BASE does.  avail_idx is just a
> cached value for notify on empty, so what this does is clear the value.

Ah, you actually refresh it every time anyway.  Hmm, could you do my poor
brain a favor and either just get_user it in vhost_trigger_irq(), or call
it 'cached_avail_idx' or something?

> >  The only reason it's not in the ring itself
> > is because I figured the other side doesn't need to see it (which is true, but
> > missed debugging opportunities as well as man-in-the-middle issues like this
> > one).  I had a patch which put this field at the end of the ring, I might
> > resurrect it to avoid this problem.  This is backwards compatible with all
> > implementations.  See patch at end.
> 
> Yes, I remember that patch. There seems to be little point though, at
> this stage.

Well, it avoids this ioctl, by exposing all the state.  We may well need it
later, to expand the ring in other ways.

> > I would drop avail_idx altogether: get_user is basically free, and simplifies
> > a lot.  As most state is in the ring, all you need is an ioctl to save/restore
> > the last_avail_idx.
> 
> avail_idx is there for notify on empty: I had this thought that it's
> better to leave the avail cache line alone when we are triggering
> interrupt to avoid bouncing it around if guest is updating it meanwhile
> on another CPU, and I think my testing showed that it helped
> performance, but could be a mistake.  You don't believe this can help?

I believe it could help, but this is YA case where it would have been nice to
have a dumb basic patch and this as a patch on top.  But I am going to ask
you to re-run that measurement, see if it stacks up (because it's an
interesting lesson if it does..)

Thanks!
Rusty.

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Re: [PATCHv8 3/3] vhost_net: a kernel-level virtio server

[Michael S. Tsirkin]

On Mon, Nov 09, 2009 at 04:47:29PM +1030, Rusty Russell wrote:
> On Sun, 8 Nov 2009 10:05:16 pm Michael S. Tsirkin wrote:
> > On Fri, Nov 06, 2009 at 03:29:17PM +1030, Rusty Russell wrote:
> > > > +/* Caller must have TX VQ lock */
> > > > +static void tx_poll_stop(struct vhost_net *net)
> > > > +{
> > > > +	if (likely(net->tx_poll_state != VHOST_NET_POLL_STARTED))
> > > > +		return;
> > > 
> > > likely?  Really?
> > 
> > Hmm ... yes. tx poll stop is called on each packet (as long as we do not
> > fill up 1/2 backend queue), the first call will stop polling
> > the rest checks state and does nothing.
> > 
> > This is because we normally do not care when the message has left the
> > queue in backend device: we tell backend to send it and forget. We only
> > start polling when backend tx queue fills up.
> 
> OK, good.
> 
> > > > +static void vhost_net_set_features(struct vhost_net *n, u64 features)
> > > > +{
> > > > +	size_t hdr_size = features & (1 << VHOST_NET_F_VIRTIO_NET_HDR) ?
> > > > +		sizeof(struct virtio_net_hdr) : 0;
> > > > +	int i;
> > > > +	mutex_lock(&n->dev.mutex);
> > > > +	n->dev.acked_features = features;
> > > 
> > > Why is this called "acked_features"?  Not just "features"?  I expected
> > > to see code which exposed these back to userspace, and didn't.
> > 
> > Not sure how do you mean. Userspace sets them, why
> > does it want to get them exposed back?
> 
> There's something about the 'acked' which rubs me the wrong way.
> "enabled_features" is perhaps a better term than "acked_features"; "acked"
> seems more a user point-of-view, "enabled" seems more driver POV?
> 
> set_features matches your ioctl names, but it sounds like a fn name :(

Hmm. Are you happy with the ioctl name? If yes I think being consistent
with that is important.

> It's marginal.  And 'features' is shorter than both.

I started with this but I was always getting confused whether this
includes all features or just acked features.  I'll go with
enabled_features.

> > > > +	switch (ioctl) {
> > > > +	case VHOST_SET_VRING_NUM:
> > > 
> > > I haven't looked at your userspace implementation, but does a generic
> > > VHOST_SET_VRING_STATE & VHOST_GET_VRING_STATE with a struct make more
> > > sense?  It'd be simpler here,
> > 
> > Not by much though, right?
> > 
> > > but not sure if it'd be simpler to use?
> > 
> > The problem is with VHOST_SET_VRING_BASE as well. I want it to be
> > separate because I want to make it possible to relocate e.g. used ring
> > to another address while ring is running. This would be a good debugging
> > tool (you look at kernel's used ring, check descriptor, then update
> > guest's used ring) and also possibly an extra way to do migration.  And
> > it's nicer to have vring size separate as well, because it is
> > initialized by host and never changed, right?
> 
> Actually, this looks wrong to me:
> 
> +	case VHOST_SET_VRING_BASE:
> ...
> +		vq->avail_idx = vq->last_avail_idx = s.num;
> 
> The last_avail_idx is part of the state of the driver.  It needs to be saved
> and restored over susp/resume.

Exactly. That's what VHOST_GET/SET_VRING_BASE does.  avail_idx is just a
cached value for notify on empty, so what this does is clear the value.
What exactly do you refer to when you say "this looks wrong"?
This could trigger an extra notification if I ever called
trigger_irq without get first. As I don't, it in fact has no effect.

>  The only reason it's not in the ring itself
> is because I figured the other side doesn't need to see it (which is true, but
> missed debugging opportunities as well as man-in-the-middle issues like this
> one).  I had a patch which put this field at the end of the ring, I might
> resurrect it to avoid this problem.  This is backwards compatible with all
> implementations.  See patch at end.

Yes, I remember that patch. There seems to be little point though, at
this stage.

> 
> I would drop avail_idx altogether: get_user is basically free, and
> simplifies a lot.  As most state is in the ring, all you need is an
> ioctl to save/restore the last_avail_idx.

avail_idx is there for notify on empty: I had this thought that it's
better to leave the avail cache line alone when we are triggering
interrupt to avoid bouncing it around if guest is updating it meanwhile
on another CPU, and I think my testing showed that it helped
performance, but could be a mistake.  You don't believe this can help?

> > We could merge DESC, AVAIL, USED, and it will reduce the amount of code
> > in userspace. With both base, size and fds separate, it seemed a bit
> > more symmetrical to have desc/avail/used separate as well.
> > What's your opinion?
> 
> Well, DESC, AVAIL, and USED could easily be turned into SET/GET_LAYOUT.

OK, I'll do this.

> > > For future reference, this is *exactly* the kind of thing which would have
> > > been nice as a followup patch.  Easy to separate, easy to review, not critical
> > > to the core.
> > 
> > Yes. It's not too late to split it out though: should I do it yet?
> 
> Only if you're feeling enthused.  It's lightly reviewed now.

Not really :) I'll keep this in mind for the future.
Thanks!

> Cheers,
> Rusty.
> 
> virtio: put last_used and last_avail index into ring itself.
> 
> Generally, the other end of the virtio ring doesn't need to see where
> you're up to in consuming the ring.  However, to completely understand
> what's going on from the outside, this information must be exposed.
> For example, if you want to save and restore a virtio_ring, but you're
> not the consumer because the kernel is using it directly.
> 
> Fortunately, we have room to expand: the ring is always a whole number
> of pages and there's hundreds of bytes of padding after the avail ring
> and the used ring, whatever the number of descriptors (which must be a
> power of 2).
> 
> We add a feature bit so the guest can tell the host that it's writing
> out the current value there, if it wants to use that.
> 
> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
> ---
>  drivers/virtio/virtio_ring.c |   23 +++++++++++++++--------
>  include/linux/virtio_ring.h  |   12 +++++++++++-
>  2 files changed, 26 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -71,9 +71,6 @@ struct vring_virtqueue
>  	/* Number we've added since last sync. */
>  	unsigned int num_added;
>  
> -	/* Last used index we've seen. */
> -	u16 last_used_idx;
> -
>  	/* How to notify other side. FIXME: commonalize hcalls! */
>  	void (*notify)(struct virtqueue *vq);
>  
> @@ -278,12 +275,13 @@ static void detach_buf(struct vring_virt
>  
>  static inline bool more_used(const struct vring_virtqueue *vq)
>  {
> -	return vq->last_used_idx != vq->vring.used->idx;
> +	return vring_last_used(&vq->vring) != vq->vring.used->idx;
>  }
>  
>  static void *vring_get_buf(struct virtqueue *_vq, unsigned int *len)
>  {
>  	struct vring_virtqueue *vq = to_vvq(_vq);
> +	struct vring_used_elem *u;
>  	void *ret;
>  	unsigned int i;
>  
> @@ -300,8 +298,11 @@ static void *vring_get_buf(struct virtqu
>  		return NULL;
>  	}
>  
> -	i = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].id;
> -	*len = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].len;
> +	u = &vq->vring.used->ring[vring_last_used(&vq->vring) % vq->vring.num];
> +	i = u->id;
> +	*len = u->len;
> +	/* Make sure we don't reload i after doing checks. */
> +	rmb();
>  
>  	if (unlikely(i >= vq->vring.num)) {
>  		BAD_RING(vq, "id %u out of range\n", i);
> @@ -315,7 +316,8 @@ static void *vring_get_buf(struct virtqu
>  	/* detach_buf clears data, so grab it now. */
>  	ret = vq->data[i];
>  	detach_buf(vq, i);
> -	vq->last_used_idx++;
> +	vring_last_used(&vq->vring)++;
> +
>  	END_USE(vq);
>  	return ret;
>  }
> @@ -402,7 +404,6 @@ struct virtqueue *vring_new_virtqueue(un
>  	vq->vq.name = name;
>  	vq->notify = notify;
>  	vq->broken = false;
> -	vq->last_used_idx = 0;
>  	vq->num_added = 0;
>  	list_add_tail(&vq->vq.list, &vdev->vqs);
>  #ifdef DEBUG
> @@ -413,6 +414,10 @@ struct virtqueue *vring_new_virtqueue(un
>  
>  	vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC);
>  
> +	/* We publish indices whether they offer it or not: if not, it's junk
> +	 * space anyway.  But calling this acknowledges the feature. */
> +	virtio_has_feature(vdev, VIRTIO_RING_F_PUBLISH_INDICES);
> +
>  	/* No callback?  Tell other side not to bother us. */
>  	if (!callback)
>  		vq->vring.avail->flags |= VRING_AVAIL_F_NO_INTERRUPT;
> @@ -443,6 +448,8 @@ void vring_transport_features(struct vir
>  		switch (i) {
>  		case VIRTIO_RING_F_INDIRECT_DESC:
>  			break;
> +		case VIRTIO_RING_F_PUBLISH_INDICES:
> +			break;
>  		default:
>  			/* We don't understand this bit. */
>  			clear_bit(i, vdev->features);
> diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
> --- a/include/linux/virtio_ring.h
> +++ b/include/linux/virtio_ring.h
> @@ -29,6 +29,9 @@
>  /* We support indirect buffer descriptors */
>  #define VIRTIO_RING_F_INDIRECT_DESC	28
>  
> +/* We publish our last-seen used index at the end of the avail ring. */
> +#define VIRTIO_RING_F_PUBLISH_INDICES	29
> +
>  /* Virtio ring descriptors: 16 bytes.  These can chain together via "next". */
>  struct vring_desc
>  {
> @@ -87,6 +90,7 @@ struct vring {
>   *	__u16 avail_flags;
>   *	__u16 avail_idx;
>   *	__u16 available[num];
> + *	__u16 last_used_idx;
>   *
>   *	// Padding to the next align boundary.
>   *	char pad[];
> @@ -95,6 +99,7 @@ struct vring {
>   *	__u16 used_flags;
>   *	__u16 used_idx;
>   *	struct vring_used_elem used[num];
> + *	__u16 last_avail_idx;
>   * };
>   */
>  static inline void vring_init(struct vring *vr, unsigned int num, void *p,
> @@ -111,9 +116,14 @@ static inline unsigned vring_size(unsign
>  {
>  	return ((sizeof(struct vring_desc) * num + sizeof(__u16) * (2 + num)
>  		 + align - 1) & ~(align - 1))
> -		+ sizeof(__u16) * 2 + sizeof(struct vring_used_elem) * num;
> +		+ sizeof(__u16) * 2 + sizeof(struct vring_used_elem) * num + 2;
>  }
>  
> +/* We publish the last-seen used index at the end of the available ring, and
> + * vice-versa.  These are at the end for backwards compatibility. */
> +#define vring_last_used(vr) ((vr)->avail->ring[(vr)->num])
> +#define vring_last_avail(vr) (*(__u16 *)&(vr)->used->ring[(vr)->num])
> +
>  #ifdef __KERNEL__
>  #include <linux/irqreturn.h>
>  struct virtio_device;

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Re: [PATCHv8 3/3] vhost_net: a kernel-level virtio server

[Michael S. Tsirkin]

On Mon, Nov 09, 2009 at 04:47:29PM +1030, Rusty Russell wrote:
> Actually, this looks wrong to me:
> 
> +	case VHOST_SET_VRING_BASE:
> ...
> +		vq->avail_idx = vq->last_avail_idx = s.num;
> 
> The last_avail_idx is part of the state of the driver.  It needs to be saved
> and restored over susp/resume.  The only reason it's not in the ring itself
> is because I figured the other side doesn't need to see it (which is true, but
> missed debugging opportunities as well as man-in-the-middle issues like this
> one).  I had a patch which put this field at the end of the ring, I might
> resurrect it to avoid this problem.  This is backwards compatible with all
> implementations.  See patch at end.
> 
> I would drop avail_idx altogether: get_user is basically free, and simplifies
> a lot.  As most state is in the ring, all you need is an ioctl to save/restore
> the last_avail_idx.

I remembered another reason for caching head in avail_idx.  Basically,
avail index could change between when I poll for descriptors and when I
want to notify guest.

So we could have:
	- poll descriptors until empty
	- notify
		detects not empty so does not notify

And the way to solve it would be to return flag from
notify telling us to restart the polling loop.

But, this will be more code, on data path, than
what happens today where I simply keep state
from descriptor polling and use that to notify.

I also suspect that somehow this race in practice can not create
deadlocks ... but I prefer to avoid it, these things are very tricky: if
I see an empty ring, and stop processing descriptors, I want to trigger
notify on empty.

So if we want to avoid keeping "empty" state, IMO the best way would be
to pass a flag to vhost_signal that tells it that ring is empty.
Makes sense?

-- 
MST

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Re: virtio: put last_used and last_avail index into ring itself.

[Michael S. Tsirkin]

> virtio: put last_used and last_avail index into ring itself.
> 
> Generally, the other end of the virtio ring doesn't need to see where
> you're up to in consuming the ring.  However, to completely understand
> what's going on from the outside, this information must be exposed.
> For example, if you want to save and restore a virtio_ring, but you're
> not the consumer because the kernel is using it directly.
> 
> Fortunately, we have room to expand: the ring is always a whole number
> of pages and there's hundreds of bytes of padding after the avail ring
> and the used ring, whatever the number of descriptors (which must be a
> power of 2).
> 
> We add a feature bit so the guest can tell the host that it's writing
> out the current value there, if it wants to use that.
> 
> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

I've been looking at this patch some more (more on why
later), and I wonder: would it be better to add some
alignment to the last used index address, so that
if we later add more stuff at the tail, it all
fits in a single cache line?

We use a new feature bit anyway, so layout change should not be
a problem.

Since I raised the question of caches: for used ring,
the ring is not aligned to 64 bit, so on CPUs with 64 bit
or larger cache lines, used entries will often cross
cache line boundaries. Am I right and might it
have been better to align ring entries to cache line boundaries?

What do you think?

> ---
>  drivers/virtio/virtio_ring.c |   23 +++++++++++++++--------
>  include/linux/virtio_ring.h  |   12 +++++++++++-
>  2 files changed, 26 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -71,9 +71,6 @@ struct vring_virtqueue
>  	/* Number we've added since last sync. */
>  	unsigned int num_added;
>  
> -	/* Last used index we've seen. */
> -	u16 last_used_idx;
> -
>  	/* How to notify other side. FIXME: commonalize hcalls! */
>  	void (*notify)(struct virtqueue *vq);
>  
> @@ -278,12 +275,13 @@ static void detach_buf(struct vring_virt
>  
>  static inline bool more_used(const struct vring_virtqueue *vq)
>  {
> -	return vq->last_used_idx != vq->vring.used->idx;
> +	return vring_last_used(&vq->vring) != vq->vring.used->idx;
>  }
>  
>  static void *vring_get_buf(struct virtqueue *_vq, unsigned int *len)
>  {
>  	struct vring_virtqueue *vq = to_vvq(_vq);
> +	struct vring_used_elem *u;
>  	void *ret;
>  	unsigned int i;
>  
> @@ -300,8 +298,11 @@ static void *vring_get_buf(struct virtqu
>  		return NULL;
>  	}
>  
> -	i = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].id;
> -	*len = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].len;
> +	u = &vq->vring.used->ring[vring_last_used(&vq->vring) % vq->vring.num];
> +	i = u->id;
> +	*len = u->len;
> +	/* Make sure we don't reload i after doing checks. */
> +	rmb();
>  
>  	if (unlikely(i >= vq->vring.num)) {
>  		BAD_RING(vq, "id %u out of range\n", i);
> @@ -315,7 +316,8 @@ static void *vring_get_buf(struct virtqu
>  	/* detach_buf clears data, so grab it now. */
>  	ret = vq->data[i];
>  	detach_buf(vq, i);
> -	vq->last_used_idx++;
> +	vring_last_used(&vq->vring)++;
> +
>  	END_USE(vq);
>  	return ret;
>  }
> @@ -402,7 +404,6 @@ struct virtqueue *vring_new_virtqueue(un
>  	vq->vq.name = name;
>  	vq->notify = notify;
>  	vq->broken = false;
> -	vq->last_used_idx = 0;
>  	vq->num_added = 0;
>  	list_add_tail(&vq->vq.list, &vdev->vqs);
>  #ifdef DEBUG
> @@ -413,6 +414,10 @@ struct virtqueue *vring_new_virtqueue(un
>  
>  	vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC);
>  
> +	/* We publish indices whether they offer it or not: if not, it's junk
> +	 * space anyway.  But calling this acknowledges the feature. */
> +	virtio_has_feature(vdev, VIRTIO_RING_F_PUBLISH_INDICES);
> +
>  	/* No callback?  Tell other side not to bother us. */
>  	if (!callback)
>  		vq->vring.avail->flags |= VRING_AVAIL_F_NO_INTERRUPT;
> @@ -443,6 +448,8 @@ void vring_transport_features(struct vir
>  		switch (i) {
>  		case VIRTIO_RING_F_INDIRECT_DESC:
>  			break;
> +		case VIRTIO_RING_F_PUBLISH_INDICES:
> +			break;
>  		default:
>  			/* We don't understand this bit. */
>  			clear_bit(i, vdev->features);
> diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
> --- a/include/linux/virtio_ring.h
> +++ b/include/linux/virtio_ring.h
> @@ -29,6 +29,9 @@
>  /* We support indirect buffer descriptors */
>  #define VIRTIO_RING_F_INDIRECT_DESC	28
>  
> +/* We publish our last-seen used index at the end of the avail ring. */
> +#define VIRTIO_RING_F_PUBLISH_INDICES	29
> +
>  /* Virtio ring descriptors: 16 bytes.  These can chain together via "next". */
>  struct vring_desc
>  {
> @@ -87,6 +90,7 @@ struct vring {
>   *	__u16 avail_flags;
>   *	__u16 avail_idx;
>   *	__u16 available[num];
> + *	__u16 last_used_idx;
>   *
>   *	// Padding to the next align boundary.
>   *	char pad[];
> @@ -95,6 +99,7 @@ struct vring {
>   *	__u16 used_flags;
>   *	__u16 used_idx;
>   *	struct vring_used_elem used[num];
> + *	__u16 last_avail_idx;
>   * };
>   */
>  static inline void vring_init(struct vring *vr, unsigned int num, void *p,
> @@ -111,9 +116,14 @@ static inline unsigned vring_size(unsign
>  {
>  	return ((sizeof(struct vring_desc) * num + sizeof(__u16) * (2 + num)
>  		 + align - 1) & ~(align - 1))
> -		+ sizeof(__u16) * 2 + sizeof(struct vring_used_elem) * num;
> +		+ sizeof(__u16) * 2 + sizeof(struct vring_used_elem) * num + 2;
>  }
>  
> +/* We publish the last-seen used index at the end of the available ring, and
> + * vice-versa.  These are at the end for backwards compatibility. */
> +#define vring_last_used(vr) ((vr)->avail->ring[(vr)->num])
> +#define vring_last_avail(vr) (*(__u16 *)&(vr)->used->ring[(vr)->num])
> +
>  #ifdef __KERNEL__
>  #include <linux/irqreturn.h>
>  struct virtio_device;

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Re: virtio: put last_used and last_avail index into ring itself.

[Rusty Russell]

On Wed, 5 May 2010 03:52:36 am Michael S. Tsirkin wrote:
> > virtio: put last_used and last_avail index into ring itself.
> > 
> > Generally, the other end of the virtio ring doesn't need to see where
> > you're up to in consuming the ring.  However, to completely understand
> > what's going on from the outside, this information must be exposed.
> > For example, if you want to save and restore a virtio_ring, but you're
> > not the consumer because the kernel is using it directly.
> > 
> > Fortunately, we have room to expand: the ring is always a whole number
> > of pages and there's hundreds of bytes of padding after the avail ring
> > and the used ring, whatever the number of descriptors (which must be a
> > power of 2).
> > 
> > We add a feature bit so the guest can tell the host that it's writing
> > out the current value there, if it wants to use that.
> > 
> > Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
> 
> I've been looking at this patch some more (more on why
> later), and I wonder: would it be better to add some
> alignment to the last used index address, so that
> if we later add more stuff at the tail, it all
> fits in a single cache line?

In theory, but not in practice.  We don't have many rings, so the
difference between 1 and 2 cache lines is not very much.

> We use a new feature bit anyway, so layout change should not be
> a problem.
> 
> Since I raised the question of caches: for used ring,
> the ring is not aligned to 64 bit, so on CPUs with 64 bit
> or larger cache lines, used entries will often cross
> cache line boundaries. Am I right and might it
> have been better to align ring entries to cache line boundaries?
> 
> What do you think?

I think everyone is settled on 128 byte cache lines for the forseeable
future, so it's not really an issue.

Cheers,
Rusty.

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Re: virtio: put last_used and last_avail index into ring itself.

[Michael S. Tsirkin]

On Thu, May 06, 2010 at 10:22:12AM +0930, Rusty Russell wrote:
> On Wed, 5 May 2010 03:52:36 am Michael S. Tsirkin wrote:
> > > virtio: put last_used and last_avail index into ring itself.
> > > 
> > > Generally, the other end of the virtio ring doesn't need to see where
> > > you're up to in consuming the ring.  However, to completely understand
> > > what's going on from the outside, this information must be exposed.
> > > For example, if you want to save and restore a virtio_ring, but you're
> > > not the consumer because the kernel is using it directly.
> > > 
> > > Fortunately, we have room to expand: the ring is always a whole number
> > > of pages and there's hundreds of bytes of padding after the avail ring
> > > and the used ring, whatever the number of descriptors (which must be a
> > > power of 2).
> > > 
> > > We add a feature bit so the guest can tell the host that it's writing
> > > out the current value there, if it wants to use that.
> > > 
> > > Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
> > 
> > I've been looking at this patch some more (more on why
> > later), and I wonder: would it be better to add some
> > alignment to the last used index address, so that
> > if we later add more stuff at the tail, it all
> > fits in a single cache line?
> 
> In theory, but not in practice.  We don't have many rings, so the
> difference between 1 and 2 cache lines is not very much.

Fair enough.

> > We use a new feature bit anyway, so layout change should not be
> > a problem.
> > 
> > Since I raised the question of caches: for used ring,
> > the ring is not aligned to 64 bit, so on CPUs with 64 bit
> > or larger cache lines, used entries will often cross
> > cache line boundaries. Am I right and might it
> > have been better to align ring entries to cache line boundaries?
> > 
> > What do you think?
> 
> I think everyone is settled on 128 byte cache lines for the forseeable
> future, so it's not really an issue.
> 
> Cheers,
> Rusty.

You mean with 64 bit descriptors we will be bouncing a cache line
between host and guest, anyway?

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Re: virtio: put last_used and last_avail index into ring itself.

[Rusty Russell]

On Thu, 6 May 2010 03:57:55 pm Michael S. Tsirkin wrote:
> On Thu, May 06, 2010 at 10:22:12AM +0930, Rusty Russell wrote:
> > On Wed, 5 May 2010 03:52:36 am Michael S. Tsirkin wrote:
> > > What do you think?
> > 
> > I think everyone is settled on 128 byte cache lines for the forseeable
> > future, so it's not really an issue.
> 
> You mean with 64 bit descriptors we will be bouncing a cache line
> between host and guest, anyway?

I'm confused by this entire thread.

Descriptors are 16 bytes.  They are at the start, so presumably aligned to
cache boundaries.

Available ring follows that at 2 bytes per entry, so it's also packed nicely
into cachelines.

Then there's padding to page boundary.  That puts us on a cacheline again
for the used ring; also 2 bytes per entry.

I don't see how any change in layout could be more cache friendly?
Rusty.

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Re: virtio: put last_used and last_avail index into ring itself.

[Michael S. Tsirkin]

On Fri, May 07, 2010 at 12:35:39PM +0930, Rusty Russell wrote:
> On Thu, 6 May 2010 03:57:55 pm Michael S. Tsirkin wrote:
> > On Thu, May 06, 2010 at 10:22:12AM +0930, Rusty Russell wrote:
> > > On Wed, 5 May 2010 03:52:36 am Michael S. Tsirkin wrote:
> > > > What do you think?
> > > 
> > > I think everyone is settled on 128 byte cache lines for the forseeable
> > > future, so it's not really an issue.
> > 
> > You mean with 64 bit descriptors we will be bouncing a cache line
> > between host and guest, anyway?
> 
> I'm confused by this entire thread.
> 
> Descriptors are 16 bytes.  They are at the start, so presumably aligned to
> cache boundaries.
> 
> Available ring follows that at 2 bytes per entry, so it's also packed nicely
> into cachelines.
> 
> Then there's padding to page boundary.  That puts us on a cacheline again
> for the used ring; also 2 bytes per entry.
> 

Hmm, is used ring really 2 bytes per entry?


/* u32 is used here for ids for padding reasons. */
struct vring_used_elem {
        /* Index of start of used descriptor chain. */
        __u32 id;
        /* Total length of the descriptor chain which was used (written to) */
        __u32 len;
};

struct vring_used {
        __u16 flags;
        __u16 idx;
        struct vring_used_elem ring[];
};

> I don't see how any change in layout could be more cache friendly?
> Rusty.

I thought that used ring has 8 bytes per entry, and that struct
vring_used is aligned at page boundary, this
would mean that ring element is at offset 4 bytes from page boundary.
Thus with cacheline size 128 bytes, each 4th element crosses
a cacheline boundary. If we had a 4 byte padding after idx, each
used element would always be completely within a single cacheline.

What am I missing?
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Re: virtio: put last_used and last_avail index into ring itself.

[Rusty Russell]

On Sun, 9 May 2010 06:27:33 pm Michael S. Tsirkin wrote:
> On Fri, May 07, 2010 at 12:35:39PM +0930, Rusty Russell wrote:
> > Then there's padding to page boundary.  That puts us on a cacheline again
> > for the used ring; also 2 bytes per entry.
> > 
> 
> Hmm, is used ring really 2 bytes per entry?

Err, no, I am an idiot.

> /* u32 is used here for ids for padding reasons. */
> struct vring_used_elem {
>         /* Index of start of used descriptor chain. */
>         __u32 id;
>         /* Total length of the descriptor chain which was used (written to) */
>         __u32 len;
> };
> 
> struct vring_used {
>         __u16 flags;
>         __u16 idx;
>         struct vring_used_elem ring[];
> };

OK, now I get it.  Sorry, I was focussed on the avail ring.

> I thought that used ring has 8 bytes per entry, and that struct
> vring_used is aligned at page boundary, this
> would mean that ring element is at offset 4 bytes from page boundary.
> Thus with cacheline size 128 bytes, each 4th element crosses
> a cacheline boundary. If we had a 4 byte padding after idx, each
> used element would always be completely within a single cacheline.

I think the numbers are: every 16th entry hits two cachelines.  So currently
the first 15 entries are "free" (assuming we hit the idx cacheline anyway),
then 1 in 16 cost 2 cachelines.  That makes the aligned version win when
N > 240.

But, we access the array linearly.  So the extra cacheline cost is in fact
amortized.  I doubt it could be measured, but maybe vring_get_buf() should
prefetch?  While you're there, we could use an & rather than a mod on the
calculation, which may actually be measurable :)

Cheers,
Rusty.

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