[PATCH 6/6] virtio ring helper

Rusty Russell
2007-09-20T12:27:14Z

These helper routines supply most of the virtqueue_ops for hypervisors
which want to use a ring for virtio. Unlike the previous lguest
implementation:

1) The rings are variable sized (2^n-1 elements).
2) They have an unfortunate limit of 65535 bytes per sg element.
3) The page numbers are always 64 bit (PAE anyone?)
4) They no longer place used[] on a separate page, just a separate
cacheline.
5) We do a modulo on a variable. We could be tricky if we cared.

Users need only implement the new_vq and free_vq hooks (KVM wants the
guest to allocate the rings, lguest does it sanely).

Signed-off-by: Rusty Russell <rusty-8n+1lVoiYb80n/F98K4Iww@public.gmane.org>
---
arch/i386/lguest/Kconfig | 1
drivers/virtio/Kconfig | 5
drivers/virtio/Makefile | 1
drivers/virtio/virtio_ring.c | 301 ++++++++++++++++++++++++++++++++++++++++++
drivers/virtio/vring.h | 50 ++++++
include/linux/virtio_ring.h | 96 +++++++++++++
6 files changed, 454 insertions(+)

===================================================================
--- a/arch/i386/lguest/Kconfig
+++ b/arch/i386/lguest/Kconfig
@@ -2,6 +2,7 @@ config LGUEST_GUEST
bool "Lguest guest support"
select PARAVIRT
depends on !X86_PAE
+ select VIRTIO_RING
help
Lguest is a tiny in-kernel hypervisor. Selecting this will
allow your kernel to boot under lguest. This option will increase
===================================================================
--- a/drivers/virtio/Kconfig
+++ b/drivers/virtio/Kconfig
@@ -1,3 +1,8 @@
# Virtio always gets selected by whoever wants it.
config VIRTIO
bool
+
+# Similarly the virtio ring implementation.
+config VIRTIO_RING
+ bool
+ depends on VIRTIO
===================================================================
--- a/drivers/virtio/Makefile
+++ b/drivers/virtio/Makefile
@@ -1,1 +1,2 @@ obj-$(CONFIG_VIRTIO) += virtio.o config.
obj-$(CONFIG_VIRTIO) += virtio.o config.o
+obj-$(CONFIG_VIRTIO_RING) += virtio_ring.o
===================================================================
--- /dev/null
+++ b/drivers/virtio/virtio_ring.c
@@ -0,0 +1,301 @@
+/* Virtio ring implementation.
+ *
+ * Copyright 2007 Rusty Russell IBM Corporation
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+#include <linux/virtio.h>
+#include <linux/virtio_ring.h>
+#include <linux/device.h>
+#include "vring.h"
+
+#ifdef DEBUG
+/* For development, we want to crash whenever the ring is screwed. */
+#define BAD_RING(vvq, fmt...) \
+ do { dev_err(vvq->dev, fmt); BUG(); } while(0)
+#define START_USE(vvq) \
+ do { if ((vvq)->in_use) panic("in_use = %i\n", (vvq)->in_use); (vvq)->in_use = __LINE__; mb(); } while(0)
+#define END_USE(vvq) \
+ do { BUG_ON(!(vvq)->in_use); (vvq)->in_use = 0; mb(); } while(0)
+#else
+#define BAD_RING(vvq, fmt...) \
+ do { dev_err(vvq->dev, fmt); (vvq)->broken = true; } while(0)
+#define START_USE(vvq)
+#define END_USE(vvq)
+#endif
+
+struct virtqueue
+{
+ /* Callback for driver when data is consumed. */
+ bool (*callback)(void *);
+ void *cb_data;
+
+ /* Who to blame this on. */
+ struct device *dev;
+
+ /* Actual memory layout for this queue */
+ struct vring vring;
+
+ /* Other side has made a mess, don't try any more. */
+ bool broken;
+
+ /* Number of free buffers */
+ unsigned int num_free;
+ /* Head of free buffer list. */
+ unsigned int free_head;
+ /* Number we've added since last sync. */
+ unsigned int num_added;
+
+ /* Last used index we've seen. */
+ unsigned int last_used_idx;
+
+ /* How to notify other side. FIXME: commonalize hcalls! */
+ void (*notify)(void *priv);
+ void *priv;
+
+ /* Unless they told us to stop */
+ bool running;
+
+#ifdef DEBUG
+ /* They're supposed to lock for us. */
+ unsigned int in_use;
+#endif
+
+ /* Tokens for callbacks. */
+ void *data[];
+};
+
+void *vring_priv(struct virtqueue *vq)
+{
+ return vq->priv;
+}
+
+int vring_add_buf(struct virtqueue *vq,
+ struct scatterlist sg[],
+ unsigned int out,
+ unsigned int in,
+ void *data)
+{
+ unsigned int i, avail, head, uninitialized_var(prev);
+
+ BUG_ON(data == NULL);
+ BUG_ON(out + in > vq->vring.num);
+ BUG_ON(out + in == 0);
+
+ START_USE(vq);
+
+ if (vq->num_free < out + in) {
+ pr_debug("Can't add buf len %i - avail = %i\n",
+ out + in, vq->num_free);
+ END_USE(vq);
+ return -ENOSPC;
+ }
+
+ /* We're about to use some buffers from the free list. */
+ vq->num_free -= out + in;
+
+ head = vq->free_head;
+ for (i = vq->free_head; out; i = vq->vring.desc[i].next, out--) {
+ vq->vring.desc[i].flags = VRING_DESC_F_NEXT;
+ vq->vring.desc[i].pfn = page_to_pfn(sg[0].page);
+ BUG_ON(sg[0].offset > 65535);
+ BUG_ON(sg[0].length > 65535);
+ vq->vring.desc[i].offset = sg[0].offset;
+ vq->vring.desc[i].len = sg[0].length;
+ prev = i;
+ sg++;
+ }
+ for (; in; i = vq->vring.desc[i].next, in--) {
+ vq->vring.desc[i].flags=VRING_DESC_F_NEXT|VRING_DESC_F_WRITE;
+ vq->vring.desc[i].pfn = page_to_pfn(sg[0].page);
+ BUG_ON(sg[0].offset > 65535);
+ BUG_ON(sg[0].length > 65535);
+ vq->vring.desc[i].offset = sg[0].offset;
+ vq->vring.desc[i].len = sg[0].length;
+ prev = i;
+ sg++;
+ }
+ /* Last one doesn't continue. */
+ vq->vring.desc[prev].flags &= ~VRING_DESC_F_NEXT;
+
+ /* Update free pointer */
+ vq->free_head = i;
+
+ /* Set token. */
+ vq->data[head] = data;
+
+ /* Put entry in available array (but don't update avail->idx until they
+ * do sync). FIXME: avoid modulus here? */
+ avail = (vq->vring.avail->idx + vq->num_added++) % vq->vring.num;
+ vq->vring.avail->ring[avail] = head;
+
+ pr_debug("Added buffer head %i to %p\n", head, vq);
+ END_USE(vq);
+ return 0;
+}
+
+void vring_kick(struct virtqueue *vq)
+{
+ START_USE(vq);
+ /* Descriptors and available array need to be set before we expose the
+ * new available array entries. */
+ wmb();
+
+ vq->vring.avail->idx += vq->num_added;
+ vq->num_added = 0;
+
+ /* Prod other side to tell it about changes. */
+ vq->notify(vq->priv);
+ END_USE(vq);
+}
+
+static void detach_buf(struct virtqueue *vq, unsigned int head)
+{
+ unsigned int i;
+
+ /* Clear data ptr. */
+ vq->data[head] = NULL;
+
+ /* Put back on free list: find end */
+ i = head;
+ while (vq->vring.desc[i].flags & VRING_DESC_F_NEXT) {
+ i = vq->vring.desc[i].next;
+ vq->num_free++;
+ }
+
+ vq->vring.desc[i].next = vq->free_head;
+ vq->free_head = head;
+ /* Plus final descriptor */
+ vq->num_free++;
+}
+
+/* FIXME: We need to tell other side about removal, to synchronize. */
+void vring_shutdown(struct virtqueue *vq)
+{
+ unsigned int i;
+
+ for (i = 0; i < vq->vring.num; i++)
+ detach_buf(vq, i);
+}
+
+static bool more_used(const struct virtqueue *vq)
+{
+ return vq->last_used_idx != vq->vring.used->idx;
+}
+
+void *vring_get_buf(struct virtqueue *vq, unsigned int *len)
+{
+ void *ret;
+ unsigned int i;
+
+ START_USE(vq);
+
+ if (!more_used(vq)) {
+ pr_debug("No more buffers in queue\n");
+ END_USE(vq);
+ 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;
+
+ if (unlikely(i >= vq->vring.num)) {
+ BAD_RING(vq, "id %u out of range\n", i);
+ return NULL;
+ }
+ if (unlikely(!vq->data[i])) {
+ BAD_RING(vq, "id %u is not a head!\n", i);
+ return NULL;
+ }
+
+ /* detach_buf clears data, so grab it now. */
+ ret = vq->data[i];
+ detach_buf(vq, i);
+ vq->last_used_idx++;
+ END_USE(vq);
+ return ret;
+}
+
+/* FIXME: Put running in shmem */
+bool vring_restart(struct virtqueue *vq)
+{
+ START_USE(vq);
+ BUG_ON(vq->running);
+
+ if (likely(!more_used(vq)) || unlikely(vq->broken))
+ vq->running = true;
+
+ END_USE(vq);
+ return vq->running;
+}
+
+irqreturn_t vring_interrupt(int irq, void *_vq)
+{
+ struct virtqueue *vq = _vq;
+
+ pr_debug("virtqueue interrupt for %p\n", vq);
+
+ if (unlikely(vq->broken))
+ return IRQ_HANDLED;
+
+ if (vq->running && more_used(vq)) {
+ pr_debug("virtqueue callback for %p (%p)\n", vq, vq->callback);
+ vq->running = vq->callback(vq->cb_data);
+ } else
+ pr_debug("virtqueue %p no more used\n", vq);
+
+ return IRQ_HANDLED;
+}
+
+struct virtqueue *vring_new_virtqueue(unsigned int num,
+ struct device *dev,
+ void *pages,
+ void (*notify)(void *priv), void *priv,
+ bool (*callback)(void *), void *cb_data)
+{
+ struct virtqueue *vq;
+ unsigned int i;
+
+ vq = kmalloc(sizeof(*vq) + sizeof(void *)*num, GFP_KERNEL);
+ if (!vq)
+ return NULL;
+
+ vring_init(&vq->vring, num, pages);
+ vq->callback = callback;
+ vq->cb_data = cb_data;
+ vq->dev = dev;
+ vq->notify = notify;
+ vq->priv = priv;
+ vq->broken = false;
+ vq->last_used_idx = 0;
+ vq->num_added = 0;
+ vq->running = true;
+#ifdef DEBUG
+ vq->in_use = false;
+#endif
+
+ /* Put everything in free lists. */
+ vq->num_free = num;
+ vq->free_head = 0;
+ for (i = 0; i < num-1; i++)
+ vq->vring.desc[i].next = i+1;
+
+ return vq;
+}
+
+void vring_destroy_virtqueue(struct virtqueue *vq)
+{
+ kfree(vq);
+}
===================================================================
--- /dev/null
+++ b/drivers/virtio/vring.h
@@ -0,0 +1,50 @@
+#ifndef _VRING_VRING_H
+#define _VRING_VRING_H
+#include <linux/virtio.h>
+#include <linux/irqreturn.h>
+
+/* A vring consists of a continuous chunk of memory which looks like so:
+ * (use num == 2^n-1 to make it nicely cacheline aligned).
+ *
+ * struct vring
+ * {
+ * // The actual descriptors.
+ * struct vring_desc desc[num];
+ *
+ * // A ring of available descriptor heads with free-running index.
+ * __u16 avail_idx;
+ * __u16 available[num];
+ *
+ * // Padding so a correctly-chosen num value will cache-align used_idx.
+ * char pad[sizeof(struct vring_desc) - sizeof(__u16)];
+ *
+ * // A ring of used descriptor heads with free-running index.
+ * __u16 used_idx;
+ * struct vring_used_elem used[num];
+ * };
+ */
+struct virtqueue *vring_new_virtqueue(unsigned int num,
+ struct device *dev,
+ void *pages,
+ void (*notify)(void *priv), void *priv,
+ bool (*callback)(void *), void *cb_data);
+void vring_destroy_virtqueue(struct virtqueue *vq);
+
+irqreturn_t vring_interrupt(int irq, void *_vq);
+
+void *vring_priv(struct virtqueue *vq);
+
+int vring_add_buf(struct virtqueue *vq,
+ struct scatterlist sg[],
+ unsigned int out,
+ unsigned int in,
+ void *data);
+
+void vring_kick(struct virtqueue *vq);
+
+void *vring_get_buf(struct virtqueue *vq, unsigned int *len);
+
+bool vring_restart(struct virtqueue *vq);
+
+void vring_shutdown(struct virtqueue *vq);
+#endif /* _VRING_VRING_H */
===================================================================
--- /dev/null
+++ b/include/linux/virtio_ring.h
@@ -0,0 +1,96 @@
+#ifndef _LINUX_VIRTIO_RING_H
+#define _LINUX_VIRTIO_RING_H
+/* An interface for efficient virtio implementation, currently for use by KVM
+ * and lguest, but hopefully others soon. Do NOT change this since it will
+ * break existing servers and clients.
+ *
+ * This header is BSD licensed so anyone can use the definitions to implement
+ * compatible drivers/servers.
+ *
+ * Copyright Rusty Russell IBM Corporation 2007. */
+#include <linux/types.h>
+
+/* This marks a buffer as continuing via the next field. */
+#define VRING_DESC_F_NEXT 1
+/* This marks a buffer as write-only (otherwise read-only). */
+#define VRING_DESC_F_WRITE 2
+
+/* Virtio ring descriptors: 16 bytes. These can chain together via "next". */
+struct vring_desc
+{
+ /* Page number. */
+ __u64 pfn;
+ /* Length and offset. */
+ __u16 len;
+ __u16 offset;
+ /* The flags as indicated above. */
+ __u16 flags;
+ /* We chain unused descriptors via this, too */
+ __u16 next;
+};
+
+struct vring_avail
+{
+ __u16 idx;
+ __u16 ring[];
+};
+
+/* 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
+{
+ __u32 idx;
+ struct vring_used_elem ring[];
+};
+
+struct vring {
+ unsigned int num;
+
+ struct vring_desc *desc;
+
+ struct vring_avail *avail;
+
+ struct vring_used *used;
+};
+
+/* The standard layout for the ring is a continuous chunk of memory which looks
+ * like this. The used fields will be aligned to a "num+1" boundary.
+ *
+ * struct vring
+ * {
+ * // The actual descriptors (16 bytes each)
+ * struct vring_desc desc[num];
+ *
+ * // A ring of available descriptor heads with free-running index.
+ * __u16 avail_idx;
+ * __u16 available[num];
+ *
+ * // Padding so a correctly-chosen num value will cache-align used_idx.
+ * char pad[sizeof(struct vring_desc)];
+ *
+ * // A ring of used descriptor heads with free-running index.
+ * __u32 used_idx;
+ * struct vring_used_elem used[num];
+ * };
+ */
+static inline void vring_init(struct vring *vr, unsigned int num, void *p)
+{
+ vr->num = num;
+ vr->desc = p;
+ vr->avail = p + num*sizeof(struct vring);
+ vr->used = p + (num+1)*(sizeof(struct vring) + sizeof(__u16));
+}
+
+static inline unsigned vring_size(unsigned int num)
+{
+ return (num + 1) * (sizeof(struct vring_desc) + sizeof(__u16))
+ + sizeof(__u32) + num * sizeof(struct vring_used_elem);
+}
+#endif /* _LINUX_VIRTIO_RING_H */