[RFC PATCH 0/1] vhost-blk: in-kernel accelerator for virtio-blk guests

[RFC PATCH 0/1] vhost-blk: in-kernel accelerator for virtio-blk guests

[Andrey Zhadchenko]

Although QEMU virtio-blk is quite fast, there is still some room for
improvements. Disk latency can be reduced if we handle virito-blk requests
in host kernel so we avoid a lot of syscalls and context switches.
The idea is quite simple - QEMU gives us block device and we translate
any incoming virtio requests into bio and push them into bdev.
The biggest disadvantage of this vhost-blk flavor is raw format.
Luckily Kirill Thai proposed device mapper driver for QCOW2 format to attach
files as block devices: https://www.spinics.net/lists/kernel/msg4292965.html

Also by using kernel modules we can bypass iothread limitation and finaly scale
block requests with cpus for high-performance devices.


There have already been several attempts to write vhost-blk:

Asias' version: https://lkml.org/lkml/2012/12/1/174
Badari's version: https://lwn.net/Articles/379864/
Vitaly's https://lwn.net/Articles/770965/

The main difference between them is API to access backend file. The fastest
one is Asias's version with bio flavor. It is also the most reviewed and
have the most features. So his module is partially based on it. Multiple
virtqueue support was addded, some places reworked.

test setup and results:
fio --direct=1 --rw=randread  --bs=4k  --ioengine=libaio --iodepth=128
QEMU drive options: cache=none
filesystem: xfs

SSD:
               | randread, IOPS  | randwrite, IOPS |
Host           |      95.8k	 |	85.3k	   |
QEMU virtio    |      57.5k	 |	79.4k	   |
QEMU vhost-blk |      95.6k	 |	84.3k	   |

RAMDISK (vq == vcpu):
                 | randread, IOPS | randwrite, IOPS |
virtio, 1vcpu    |	123k	  |	 129k       |
virtio, 2vcpu    |	253k (??) |	 250k (??)  |
virtio, 4vcpu    |	158k	  |	 154k       |
vhost-blk, 1vcpu |	110k	  |	 113k       |
vhost-blk, 2vcpu |	247k	  |	 252k       |
vhost-blk, 4vcpu |	576k	  |	 567k       |

Major features planned for the next versions:
 - DISCARD\WRITE_ZEROES support
 - multiple vhost workers

Andrey Zhadchenko (1):
  drivers/vhost: vhost-blk accelerator for virtio-blk guests

 drivers/vhost/Kconfig      |  12 +
 drivers/vhost/Makefile     |   3 +
 drivers/vhost/blk.c        | 831 +++++++++++++++++++++++++++++++++++++
 include/uapi/linux/vhost.h |   5 +
 4 files changed, 851 insertions(+)
 create mode 100644 drivers/vhost/blk.c

-- 
2.31.1

[RFC PATCH 1/1] drivers/vhost: vhost-blk accelerator for virtio-blk guests

[Andrey Zhadchenko]

Although QEMU virtio is quite fast, there is still some room for
improvements. Disk latency can be reduced if we handle virito-blk requests
in host kernel istead of passing them to QEMU. The patch adds vhost-blk
kernel module to do so.

Some test setups:
fio --direct=1 --rw=randread  --bs=4k  --ioengine=libaio --iodepth=128
QEMU drive options: cache=none
filesystem: xfs

SSD:
               | randread, IOPS  | randwrite, IOPS |
Host           |      95.8k      |      85.3k      |
QEMU virtio    |      57.5k      |      79.4k      |
QEMU vhost-blk |      95.6k      |      84.3k      |

RAMDISK (vq == vcpu):
                 | randread, IOPS | randwrite, IOPS |
virtio, 1vcpu    |      123k      |      129k       |
virtio, 2vcpu    |      253k (??) |      250k (??)  |
virtio, 4vcpu    |      158k      |      154k       |
vhost-blk, 1vcpu |      110k      |      113k       |
vhost-blk, 2vcpu |      247k      |      252k       |
vhost-blk, 4vcpu |      576k      |      567k       |

Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>
---
 drivers/vhost/Kconfig      |  12 +
 drivers/vhost/Makefile     |   3 +
 drivers/vhost/blk.c        | 831 +++++++++++++++++++++++++++++++++++++
 include/uapi/linux/vhost.h |   4 +
 4 files changed, 850 insertions(+)
 create mode 100644 drivers/vhost/blk.c

diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
index 587fbae06182..bcd21a434885 100644
--- a/drivers/vhost/Kconfig
+++ b/drivers/vhost/Kconfig
@@ -89,4 +89,16 @@ config VHOST_CROSS_ENDIAN_LEGACY
 
 	  If unsure, say "N".
 
+config VHOST_BLK
+	tristate "Host kernel accelerator for virtio-blk (EXPERIMENTAL)"
+	depends on BLOCK && EVENTFD
+	select VHOST
+	default n
+	help
+	  This kernel module can be loaded in host kernel to accelerate
+	  guest vm with virtio-blk driver.
+
+	  To compile this driver as a module, choose M here: the module will
+	  be called vhost_blk.
+
 endif
diff --git a/drivers/vhost/Makefile b/drivers/vhost/Makefile
index f3e1897cce85..c76cc4f5fcd8 100644
--- a/drivers/vhost/Makefile
+++ b/drivers/vhost/Makefile
@@ -17,3 +17,6 @@ obj-$(CONFIG_VHOST)	+= vhost.o
 
 obj-$(CONFIG_VHOST_IOTLB) += vhost_iotlb.o
 vhost_iotlb-y := iotlb.o
+
+obj-$(CONFIG_VHOST_BLK) += vhost_blk.o
+vhost_blk-y := blk.o
diff --git a/drivers/vhost/blk.c b/drivers/vhost/blk.c
new file mode 100644
index 000000000000..2c3cda47b54f
--- /dev/null
+++ b/drivers/vhost/blk.c
@@ -0,0 +1,831 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2011 Taobao, Inc.
+ * Author: Liu Yuan <tailai.ly@taobao.com>
+ *
+ * Copyright (C) 2012 Red Hat, Inc.
+ * Author: Asias He <asias@redhat.com>
+ *
+ * Copyright (c) 2022 Virtuozzo International GmbH.
+ * Author: Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>
+ *
+ * virtio-blk host kernel accelerator.
+ */
+
+#include <linux/miscdevice.h>
+#include <linux/module.h>
+#include <linux/vhost.h>
+#include <linux/virtio_blk.h>
+#include <linux/mutex.h>
+#include <linux/file.h>
+#include <linux/kthread.h>
+#include <linux/blkdev.h>
+#include <linux/llist.h>
+
+#include "vhost.h"
+
+enum {
+	VHOST_BLK_FEATURES = VHOST_FEATURES |
+			     (1ULL << VIRTIO_RING_F_INDIRECT_DESC) |
+			     (1ULL << VIRTIO_RING_F_EVENT_IDX) |
+			     (1ULL << VIRTIO_BLK_F_MQ) |
+			     (1ULL << VIRTIO_BLK_F_FLUSH),
+};
+
+/*
+ * Max number of bytes transferred before requeueing the job.
+ * Using this limit prevents one virtqueue from starving others.
+ */
+#define VHOST_DEV_WEIGHT 0x80000
+
+/*
+ * Max number of packets transferred before requeueing the job.
+ * Using this limit prevents one virtqueue from starving others with
+ * pkts.
+ */
+#define VHOST_DEV_PKT_WEIGHT 256
+
+enum {
+	VHOST_BLK_VQ = 0,
+	VHOST_BLK_VQ_MAX = 8,
+};
+
+#define VHOST_MAX_METADATA_IOV 1
+
+struct req_page_list {
+	struct page **pages;
+	int pages_nr;
+};
+
+#define NR_INLINE 16
+
+struct vhost_blk_req {
+	struct req_page_list inline_pl[NR_INLINE];
+	struct page *inline_page[NR_INLINE];
+	struct bio *inline_bio[NR_INLINE];
+	struct req_page_list *pl;
+	int during_flush;
+	bool use_inline;
+
+	struct llist_node llnode;
+
+	struct vhost_blk *blk;
+
+	struct iovec *iov;
+	int iov_nr;
+
+	struct bio **bio;
+	atomic_t bio_nr;
+
+	struct iovec status[VHOST_MAX_METADATA_IOV];
+
+	sector_t sector;
+	int bi_opf;
+	u16 head;
+	long len;
+	int bio_err;
+
+	struct vhost_blk_vq *blk_vq;
+};
+
+struct vhost_blk_vq {
+	struct vhost_virtqueue vq;
+	struct vhost_blk_req *req;
+	struct iovec iov[UIO_MAXIOV];
+	struct llist_head llhead;
+	struct vhost_work work;
+};
+
+struct vhost_blk {
+	wait_queue_head_t flush_wait;
+	struct vhost_blk_vq vqs[VHOST_BLK_VQ_MAX];
+	atomic_t req_inflight[2];
+	spinlock_t flush_lock;
+	struct vhost_dev dev;
+	int during_flush;
+	struct file *backend;
+	int index;
+};
+
+static int gen;
+
+static int move_iovec(struct iovec *from, struct iovec *to,
+		      size_t len, int iov_count_from, int iov_count_to)
+{
+	int moved_seg = 0, spent_seg = 0;
+	size_t size;
+
+	while (len && spent_seg < iov_count_from && moved_seg < iov_count_to) {
+		if (from->iov_len == 0) {
+			++from;
+			++spent_seg;
+			continue;
+		}
+		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;
+		++moved_seg;
+		++spent_seg;
+	}
+
+	return len ? -1 : moved_seg;
+}
+
+static inline int iov_num_pages(struct iovec *iov)
+{
+	return (PAGE_ALIGN((unsigned long)iov->iov_base + iov->iov_len) -
+	       ((unsigned long)iov->iov_base & PAGE_MASK)) >> PAGE_SHIFT;
+}
+
+static inline int vhost_blk_set_status(struct vhost_blk_req *req, u8 status)
+{
+	struct iov_iter iter;
+	int ret;
+
+	iov_iter_init(&iter, WRITE, req->status, ARRAY_SIZE(req->status), sizeof(status));
+	ret = copy_to_iter(&status, sizeof(status), &iter);
+	if (ret != sizeof(status)) {
+		vq_err(&req->blk_vq->vq, "Failed to write status\n");
+		return -EFAULT;
+	}
+
+	return 0;
+}
+
+static void vhost_blk_req_done(struct bio *bio)
+{
+	struct vhost_blk_req *req = bio->bi_private;
+	struct vhost_blk *blk = req->blk;
+
+	req->bio_err = blk_status_to_errno(bio->bi_status);
+
+	if (atomic_dec_and_test(&req->bio_nr)) {
+		llist_add(&req->llnode, &req->blk_vq->llhead);
+		vhost_work_queue(&blk->dev, &req->blk_vq->work);
+	}
+
+	bio_put(bio);
+}
+
+static void vhost_blk_req_umap(struct vhost_blk_req *req)
+{
+	struct req_page_list *pl;
+	int i, j;
+
+	if (req->pl) {
+		for (i = 0; i < req->iov_nr; i++) {
+			pl = &req->pl[i];
+
+			for (j = 0; j < pl->pages_nr; j++) {
+				if (!req->bi_opf)
+					set_page_dirty_lock(pl->pages[j]);
+				put_page(pl->pages[j]);
+			}
+		}
+	}
+
+	if (!req->use_inline)
+		kfree(req->pl);
+}
+
+static int vhost_blk_bio_make_simple(struct vhost_blk_req *req,
+				     struct block_device *bdev)
+{
+	struct bio *bio;
+
+	req->use_inline = true;
+	req->pl = NULL;
+	req->bio = req->inline_bio;
+
+	bio = bio_alloc(GFP_KERNEL, 1);
+	if (!bio)
+		return -ENOMEM;
+
+	bio->bi_iter.bi_sector = req->sector;
+	bio_set_dev(bio, bdev);
+	bio->bi_private = req;
+	bio->bi_end_io  = vhost_blk_req_done;
+	bio->bi_opf	= req->bi_opf;
+	req->bio[0] = bio;
+
+	atomic_set(&req->bio_nr, 1);
+
+	return 0;
+}
+
+static struct page **vhost_blk_prepare_req(struct vhost_blk_req *req,
+				 int total_pages, int iov_nr)
+{
+	int pl_len, page_len, bio_len;
+	void *buf;
+
+	req->use_inline = false;
+	pl_len = iov_nr * sizeof(req->pl[0]);
+	page_len = total_pages * sizeof(struct page *);
+	bio_len = total_pages * sizeof(struct bio *);
+
+	buf = kmalloc(pl_len + page_len + bio_len, GFP_KERNEL);
+	if (!buf)
+		return NULL;
+
+	req->pl	= buf;
+	req->bio = buf + pl_len + page_len;
+
+	return buf + pl_len;
+}
+
+static int vhost_blk_bio_make(struct vhost_blk_req *req,
+			      struct block_device *bdev)
+{
+	int pages_nr_total, i, j, ret;
+	struct iovec *iov = req->iov;
+	int iov_nr = req->iov_nr;
+	struct page **pages, *page;
+	struct bio *bio = NULL;
+	int bio_nr = 0;
+
+	if (unlikely(req->bi_opf == REQ_OP_FLUSH))
+		return vhost_blk_bio_make_simple(req, bdev);
+
+	pages_nr_total = 0;
+	for (i = 0; i < iov_nr; i++)
+		pages_nr_total += iov_num_pages(&iov[i]);
+
+	if (pages_nr_total > NR_INLINE) {
+		pages = vhost_blk_prepare_req(req, pages_nr_total, iov_nr);
+		if (!pages)
+			return -ENOMEM;
+	} else {
+		req->use_inline = true;
+		req->pl = req->inline_pl;
+		pages = req->inline_page;
+		req->bio = req->inline_bio;
+	}
+
+	req->iov_nr = 0;
+	for (i = 0; i < iov_nr; i++) {
+		int pages_nr = iov_num_pages(&iov[i]);
+		unsigned long iov_base, iov_len;
+		struct req_page_list *pl;
+
+		iov_base = (unsigned long)iov[i].iov_base;
+		iov_len  = (unsigned long)iov[i].iov_len;
+
+		ret = get_user_pages_fast(iov_base, pages_nr,
+					  !req->bi_opf, pages);
+		if (ret != pages_nr)
+			goto fail;
+
+		req->iov_nr++;
+		pl = &req->pl[i];
+		pl->pages_nr = pages_nr;
+		pl->pages = pages;
+
+		for (j = 0; j < pages_nr; j++) {
+			unsigned int off, len;
+
+			page = pages[j];
+			off = iov_base & ~PAGE_MASK;
+			len = PAGE_SIZE - off;
+			if (len > iov_len)
+				len = iov_len;
+
+			bio = bio_alloc(GFP_KERNEL, 1);
+			if (!bio)
+				goto fail;
+			bio->bi_iter.bi_sector  = req->sector;
+			bio_set_dev(bio, bdev);
+			bio->bi_private = req;
+			bio->bi_end_io  = vhost_blk_req_done;
+			bio->bi_opf	= req->bi_opf;
+			req->bio[bio_nr++] = bio;
+			bio_add_page(bio, page, len, off);
+			req->sector	+= len >> 9;
+			iov_base	+= len;
+			iov_len		-= len;
+		}
+
+		pages += pages_nr;
+	}
+	atomic_set(&req->bio_nr, bio_nr);
+	return 0;
+
+fail:
+	for (i = 0; i < bio_nr; i++)
+		bio_put(req->bio[i]);
+	vhost_blk_req_umap(req);
+	return -ENOMEM;
+}
+
+static inline void vhost_blk_bio_send(struct vhost_blk_req *req)
+{
+	struct blk_plug plug;
+	int i, bio_nr;
+
+	bio_nr = atomic_read(&req->bio_nr);
+	blk_start_plug(&plug);
+	for (i = 0; i < bio_nr; i++)
+		submit_bio(req->bio[i]);
+
+	blk_finish_plug(&plug);
+}
+
+static int vhost_blk_req_submit(struct vhost_blk_req *req, struct file *file)
+{
+
+	struct inode *inode = file->f_mapping->host;
+	struct block_device *bdev = I_BDEV(inode);
+	int ret;
+
+	ret = vhost_blk_bio_make(req, bdev);
+	if (ret < 0)
+		return ret;
+
+	vhost_blk_bio_send(req);
+
+	spin_lock(&req->blk->flush_lock);
+	req->during_flush = req->blk->during_flush;
+	atomic_inc(&req->blk->req_inflight[req->during_flush]);
+	spin_unlock(&req->blk->flush_lock);
+
+	return ret;
+}
+
+static int vhost_blk_req_handle(struct vhost_virtqueue *vq,
+				struct virtio_blk_outhdr *hdr,
+				u16 head, u16 total_iov_nr,
+				struct file *file)
+{
+	struct vhost_blk *blk = container_of(vq->dev, struct vhost_blk, dev);
+	struct vhost_blk_vq *blk_vq = container_of(vq, struct vhost_blk_vq, vq);
+	unsigned char id[VIRTIO_BLK_ID_BYTES];
+	struct vhost_blk_req *req;
+	struct iov_iter iter;
+	int ret, len;
+	u8 status;
+
+	req		= &blk_vq->req[head];
+	req->blk_vq	= blk_vq;
+	req->head	= head;
+	req->blk	= blk;
+	req->sector	= hdr->sector;
+	req->iov	= blk_vq->iov;
+
+	req->len	= iov_length(vq->iov, total_iov_nr) - sizeof(status);
+	req->iov_nr	= move_iovec(vq->iov, req->iov, req->len, total_iov_nr,
+				     ARRAY_SIZE(blk_vq->iov));
+
+	move_iovec(vq->iov, req->status, sizeof(status), total_iov_nr, ARRAY_SIZE(req->status));
+	if (req->iov_nr < 0)
+		return -EINVAL;
+
+	switch (hdr->type) {
+	case VIRTIO_BLK_T_OUT:
+		req->bi_opf = REQ_OP_WRITE;
+		ret = vhost_blk_req_submit(req, file);
+		break;
+	case VIRTIO_BLK_T_IN:
+		req->bi_opf = REQ_OP_READ;
+		ret = vhost_blk_req_submit(req, file);
+		break;
+	case VIRTIO_BLK_T_FLUSH:
+		req->bi_opf = REQ_OP_FLUSH;
+		ret = vhost_blk_req_submit(req, file);
+		break;
+	case VIRTIO_BLK_T_GET_ID:
+		ret = snprintf(id, VIRTIO_BLK_ID_BYTES,
+			       "vhost-blk%d", blk->index);
+		if (ret < 0)
+			break;
+		len = ret;
+		iov_iter_init(&iter, WRITE, req->iov, req->iov_nr, req->len);
+		ret = copy_to_iter(id, len, &iter);
+		status = ret < 0 ? VIRTIO_BLK_S_IOERR : VIRTIO_BLK_S_OK;
+		ret = vhost_blk_set_status(req, status);
+		if (ret)
+			break;
+		vhost_add_used_and_signal(&blk->dev, vq, head, len);
+		break;
+	default:
+		vq_err(vq, "Unsupported request type %d\n", hdr->type);
+		status = VIRTIO_BLK_S_UNSUPP;
+		ret = vhost_blk_set_status(req, status);
+		if (ret)
+			break;
+		vhost_add_used_and_signal(&blk->dev, vq, head, 0);
+	}
+
+	return ret;
+}
+
+static void vhost_blk_handle_guest_kick(struct vhost_work *work)
+{
+	struct virtio_blk_outhdr hdr;
+	struct vhost_blk_vq *blk_vq;
+	struct vhost_virtqueue *vq;
+	struct iovec hdr_iovec[VHOST_MAX_METADATA_IOV];
+	struct vhost_blk *blk;
+	struct iov_iter iter;
+	int in, out, ret;
+	struct file *f;
+	u16 head;
+
+	vq = container_of(work, struct vhost_virtqueue, poll.work);
+	blk = container_of(vq->dev, struct vhost_blk, dev);
+	blk_vq = container_of(vq, struct vhost_blk_vq, vq);
+
+	f = vhost_vq_get_backend(vq);
+	if (!f)
+		return;
+
+	vhost_disable_notify(&blk->dev, vq);
+	for (;;) {
+		head = vhost_get_vq_desc(vq, vq->iov,
+					 ARRAY_SIZE(vq->iov),
+					 &out, &in, NULL, NULL);
+		if (unlikely(head < 0))
+			break;
+
+		if (unlikely(head == vq->num)) {
+			if (unlikely(vhost_enable_notify(&blk->dev, vq))) {
+				vhost_disable_notify(&blk->dev, vq);
+				continue;
+			}
+			break;
+		}
+
+		ret = move_iovec(vq->iov, hdr_iovec, sizeof(hdr), in + out, ARRAY_SIZE(hdr_iovec));
+		if (ret < 0) {
+			vq_err(vq, "virtio_blk_hdr is too split!");
+			vhost_discard_vq_desc(vq, 1);
+			break;
+		}
+
+		iov_iter_init(&iter, READ, hdr_iovec, ARRAY_SIZE(hdr_iovec), sizeof(hdr));
+		ret = copy_from_iter(&hdr, sizeof(hdr), &iter);
+		if (ret != sizeof(hdr)) {
+			vq_err(vq, "Failed to get block header: read %d bytes instead of %ld!\n",
+			       ret, sizeof(hdr));
+			vhost_discard_vq_desc(vq, 1);
+			break;
+		}
+
+		if (vhost_blk_req_handle(vq, &hdr, head, out + in, f) < 0)
+			break;
+
+		if (!llist_empty(&blk_vq->llhead)) {
+			vhost_poll_queue(&vq->poll);
+			break;
+		}
+	}
+}
+
+static void vhost_blk_handle_host_kick(struct vhost_work *work)
+{
+	struct vhost_blk_vq *blk_vq;
+	struct vhost_virtqueue *vq;
+	struct vhost_blk_req *req;
+	struct llist_node *llnode;
+	struct vhost_blk *blk = NULL;
+	bool added, zero;
+	u8 status;
+	int ret;
+
+	blk_vq = container_of(work, struct vhost_blk_vq, work);
+	vq = &blk_vq->vq;
+	llnode = llist_del_all(&blk_vq->llhead);
+	added = false;
+	while (llnode) {
+		req = llist_entry(llnode, struct vhost_blk_req, llnode);
+		llnode = llist_next(llnode);
+
+		if (!blk)
+			blk = req->blk;
+
+		vhost_blk_req_umap(req);
+
+		status = req->bio_err == 0 ?  VIRTIO_BLK_S_OK : VIRTIO_BLK_S_IOERR;
+		ret = vhost_blk_set_status(req, status);
+		if (unlikely(ret))
+			continue;
+
+		vhost_add_used(vq, req->head, req->len);
+		added = true;
+
+		spin_lock(&req->blk->flush_lock);
+		zero = atomic_dec_and_test(
+				&req->blk->req_inflight[req->during_flush]);
+		if (zero && !req->during_flush)
+			wake_up(&blk->flush_wait);
+		spin_unlock(&req->blk->flush_lock);
+
+	}
+
+	if (likely(added))
+		vhost_signal(&blk->dev, vq);
+}
+
+static void vhost_blk_flush(struct vhost_blk *blk)
+{
+	int i;
+
+	spin_lock(&blk->flush_lock);
+	blk->during_flush = 1;
+	spin_unlock(&blk->flush_lock);
+
+	for (i = 0; i < VHOST_BLK_VQ_MAX; i++)
+		vhost_poll_flush(&blk->vqs[i].vq.poll);
+	vhost_work_dev_flush(&blk->dev);
+	/*
+	 * Wait until requests fired before the flush to be finished
+	 * req_inflight[0] is used to track the requests fired before the flush
+	 * req_inflight[1] is used to track the requests fired during the flush
+	 */
+	wait_event(blk->flush_wait, !atomic_read(&blk->req_inflight[0]));
+
+	spin_lock(&blk->flush_lock);
+	blk->during_flush = 0;
+	spin_unlock(&blk->flush_lock);
+}
+
+static inline void vhost_blk_drop_backends(struct vhost_blk *blk)
+{
+	struct vhost_virtqueue *vq;
+	int i;
+
+	for (i = 0; i < VHOST_BLK_VQ_MAX; i++) {
+		vq = &blk->vqs[i].vq;
+
+		mutex_lock(&vq->mutex);
+		vhost_vq_set_backend(vq, NULL);
+		mutex_unlock(&vq->mutex);
+	}
+}
+
+static int vhost_blk_open(struct inode *inode, struct file *file)
+{
+	struct vhost_blk *blk;
+	struct vhost_virtqueue **vqs;
+	int ret = 0, i = 0;
+
+	blk = kvzalloc(sizeof(*blk), GFP_KERNEL);
+	if (!blk) {
+		ret = -ENOMEM;
+		goto out;
+	}
+
+	vqs = kcalloc(VHOST_BLK_VQ_MAX, sizeof(*vqs), GFP_KERNEL);
+	if (!vqs) {
+		ret = -ENOMEM;
+		goto out_blk;
+	}
+
+	for (i = 0; i < VHOST_BLK_VQ_MAX; i++) {
+		blk->vqs[i].vq.handle_kick = vhost_blk_handle_guest_kick;
+		vqs[i] = &blk->vqs[i].vq;
+	}
+
+	blk->index = gen++;
+
+	atomic_set(&blk->req_inflight[0], 0);
+	atomic_set(&blk->req_inflight[1], 0);
+	blk->during_flush = 0;
+	spin_lock_init(&blk->flush_lock);
+	init_waitqueue_head(&blk->flush_wait);
+
+	vhost_dev_init(&blk->dev, vqs, VHOST_BLK_VQ_MAX, UIO_MAXIOV,
+		       VHOST_DEV_WEIGHT, VHOST_DEV_PKT_WEIGHT, true, NULL);
+	file->private_data = blk;
+
+	for (i = 0; i < VHOST_BLK_VQ_MAX; i++)
+		vhost_work_init(&blk->vqs[i].work, vhost_blk_handle_host_kick);
+
+	return ret;
+out_blk:
+	kvfree(blk);
+out:
+	return ret;
+}
+
+static int vhost_blk_release(struct inode *inode, struct file *f)
+{
+	struct vhost_blk *blk = f->private_data;
+	int i;
+
+	vhost_blk_drop_backends(blk);
+	vhost_blk_flush(blk);
+	vhost_dev_stop(&blk->dev);
+	if (blk->backend)
+		fput(blk->backend);
+	vhost_dev_cleanup(&blk->dev);
+	for (i = 0; i < VHOST_BLK_VQ_MAX; i++)
+		kvfree(blk->vqs[i].req);
+	kfree(blk->dev.vqs);
+	kvfree(blk);
+
+	return 0;
+}
+
+static int vhost_blk_set_features(struct vhost_blk *blk, u64 features)
+{
+	struct vhost_virtqueue *vq;
+	int i;
+
+	mutex_lock(&blk->dev.mutex);
+	if ((features & (1 << VHOST_F_LOG_ALL)) &&
+	    !vhost_log_access_ok(&blk->dev)) {
+		mutex_unlock(&blk->dev.mutex);
+		return -EFAULT;
+	}
+
+	for (i = 0; i < VHOST_BLK_VQ_MAX; i++) {
+		vq = &blk->vqs[i].vq;
+		mutex_lock(&vq->mutex);
+		vq->acked_features = features & (VHOST_BLK_FEATURES);
+		mutex_unlock(&vq->mutex);
+	}
+
+	vhost_blk_flush(blk);
+	mutex_unlock(&blk->dev.mutex);
+
+	return 0;
+}
+
+static long vhost_blk_set_backend(struct vhost_blk *blk, int fd)
+{
+	struct vhost_virtqueue *vq;
+	struct file *file, *oldfile = NULL;
+	struct inode *inode;
+	int ret, i;
+
+	mutex_lock(&blk->dev.mutex);
+	ret = vhost_dev_check_owner(&blk->dev);
+	if (ret)
+		goto out_dev;
+
+	file = fget(fd);
+	if (IS_ERR(file)) {
+		ret = PTR_ERR(file);
+		goto out_dev;
+	}
+
+	if (file == blk->backend) {
+		fput(file);
+		goto out_dev;
+	}
+
+	oldfile = blk->backend;
+	blk->backend = file;
+
+	inode = file->f_mapping->host;
+	if (!S_ISBLK(inode->i_mode)) {
+		ret = -EFAULT;
+		goto out_file;
+	}
+
+	for (i = 0; i < VHOST_BLK_VQ_MAX; i++) {
+		vq = &blk->vqs[i].vq;
+		if (!vhost_vq_access_ok(vq)) {
+			ret = -EFAULT;
+			goto out_drop;
+		}
+
+		mutex_lock(&vq->mutex);
+		vhost_vq_set_backend(vq, file);
+		ret = vhost_vq_init_access(vq);
+		mutex_unlock(&vq->mutex);
+	}
+
+	if (oldfile) {
+		vhost_blk_flush(blk);
+		fput(oldfile);
+	}
+
+	mutex_unlock(&blk->dev.mutex);
+	return 0;
+
+out_drop:
+	vhost_blk_drop_backends(blk);
+out_file:
+	fput(file);
+	blk->backend = NULL;
+out_dev:
+	mutex_unlock(&blk->dev.mutex);
+	return ret;
+}
+
+static long vhost_blk_reset_owner(struct vhost_blk *blk)
+{
+	struct vhost_iotlb *umem;
+	int err, i;
+
+	mutex_lock(&blk->dev.mutex);
+	err = vhost_dev_check_owner(&blk->dev);
+	if (err)
+		goto done;
+	umem = vhost_dev_reset_owner_prepare();
+	if (!umem) {
+		err = -ENOMEM;
+		goto done;
+	}
+	vhost_blk_drop_backends(blk);
+	if (blk->backend) {
+		fput(blk->backend);
+		blk->backend = NULL;
+	}
+	vhost_blk_flush(blk);
+	vhost_dev_stop(&blk->dev);
+	vhost_dev_reset_owner(&blk->dev, umem);
+
+	for (i = 0; i < VHOST_BLK_VQ_MAX; i++) {
+		kvfree(blk->vqs[i].req);
+		blk->vqs[i].req = NULL;
+	}
+
+done:
+	mutex_unlock(&blk->dev.mutex);
+	return err;
+}
+
+static int vhost_blk_setup(struct vhost_blk *blk, void __user *argp)
+{
+	struct vhost_vring_state s;
+
+	if (copy_from_user(&s, argp, sizeof(s)))
+		return -EFAULT;
+
+	if (blk->vqs[s.index].req)
+		return 0;
+
+	blk->vqs[s.index].req = kvmalloc(sizeof(struct vhost_blk_req) * s.num, GFP_KERNEL);
+	if (!blk->vqs[s.index].req)
+		return -ENOMEM;
+
+	return 0;
+}
+
+static long vhost_blk_ioctl(struct file *f, unsigned int ioctl,
+			    unsigned long arg)
+{
+	struct vhost_blk *blk = f->private_data;
+	void __user *argp = (void __user *)arg;
+	struct vhost_vring_file backend;
+	u64 __user *featurep = argp;
+	u64 features;
+	int ret;
+
+	switch (ioctl) {
+	case VHOST_BLK_SET_BACKEND:
+		if (copy_from_user(&backend, argp, sizeof(backend)))
+			return -EFAULT;
+		return vhost_blk_set_backend(blk, backend.fd);
+	case VHOST_GET_FEATURES:
+		features = VHOST_BLK_FEATURES;
+		if (copy_to_user(featurep, &features, sizeof(features)))
+			return -EFAULT;
+		return 0;
+	case VHOST_SET_FEATURES:
+		if (copy_from_user(&features, featurep, sizeof(features)))
+			return -EFAULT;
+		if (features & ~VHOST_BLK_FEATURES)
+			return -EOPNOTSUPP;
+		return vhost_blk_set_features(blk, features);
+	case VHOST_RESET_OWNER:
+		return vhost_blk_reset_owner(blk);
+	default:
+		mutex_lock(&blk->dev.mutex);
+		ret = vhost_dev_ioctl(&blk->dev, ioctl, argp);
+		if (ret == -ENOIOCTLCMD)
+			ret = vhost_vring_ioctl(&blk->dev, ioctl, argp);
+		if (!ret && ioctl == VHOST_SET_VRING_NUM)
+			ret = vhost_blk_setup(blk, argp);
+		vhost_blk_flush(blk);
+		mutex_unlock(&blk->dev.mutex);
+		return ret;
+	}
+}
+
+static const struct file_operations vhost_blk_fops = {
+	.owner          = THIS_MODULE,
+	.open           = vhost_blk_open,
+	.release        = vhost_blk_release,
+	.llseek		= noop_llseek,
+	.unlocked_ioctl = vhost_blk_ioctl,
+};
+
+static struct miscdevice vhost_blk_misc = {
+	MISC_DYNAMIC_MINOR,
+	"vhost-blk",
+	&vhost_blk_fops,
+};
+module_misc_device(vhost_blk_misc);
+
+MODULE_VERSION("0.0.1");
+MODULE_LICENSE("GPL v2");
+MODULE_AUTHOR("Andrey Zhadchenko");
+MODULE_DESCRIPTION("Host kernel accelerator for virtio_blk");
diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
index cab645d4a645..a54861e9633e 100644
--- a/include/uapi/linux/vhost.h
+++ b/include/uapi/linux/vhost.h
@@ -171,4 +171,8 @@
 #define VHOST_VDPA_SET_GROUP_ASID	_IOW(VHOST_VIRTIO, 0x7C, \
 					     struct vhost_vring_state)
 
+/* VHOST_BLK specific defines */
+#define VHOST_BLK_SET_BACKEND		_IOW(VHOST_VIRTIO, 0x90, \
+					     struct vhost_vring_file)
+
 #endif
-- 
2.31.1

Re: [RFC PATCH 1/1] drivers/vhost: vhost-blk accelerator for virtio-blk guests

[Michael S. Tsirkin]

On Mon, Jul 25, 2022 at 11:27:53PM +0300, Andrey Zhadchenko wrote:
> Although QEMU virtio is quite fast, there is still some room for
> improvements. Disk latency can be reduced if we handle virito-blk requests
> in host kernel istead of passing them to QEMU. The patch adds vhost-blk
> kernel module to do so.
> 
> Some test setups:
> fio --direct=1 --rw=randread  --bs=4k  --ioengine=libaio --iodepth=128
> QEMU drive options: cache=none
> filesystem: xfs
> 
> SSD:
>                | randread, IOPS  | randwrite, IOPS |
> Host           |      95.8k      |      85.3k      |
> QEMU virtio    |      57.5k      |      79.4k      |
> QEMU vhost-blk |      95.6k      |      84.3k      |
> 
> RAMDISK (vq == vcpu):
>                  | randread, IOPS | randwrite, IOPS |
> virtio, 1vcpu    |      123k      |      129k       |
> virtio, 2vcpu    |      253k (??) |      250k (??)  |
> virtio, 4vcpu    |      158k      |      154k       |
> vhost-blk, 1vcpu |      110k      |      113k       |
> vhost-blk, 2vcpu |      247k      |      252k       |
> vhost-blk, 4vcpu |      576k      |      567k       |
> 
> Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>


Sounds good to me. What this depends on is whether some userspace
will actually use it. In the past QEMU rejected support for vhost-blk,
if this time it fares better then I won't have a problem merging
the kernel bits.

> ---
>  drivers/vhost/Kconfig      |  12 +
>  drivers/vhost/Makefile     |   3 +
>  drivers/vhost/blk.c        | 831 +++++++++++++++++++++++++++++++++++++
>  include/uapi/linux/vhost.h |   4 +
>  4 files changed, 850 insertions(+)
>  create mode 100644 drivers/vhost/blk.c
> 
> diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
> index 587fbae06182..bcd21a434885 100644
> --- a/drivers/vhost/Kconfig
> +++ b/drivers/vhost/Kconfig
> @@ -89,4 +89,16 @@ config VHOST_CROSS_ENDIAN_LEGACY
>  
>  	  If unsure, say "N".
>  
> +config VHOST_BLK
> +	tristate "Host kernel accelerator for virtio-blk (EXPERIMENTAL)"
> +	depends on BLOCK && EVENTFD
> +	select VHOST
> +	default n
> +	help
> +	  This kernel module can be loaded in host kernel to accelerate
> +	  guest vm with virtio-blk driver.
> +
> +	  To compile this driver as a module, choose M here: the module will
> +	  be called vhost_blk.
> +
>  endif
> diff --git a/drivers/vhost/Makefile b/drivers/vhost/Makefile
> index f3e1897cce85..c76cc4f5fcd8 100644
> --- a/drivers/vhost/Makefile
> +++ b/drivers/vhost/Makefile
> @@ -17,3 +17,6 @@ obj-$(CONFIG_VHOST)	+= vhost.o
>  
>  obj-$(CONFIG_VHOST_IOTLB) += vhost_iotlb.o
>  vhost_iotlb-y := iotlb.o
> +
> +obj-$(CONFIG_VHOST_BLK) += vhost_blk.o
> +vhost_blk-y := blk.o
> diff --git a/drivers/vhost/blk.c b/drivers/vhost/blk.c
> new file mode 100644
> index 000000000000..2c3cda47b54f
> --- /dev/null
> +++ b/drivers/vhost/blk.c
> @@ -0,0 +1,831 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Copyright (C) 2011 Taobao, Inc.
> + * Author: Liu Yuan <tailai.ly@taobao.com>
> + *
> + * Copyright (C) 2012 Red Hat, Inc.
> + * Author: Asias He <asias@redhat.com>
> + *
> + * Copyright (c) 2022 Virtuozzo International GmbH.
> + * Author: Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>
> + *
> + * virtio-blk host kernel accelerator.
> + */
> +
> +#include <linux/miscdevice.h>
> +#include <linux/module.h>
> +#include <linux/vhost.h>
> +#include <linux/virtio_blk.h>
> +#include <linux/mutex.h>
> +#include <linux/file.h>
> +#include <linux/kthread.h>
> +#include <linux/blkdev.h>
> +#include <linux/llist.h>
> +
> +#include "vhost.h"
> +
> +enum {
> +	VHOST_BLK_FEATURES = VHOST_FEATURES |
> +			     (1ULL << VIRTIO_RING_F_INDIRECT_DESC) |
> +			     (1ULL << VIRTIO_RING_F_EVENT_IDX) |
> +			     (1ULL << VIRTIO_BLK_F_MQ) |
> +			     (1ULL << VIRTIO_BLK_F_FLUSH),
> +};
> +
> +/*
> + * Max number of bytes transferred before requeueing the job.
> + * Using this limit prevents one virtqueue from starving others.
> + */
> +#define VHOST_DEV_WEIGHT 0x80000
> +
> +/*
> + * Max number of packets transferred before requeueing the job.
> + * Using this limit prevents one virtqueue from starving others with
> + * pkts.
> + */
> +#define VHOST_DEV_PKT_WEIGHT 256
> +
> +enum {
> +	VHOST_BLK_VQ = 0,
> +	VHOST_BLK_VQ_MAX = 8,
> +};
> +
> +#define VHOST_MAX_METADATA_IOV 1
> +
> +struct req_page_list {
> +	struct page **pages;
> +	int pages_nr;
> +};
> +
> +#define NR_INLINE 16
> +
> +struct vhost_blk_req {
> +	struct req_page_list inline_pl[NR_INLINE];
> +	struct page *inline_page[NR_INLINE];
> +	struct bio *inline_bio[NR_INLINE];
> +	struct req_page_list *pl;
> +	int during_flush;
> +	bool use_inline;
> +
> +	struct llist_node llnode;
> +
> +	struct vhost_blk *blk;
> +
> +	struct iovec *iov;
> +	int iov_nr;
> +
> +	struct bio **bio;
> +	atomic_t bio_nr;
> +
> +	struct iovec status[VHOST_MAX_METADATA_IOV];
> +
> +	sector_t sector;
> +	int bi_opf;
> +	u16 head;
> +	long len;
> +	int bio_err;
> +
> +	struct vhost_blk_vq *blk_vq;
> +};
> +
> +struct vhost_blk_vq {
> +	struct vhost_virtqueue vq;
> +	struct vhost_blk_req *req;
> +	struct iovec iov[UIO_MAXIOV];
> +	struct llist_head llhead;
> +	struct vhost_work work;
> +};
> +
> +struct vhost_blk {
> +	wait_queue_head_t flush_wait;
> +	struct vhost_blk_vq vqs[VHOST_BLK_VQ_MAX];
> +	atomic_t req_inflight[2];
> +	spinlock_t flush_lock;
> +	struct vhost_dev dev;
> +	int during_flush;
> +	struct file *backend;
> +	int index;
> +};
> +
> +static int gen;
> +
> +static int move_iovec(struct iovec *from, struct iovec *to,
> +		      size_t len, int iov_count_from, int iov_count_to)
> +{
> +	int moved_seg = 0, spent_seg = 0;
> +	size_t size;
> +
> +	while (len && spent_seg < iov_count_from && moved_seg < iov_count_to) {
> +		if (from->iov_len == 0) {
> +			++from;
> +			++spent_seg;
> +			continue;
> +		}
> +		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;
> +		++moved_seg;
> +		++spent_seg;
> +	}
> +
> +	return len ? -1 : moved_seg;
> +}
> +
> +static inline int iov_num_pages(struct iovec *iov)
> +{
> +	return (PAGE_ALIGN((unsigned long)iov->iov_base + iov->iov_len) -
> +	       ((unsigned long)iov->iov_base & PAGE_MASK)) >> PAGE_SHIFT;
> +}
> +
> +static inline int vhost_blk_set_status(struct vhost_blk_req *req, u8 status)
> +{
> +	struct iov_iter iter;
> +	int ret;
> +
> +	iov_iter_init(&iter, WRITE, req->status, ARRAY_SIZE(req->status), sizeof(status));
> +	ret = copy_to_iter(&status, sizeof(status), &iter);
> +	if (ret != sizeof(status)) {
> +		vq_err(&req->blk_vq->vq, "Failed to write status\n");
> +		return -EFAULT;
> +	}
> +
> +	return 0;
> +}
> +
> +static void vhost_blk_req_done(struct bio *bio)
> +{
> +	struct vhost_blk_req *req = bio->bi_private;
> +	struct vhost_blk *blk = req->blk;
> +
> +	req->bio_err = blk_status_to_errno(bio->bi_status);
> +
> +	if (atomic_dec_and_test(&req->bio_nr)) {
> +		llist_add(&req->llnode, &req->blk_vq->llhead);
> +		vhost_work_queue(&blk->dev, &req->blk_vq->work);
> +	}
> +
> +	bio_put(bio);
> +}
> +
> +static void vhost_blk_req_umap(struct vhost_blk_req *req)
> +{
> +	struct req_page_list *pl;
> +	int i, j;
> +
> +	if (req->pl) {
> +		for (i = 0; i < req->iov_nr; i++) {
> +			pl = &req->pl[i];
> +
> +			for (j = 0; j < pl->pages_nr; j++) {
> +				if (!req->bi_opf)
> +					set_page_dirty_lock(pl->pages[j]);
> +				put_page(pl->pages[j]);
> +			}
> +		}
> +	}
> +
> +	if (!req->use_inline)
> +		kfree(req->pl);
> +}
> +
> +static int vhost_blk_bio_make_simple(struct vhost_blk_req *req,
> +				     struct block_device *bdev)
> +{
> +	struct bio *bio;
> +
> +	req->use_inline = true;
> +	req->pl = NULL;
> +	req->bio = req->inline_bio;
> +
> +	bio = bio_alloc(GFP_KERNEL, 1);
> +	if (!bio)
> +		return -ENOMEM;
> +
> +	bio->bi_iter.bi_sector = req->sector;
> +	bio_set_dev(bio, bdev);
> +	bio->bi_private = req;
> +	bio->bi_end_io  = vhost_blk_req_done;
> +	bio->bi_opf	= req->bi_opf;
> +	req->bio[0] = bio;
> +
> +	atomic_set(&req->bio_nr, 1);
> +
> +	return 0;
> +}
> +
> +static struct page **vhost_blk_prepare_req(struct vhost_blk_req *req,
> +				 int total_pages, int iov_nr)
> +{
> +	int pl_len, page_len, bio_len;
> +	void *buf;
> +
> +	req->use_inline = false;
> +	pl_len = iov_nr * sizeof(req->pl[0]);
> +	page_len = total_pages * sizeof(struct page *);
> +	bio_len = total_pages * sizeof(struct bio *);
> +
> +	buf = kmalloc(pl_len + page_len + bio_len, GFP_KERNEL);
> +	if (!buf)
> +		return NULL;
> +
> +	req->pl	= buf;
> +	req->bio = buf + pl_len + page_len;
> +
> +	return buf + pl_len;
> +}
> +
> +static int vhost_blk_bio_make(struct vhost_blk_req *req,
> +			      struct block_device *bdev)
> +{
> +	int pages_nr_total, i, j, ret;
> +	struct iovec *iov = req->iov;
> +	int iov_nr = req->iov_nr;
> +	struct page **pages, *page;
> +	struct bio *bio = NULL;
> +	int bio_nr = 0;
> +
> +	if (unlikely(req->bi_opf == REQ_OP_FLUSH))
> +		return vhost_blk_bio_make_simple(req, bdev);
> +
> +	pages_nr_total = 0;
> +	for (i = 0; i < iov_nr; i++)
> +		pages_nr_total += iov_num_pages(&iov[i]);
> +
> +	if (pages_nr_total > NR_INLINE) {
> +		pages = vhost_blk_prepare_req(req, pages_nr_total, iov_nr);
> +		if (!pages)
> +			return -ENOMEM;
> +	} else {
> +		req->use_inline = true;
> +		req->pl = req->inline_pl;
> +		pages = req->inline_page;
> +		req->bio = req->inline_bio;
> +	}
> +
> +	req->iov_nr = 0;
> +	for (i = 0; i < iov_nr; i++) {
> +		int pages_nr = iov_num_pages(&iov[i]);
> +		unsigned long iov_base, iov_len;
> +		struct req_page_list *pl;
> +
> +		iov_base = (unsigned long)iov[i].iov_base;
> +		iov_len  = (unsigned long)iov[i].iov_len;
> +
> +		ret = get_user_pages_fast(iov_base, pages_nr,
> +					  !req->bi_opf, pages);
> +		if (ret != pages_nr)
> +			goto fail;
> +
> +		req->iov_nr++;
> +		pl = &req->pl[i];
> +		pl->pages_nr = pages_nr;
> +		pl->pages = pages;
> +
> +		for (j = 0; j < pages_nr; j++) {
> +			unsigned int off, len;
> +
> +			page = pages[j];
> +			off = iov_base & ~PAGE_MASK;
> +			len = PAGE_SIZE - off;
> +			if (len > iov_len)
> +				len = iov_len;
> +
> +			bio = bio_alloc(GFP_KERNEL, 1);
> +			if (!bio)
> +				goto fail;
> +			bio->bi_iter.bi_sector  = req->sector;
> +			bio_set_dev(bio, bdev);
> +			bio->bi_private = req;
> +			bio->bi_end_io  = vhost_blk_req_done;
> +			bio->bi_opf	= req->bi_opf;
> +			req->bio[bio_nr++] = bio;
> +			bio_add_page(bio, page, len, off);
> +			req->sector	+= len >> 9;
> +			iov_base	+= len;
> +			iov_len		-= len;
> +		}
> +
> +		pages += pages_nr;
> +	}
> +	atomic_set(&req->bio_nr, bio_nr);
> +	return 0;
> +
> +fail:
> +	for (i = 0; i < bio_nr; i++)
> +		bio_put(req->bio[i]);
> +	vhost_blk_req_umap(req);
> +	return -ENOMEM;
> +}
> +
> +static inline void vhost_blk_bio_send(struct vhost_blk_req *req)
> +{
> +	struct blk_plug plug;
> +	int i, bio_nr;
> +
> +	bio_nr = atomic_read(&req->bio_nr);
> +	blk_start_plug(&plug);
> +	for (i = 0; i < bio_nr; i++)
> +		submit_bio(req->bio[i]);
> +
> +	blk_finish_plug(&plug);
> +}
> +
> +static int vhost_blk_req_submit(struct vhost_blk_req *req, struct file *file)
> +{
> +
> +	struct inode *inode = file->f_mapping->host;
> +	struct block_device *bdev = I_BDEV(inode);
> +	int ret;
> +
> +	ret = vhost_blk_bio_make(req, bdev);
> +	if (ret < 0)
> +		return ret;
> +
> +	vhost_blk_bio_send(req);
> +
> +	spin_lock(&req->blk->flush_lock);
> +	req->during_flush = req->blk->during_flush;
> +	atomic_inc(&req->blk->req_inflight[req->during_flush]);
> +	spin_unlock(&req->blk->flush_lock);
> +
> +	return ret;
> +}
> +
> +static int vhost_blk_req_handle(struct vhost_virtqueue *vq,
> +				struct virtio_blk_outhdr *hdr,
> +				u16 head, u16 total_iov_nr,
> +				struct file *file)
> +{
> +	struct vhost_blk *blk = container_of(vq->dev, struct vhost_blk, dev);
> +	struct vhost_blk_vq *blk_vq = container_of(vq, struct vhost_blk_vq, vq);
> +	unsigned char id[VIRTIO_BLK_ID_BYTES];
> +	struct vhost_blk_req *req;
> +	struct iov_iter iter;
> +	int ret, len;
> +	u8 status;
> +
> +	req		= &blk_vq->req[head];
> +	req->blk_vq	= blk_vq;
> +	req->head	= head;
> +	req->blk	= blk;
> +	req->sector	= hdr->sector;
> +	req->iov	= blk_vq->iov;
> +
> +	req->len	= iov_length(vq->iov, total_iov_nr) - sizeof(status);
> +	req->iov_nr	= move_iovec(vq->iov, req->iov, req->len, total_iov_nr,
> +				     ARRAY_SIZE(blk_vq->iov));
> +
> +	move_iovec(vq->iov, req->status, sizeof(status), total_iov_nr, ARRAY_SIZE(req->status));
> +	if (req->iov_nr < 0)
> +		return -EINVAL;
> +
> +	switch (hdr->type) {
> +	case VIRTIO_BLK_T_OUT:
> +		req->bi_opf = REQ_OP_WRITE;
> +		ret = vhost_blk_req_submit(req, file);
> +		break;
> +	case VIRTIO_BLK_T_IN:
> +		req->bi_opf = REQ_OP_READ;
> +		ret = vhost_blk_req_submit(req, file);
> +		break;
> +	case VIRTIO_BLK_T_FLUSH:
> +		req->bi_opf = REQ_OP_FLUSH;
> +		ret = vhost_blk_req_submit(req, file);
> +		break;
> +	case VIRTIO_BLK_T_GET_ID:
> +		ret = snprintf(id, VIRTIO_BLK_ID_BYTES,
> +			       "vhost-blk%d", blk->index);
> +		if (ret < 0)
> +			break;
> +		len = ret;
> +		iov_iter_init(&iter, WRITE, req->iov, req->iov_nr, req->len);
> +		ret = copy_to_iter(id, len, &iter);
> +		status = ret < 0 ? VIRTIO_BLK_S_IOERR : VIRTIO_BLK_S_OK;
> +		ret = vhost_blk_set_status(req, status);
> +		if (ret)
> +			break;
> +		vhost_add_used_and_signal(&blk->dev, vq, head, len);
> +		break;
> +	default:
> +		vq_err(vq, "Unsupported request type %d\n", hdr->type);
> +		status = VIRTIO_BLK_S_UNSUPP;
> +		ret = vhost_blk_set_status(req, status);
> +		if (ret)
> +			break;
> +		vhost_add_used_and_signal(&blk->dev, vq, head, 0);
> +	}
> +
> +	return ret;
> +}
> +
> +static void vhost_blk_handle_guest_kick(struct vhost_work *work)
> +{
> +	struct virtio_blk_outhdr hdr;
> +	struct vhost_blk_vq *blk_vq;
> +	struct vhost_virtqueue *vq;
> +	struct iovec hdr_iovec[VHOST_MAX_METADATA_IOV];
> +	struct vhost_blk *blk;
> +	struct iov_iter iter;
> +	int in, out, ret;
> +	struct file *f;
> +	u16 head;
> +
> +	vq = container_of(work, struct vhost_virtqueue, poll.work);
> +	blk = container_of(vq->dev, struct vhost_blk, dev);
> +	blk_vq = container_of(vq, struct vhost_blk_vq, vq);
> +
> +	f = vhost_vq_get_backend(vq);
> +	if (!f)
> +		return;
> +
> +	vhost_disable_notify(&blk->dev, vq);
> +	for (;;) {
> +		head = vhost_get_vq_desc(vq, vq->iov,
> +					 ARRAY_SIZE(vq->iov),
> +					 &out, &in, NULL, NULL);
> +		if (unlikely(head < 0))
> +			break;
> +
> +		if (unlikely(head == vq->num)) {
> +			if (unlikely(vhost_enable_notify(&blk->dev, vq))) {
> +				vhost_disable_notify(&blk->dev, vq);
> +				continue;
> +			}
> +			break;
> +		}
> +
> +		ret = move_iovec(vq->iov, hdr_iovec, sizeof(hdr), in + out, ARRAY_SIZE(hdr_iovec));
> +		if (ret < 0) {
> +			vq_err(vq, "virtio_blk_hdr is too split!");
> +			vhost_discard_vq_desc(vq, 1);
> +			break;
> +		}
> +
> +		iov_iter_init(&iter, READ, hdr_iovec, ARRAY_SIZE(hdr_iovec), sizeof(hdr));
> +		ret = copy_from_iter(&hdr, sizeof(hdr), &iter);
> +		if (ret != sizeof(hdr)) {
> +			vq_err(vq, "Failed to get block header: read %d bytes instead of %ld!\n",
> +			       ret, sizeof(hdr));
> +			vhost_discard_vq_desc(vq, 1);
> +			break;
> +		}
> +
> +		if (vhost_blk_req_handle(vq, &hdr, head, out + in, f) < 0)
> +			break;
> +
> +		if (!llist_empty(&blk_vq->llhead)) {
> +			vhost_poll_queue(&vq->poll);
> +			break;
> +		}
> +	}
> +}
> +
> +static void vhost_blk_handle_host_kick(struct vhost_work *work)
> +{
> +	struct vhost_blk_vq *blk_vq;
> +	struct vhost_virtqueue *vq;
> +	struct vhost_blk_req *req;
> +	struct llist_node *llnode;
> +	struct vhost_blk *blk = NULL;
> +	bool added, zero;
> +	u8 status;
> +	int ret;
> +
> +	blk_vq = container_of(work, struct vhost_blk_vq, work);
> +	vq = &blk_vq->vq;
> +	llnode = llist_del_all(&blk_vq->llhead);
> +	added = false;
> +	while (llnode) {
> +		req = llist_entry(llnode, struct vhost_blk_req, llnode);
> +		llnode = llist_next(llnode);
> +
> +		if (!blk)
> +			blk = req->blk;
> +
> +		vhost_blk_req_umap(req);
> +
> +		status = req->bio_err == 0 ?  VIRTIO_BLK_S_OK : VIRTIO_BLK_S_IOERR;
> +		ret = vhost_blk_set_status(req, status);
> +		if (unlikely(ret))
> +			continue;
> +
> +		vhost_add_used(vq, req->head, req->len);
> +		added = true;
> +
> +		spin_lock(&req->blk->flush_lock);
> +		zero = atomic_dec_and_test(
> +				&req->blk->req_inflight[req->during_flush]);
> +		if (zero && !req->during_flush)
> +			wake_up(&blk->flush_wait);
> +		spin_unlock(&req->blk->flush_lock);
> +
> +	}
> +
> +	if (likely(added))
> +		vhost_signal(&blk->dev, vq);
> +}
> +
> +static void vhost_blk_flush(struct vhost_blk *blk)
> +{
> +	int i;
> +
> +	spin_lock(&blk->flush_lock);
> +	blk->during_flush = 1;
> +	spin_unlock(&blk->flush_lock);
> +
> +	for (i = 0; i < VHOST_BLK_VQ_MAX; i++)
> +		vhost_poll_flush(&blk->vqs[i].vq.poll);
> +	vhost_work_dev_flush(&blk->dev);
> +	/*
> +	 * Wait until requests fired before the flush to be finished
> +	 * req_inflight[0] is used to track the requests fired before the flush
> +	 * req_inflight[1] is used to track the requests fired during the flush
> +	 */
> +	wait_event(blk->flush_wait, !atomic_read(&blk->req_inflight[0]));
> +
> +	spin_lock(&blk->flush_lock);
> +	blk->during_flush = 0;
> +	spin_unlock(&blk->flush_lock);
> +}
> +
> +static inline void vhost_blk_drop_backends(struct vhost_blk *blk)
> +{
> +	struct vhost_virtqueue *vq;
> +	int i;
> +
> +	for (i = 0; i < VHOST_BLK_VQ_MAX; i++) {
> +		vq = &blk->vqs[i].vq;
> +
> +		mutex_lock(&vq->mutex);
> +		vhost_vq_set_backend(vq, NULL);
> +		mutex_unlock(&vq->mutex);
> +	}
> +}
> +
> +static int vhost_blk_open(struct inode *inode, struct file *file)
> +{
> +	struct vhost_blk *blk;
> +	struct vhost_virtqueue **vqs;
> +	int ret = 0, i = 0;
> +
> +	blk = kvzalloc(sizeof(*blk), GFP_KERNEL);
> +	if (!blk) {
> +		ret = -ENOMEM;
> +		goto out;
> +	}
> +
> +	vqs = kcalloc(VHOST_BLK_VQ_MAX, sizeof(*vqs), GFP_KERNEL);
> +	if (!vqs) {
> +		ret = -ENOMEM;
> +		goto out_blk;
> +	}
> +
> +	for (i = 0; i < VHOST_BLK_VQ_MAX; i++) {
> +		blk->vqs[i].vq.handle_kick = vhost_blk_handle_guest_kick;
> +		vqs[i] = &blk->vqs[i].vq;
> +	}
> +
> +	blk->index = gen++;
> +
> +	atomic_set(&blk->req_inflight[0], 0);
> +	atomic_set(&blk->req_inflight[1], 0);
> +	blk->during_flush = 0;
> +	spin_lock_init(&blk->flush_lock);
> +	init_waitqueue_head(&blk->flush_wait);
> +
> +	vhost_dev_init(&blk->dev, vqs, VHOST_BLK_VQ_MAX, UIO_MAXIOV,
> +		       VHOST_DEV_WEIGHT, VHOST_DEV_PKT_WEIGHT, true, NULL);
> +	file->private_data = blk;
> +
> +	for (i = 0; i < VHOST_BLK_VQ_MAX; i++)
> +		vhost_work_init(&blk->vqs[i].work, vhost_blk_handle_host_kick);
> +
> +	return ret;
> +out_blk:
> +	kvfree(blk);
> +out:
> +	return ret;
> +}
> +
> +static int vhost_blk_release(struct inode *inode, struct file *f)
> +{
> +	struct vhost_blk *blk = f->private_data;
> +	int i;
> +
> +	vhost_blk_drop_backends(blk);
> +	vhost_blk_flush(blk);
> +	vhost_dev_stop(&blk->dev);
> +	if (blk->backend)
> +		fput(blk->backend);
> +	vhost_dev_cleanup(&blk->dev);
> +	for (i = 0; i < VHOST_BLK_VQ_MAX; i++)
> +		kvfree(blk->vqs[i].req);
> +	kfree(blk->dev.vqs);
> +	kvfree(blk);
> +
> +	return 0;
> +}
> +
> +static int vhost_blk_set_features(struct vhost_blk *blk, u64 features)
> +{
> +	struct vhost_virtqueue *vq;
> +	int i;
> +
> +	mutex_lock(&blk->dev.mutex);
> +	if ((features & (1 << VHOST_F_LOG_ALL)) &&
> +	    !vhost_log_access_ok(&blk->dev)) {
> +		mutex_unlock(&blk->dev.mutex);
> +		return -EFAULT;
> +	}
> +
> +	for (i = 0; i < VHOST_BLK_VQ_MAX; i++) {
> +		vq = &blk->vqs[i].vq;
> +		mutex_lock(&vq->mutex);
> +		vq->acked_features = features & (VHOST_BLK_FEATURES);
> +		mutex_unlock(&vq->mutex);
> +	}
> +
> +	vhost_blk_flush(blk);
> +	mutex_unlock(&blk->dev.mutex);
> +
> +	return 0;
> +}
> +
> +static long vhost_blk_set_backend(struct vhost_blk *blk, int fd)
> +{
> +	struct vhost_virtqueue *vq;
> +	struct file *file, *oldfile = NULL;
> +	struct inode *inode;
> +	int ret, i;
> +
> +	mutex_lock(&blk->dev.mutex);
> +	ret = vhost_dev_check_owner(&blk->dev);
> +	if (ret)
> +		goto out_dev;
> +
> +	file = fget(fd);
> +	if (IS_ERR(file)) {
> +		ret = PTR_ERR(file);
> +		goto out_dev;
> +	}
> +
> +	if (file == blk->backend) {
> +		fput(file);
> +		goto out_dev;
> +	}
> +
> +	oldfile = blk->backend;
> +	blk->backend = file;
> +
> +	inode = file->f_mapping->host;
> +	if (!S_ISBLK(inode->i_mode)) {
> +		ret = -EFAULT;
> +		goto out_file;
> +	}
> +
> +	for (i = 0; i < VHOST_BLK_VQ_MAX; i++) {
> +		vq = &blk->vqs[i].vq;
> +		if (!vhost_vq_access_ok(vq)) {
> +			ret = -EFAULT;
> +			goto out_drop;
> +		}
> +
> +		mutex_lock(&vq->mutex);
> +		vhost_vq_set_backend(vq, file);
> +		ret = vhost_vq_init_access(vq);
> +		mutex_unlock(&vq->mutex);
> +	}
> +
> +	if (oldfile) {
> +		vhost_blk_flush(blk);
> +		fput(oldfile);
> +	}
> +
> +	mutex_unlock(&blk->dev.mutex);
> +	return 0;
> +
> +out_drop:
> +	vhost_blk_drop_backends(blk);
> +out_file:
> +	fput(file);
> +	blk->backend = NULL;
> +out_dev:
> +	mutex_unlock(&blk->dev.mutex);
> +	return ret;
> +}
> +
> +static long vhost_blk_reset_owner(struct vhost_blk *blk)
> +{
> +	struct vhost_iotlb *umem;
> +	int err, i;
> +
> +	mutex_lock(&blk->dev.mutex);
> +	err = vhost_dev_check_owner(&blk->dev);
> +	if (err)
> +		goto done;
> +	umem = vhost_dev_reset_owner_prepare();
> +	if (!umem) {
> +		err = -ENOMEM;
> +		goto done;
> +	}
> +	vhost_blk_drop_backends(blk);
> +	if (blk->backend) {
> +		fput(blk->backend);
> +		blk->backend = NULL;
> +	}
> +	vhost_blk_flush(blk);
> +	vhost_dev_stop(&blk->dev);
> +	vhost_dev_reset_owner(&blk->dev, umem);
> +
> +	for (i = 0; i < VHOST_BLK_VQ_MAX; i++) {
> +		kvfree(blk->vqs[i].req);
> +		blk->vqs[i].req = NULL;
> +	}
> +
> +done:
> +	mutex_unlock(&blk->dev.mutex);
> +	return err;
> +}
> +
> +static int vhost_blk_setup(struct vhost_blk *blk, void __user *argp)
> +{
> +	struct vhost_vring_state s;
> +
> +	if (copy_from_user(&s, argp, sizeof(s)))
> +		return -EFAULT;
> +
> +	if (blk->vqs[s.index].req)
> +		return 0;
> +
> +	blk->vqs[s.index].req = kvmalloc(sizeof(struct vhost_blk_req) * s.num, GFP_KERNEL);
> +	if (!blk->vqs[s.index].req)
> +		return -ENOMEM;
> +
> +	return 0;
> +}
> +
> +static long vhost_blk_ioctl(struct file *f, unsigned int ioctl,
> +			    unsigned long arg)
> +{
> +	struct vhost_blk *blk = f->private_data;
> +	void __user *argp = (void __user *)arg;
> +	struct vhost_vring_file backend;
> +	u64 __user *featurep = argp;
> +	u64 features;
> +	int ret;
> +
> +	switch (ioctl) {
> +	case VHOST_BLK_SET_BACKEND:
> +		if (copy_from_user(&backend, argp, sizeof(backend)))
> +			return -EFAULT;
> +		return vhost_blk_set_backend(blk, backend.fd);
> +	case VHOST_GET_FEATURES:
> +		features = VHOST_BLK_FEATURES;
> +		if (copy_to_user(featurep, &features, sizeof(features)))
> +			return -EFAULT;
> +		return 0;
> +	case VHOST_SET_FEATURES:
> +		if (copy_from_user(&features, featurep, sizeof(features)))
> +			return -EFAULT;
> +		if (features & ~VHOST_BLK_FEATURES)
> +			return -EOPNOTSUPP;
> +		return vhost_blk_set_features(blk, features);
> +	case VHOST_RESET_OWNER:
> +		return vhost_blk_reset_owner(blk);
> +	default:
> +		mutex_lock(&blk->dev.mutex);
> +		ret = vhost_dev_ioctl(&blk->dev, ioctl, argp);
> +		if (ret == -ENOIOCTLCMD)
> +			ret = vhost_vring_ioctl(&blk->dev, ioctl, argp);
> +		if (!ret && ioctl == VHOST_SET_VRING_NUM)
> +			ret = vhost_blk_setup(blk, argp);
> +		vhost_blk_flush(blk);
> +		mutex_unlock(&blk->dev.mutex);
> +		return ret;
> +	}
> +}
> +
> +static const struct file_operations vhost_blk_fops = {
> +	.owner          = THIS_MODULE,
> +	.open           = vhost_blk_open,
> +	.release        = vhost_blk_release,
> +	.llseek		= noop_llseek,
> +	.unlocked_ioctl = vhost_blk_ioctl,
> +};
> +
> +static struct miscdevice vhost_blk_misc = {
> +	MISC_DYNAMIC_MINOR,
> +	"vhost-blk",
> +	&vhost_blk_fops,
> +};
> +module_misc_device(vhost_blk_misc);
> +
> +MODULE_VERSION("0.0.1");
> +MODULE_LICENSE("GPL v2");
> +MODULE_AUTHOR("Andrey Zhadchenko");
> +MODULE_DESCRIPTION("Host kernel accelerator for virtio_blk");
> diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
> index cab645d4a645..a54861e9633e 100644
> --- a/include/uapi/linux/vhost.h
> +++ b/include/uapi/linux/vhost.h
> @@ -171,4 +171,8 @@
>  #define VHOST_VDPA_SET_GROUP_ASID	_IOW(VHOST_VIRTIO, 0x7C, \
>  					     struct vhost_vring_state)
>  
> +/* VHOST_BLK specific defines */
> +#define VHOST_BLK_SET_BACKEND		_IOW(VHOST_VIRTIO, 0x90, \
> +					     struct vhost_vring_file)
> +
>  #endif
> -- 
> 2.31.1

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Re: [RFC PATCH 1/1] drivers/vhost: vhost-blk accelerator for virtio-blk guests

[Michael S. Tsirkin]

On Mon, Jul 25, 2022 at 11:27:53PM +0300, Andrey Zhadchenko wrote:
> Although QEMU virtio is quite fast, there is still some room for
> improvements. Disk latency can be reduced if we handle virito-blk requests
> in host kernel istead of passing them to QEMU. The patch adds vhost-blk
> kernel module to do so.
> 
> Some test setups:
> fio --direct=1 --rw=randread  --bs=4k  --ioengine=libaio --iodepth=128
> QEMU drive options: cache=none
> filesystem: xfs
> 
> SSD:
>                | randread, IOPS  | randwrite, IOPS |
> Host           |      95.8k      |      85.3k      |
> QEMU virtio    |      57.5k      |      79.4k      |
> QEMU vhost-blk |      95.6k      |      84.3k      |
> 
> RAMDISK (vq == vcpu):
>                  | randread, IOPS | randwrite, IOPS |
> virtio, 1vcpu    |      123k      |      129k       |
> virtio, 2vcpu    |      253k (??) |      250k (??)  |
> virtio, 4vcpu    |      158k      |      154k       |
> vhost-blk, 1vcpu |      110k      |      113k       |
> vhost-blk, 2vcpu |      247k      |      252k       |
> vhost-blk, 4vcpu |      576k      |      567k       |
> 
> Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>


Sounds good to me. What this depends on is whether some userspace
will actually use it. In the past QEMU rejected support for vhost-blk,
if this time it fares better then I won't have a problem merging
the kernel bits.

> ---
>  drivers/vhost/Kconfig      |  12 +
>  drivers/vhost/Makefile     |   3 +
>  drivers/vhost/blk.c        | 831 +++++++++++++++++++++++++++++++++++++
>  include/uapi/linux/vhost.h |   4 +
>  4 files changed, 850 insertions(+)
>  create mode 100644 drivers/vhost/blk.c
> 
> diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
> index 587fbae06182..bcd21a434885 100644
> --- a/drivers/vhost/Kconfig
> +++ b/drivers/vhost/Kconfig
> @@ -89,4 +89,16 @@ config VHOST_CROSS_ENDIAN_LEGACY
>  
>  	  If unsure, say "N".
>  
> +config VHOST_BLK
> +	tristate "Host kernel accelerator for virtio-blk (EXPERIMENTAL)"
> +	depends on BLOCK && EVENTFD
> +	select VHOST
> +	default n
> +	help
> +	  This kernel module can be loaded in host kernel to accelerate
> +	  guest vm with virtio-blk driver.
> +
> +	  To compile this driver as a module, choose M here: the module will
> +	  be called vhost_blk.
> +
>  endif
> diff --git a/drivers/vhost/Makefile b/drivers/vhost/Makefile
> index f3e1897cce85..c76cc4f5fcd8 100644
> --- a/drivers/vhost/Makefile
> +++ b/drivers/vhost/Makefile
> @@ -17,3 +17,6 @@ obj-$(CONFIG_VHOST)	+= vhost.o
>  
>  obj-$(CONFIG_VHOST_IOTLB) += vhost_iotlb.o
>  vhost_iotlb-y := iotlb.o
> +
> +obj-$(CONFIG_VHOST_BLK) += vhost_blk.o
> +vhost_blk-y := blk.o
> diff --git a/drivers/vhost/blk.c b/drivers/vhost/blk.c
> new file mode 100644
> index 000000000000..2c3cda47b54f
> --- /dev/null
> +++ b/drivers/vhost/blk.c
> @@ -0,0 +1,831 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Copyright (C) 2011 Taobao, Inc.
> + * Author: Liu Yuan <tailai.ly@taobao.com>
> + *
> + * Copyright (C) 2012 Red Hat, Inc.
> + * Author: Asias He <asias@redhat.com>
> + *
> + * Copyright (c) 2022 Virtuozzo International GmbH.
> + * Author: Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>
> + *
> + * virtio-blk host kernel accelerator.
> + */
> +
> +#include <linux/miscdevice.h>
> +#include <linux/module.h>
> +#include <linux/vhost.h>
> +#include <linux/virtio_blk.h>
> +#include <linux/mutex.h>
> +#include <linux/file.h>
> +#include <linux/kthread.h>
> +#include <linux/blkdev.h>
> +#include <linux/llist.h>
> +
> +#include "vhost.h"
> +
> +enum {
> +	VHOST_BLK_FEATURES = VHOST_FEATURES |
> +			     (1ULL << VIRTIO_RING_F_INDIRECT_DESC) |
> +			     (1ULL << VIRTIO_RING_F_EVENT_IDX) |
> +			     (1ULL << VIRTIO_BLK_F_MQ) |
> +			     (1ULL << VIRTIO_BLK_F_FLUSH),
> +};
> +
> +/*
> + * Max number of bytes transferred before requeueing the job.
> + * Using this limit prevents one virtqueue from starving others.
> + */
> +#define VHOST_DEV_WEIGHT 0x80000
> +
> +/*
> + * Max number of packets transferred before requeueing the job.
> + * Using this limit prevents one virtqueue from starving others with
> + * pkts.
> + */
> +#define VHOST_DEV_PKT_WEIGHT 256
> +
> +enum {
> +	VHOST_BLK_VQ = 0,
> +	VHOST_BLK_VQ_MAX = 8,
> +};
> +
> +#define VHOST_MAX_METADATA_IOV 1
> +
> +struct req_page_list {
> +	struct page **pages;
> +	int pages_nr;
> +};
> +
> +#define NR_INLINE 16
> +
> +struct vhost_blk_req {
> +	struct req_page_list inline_pl[NR_INLINE];
> +	struct page *inline_page[NR_INLINE];
> +	struct bio *inline_bio[NR_INLINE];
> +	struct req_page_list *pl;
> +	int during_flush;
> +	bool use_inline;
> +
> +	struct llist_node llnode;
> +
> +	struct vhost_blk *blk;
> +
> +	struct iovec *iov;
> +	int iov_nr;
> +
> +	struct bio **bio;
> +	atomic_t bio_nr;
> +
> +	struct iovec status[VHOST_MAX_METADATA_IOV];
> +
> +	sector_t sector;
> +	int bi_opf;
> +	u16 head;
> +	long len;
> +	int bio_err;
> +
> +	struct vhost_blk_vq *blk_vq;
> +};
> +
> +struct vhost_blk_vq {
> +	struct vhost_virtqueue vq;
> +	struct vhost_blk_req *req;
> +	struct iovec iov[UIO_MAXIOV];
> +	struct llist_head llhead;
> +	struct vhost_work work;
> +};
> +
> +struct vhost_blk {
> +	wait_queue_head_t flush_wait;
> +	struct vhost_blk_vq vqs[VHOST_BLK_VQ_MAX];
> +	atomic_t req_inflight[2];
> +	spinlock_t flush_lock;
> +	struct vhost_dev dev;
> +	int during_flush;
> +	struct file *backend;
> +	int index;
> +};
> +
> +static int gen;
> +
> +static int move_iovec(struct iovec *from, struct iovec *to,
> +		      size_t len, int iov_count_from, int iov_count_to)
> +{
> +	int moved_seg = 0, spent_seg = 0;
> +	size_t size;
> +
> +	while (len && spent_seg < iov_count_from && moved_seg < iov_count_to) {
> +		if (from->iov_len == 0) {
> +			++from;
> +			++spent_seg;
> +			continue;
> +		}
> +		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;
> +		++moved_seg;
> +		++spent_seg;
> +	}
> +
> +	return len ? -1 : moved_seg;
> +}
> +
> +static inline int iov_num_pages(struct iovec *iov)
> +{
> +	return (PAGE_ALIGN((unsigned long)iov->iov_base + iov->iov_len) -
> +	       ((unsigned long)iov->iov_base & PAGE_MASK)) >> PAGE_SHIFT;
> +}
> +
> +static inline int vhost_blk_set_status(struct vhost_blk_req *req, u8 status)
> +{
> +	struct iov_iter iter;
> +	int ret;
> +
> +	iov_iter_init(&iter, WRITE, req->status, ARRAY_SIZE(req->status), sizeof(status));
> +	ret = copy_to_iter(&status, sizeof(status), &iter);
> +	if (ret != sizeof(status)) {
> +		vq_err(&req->blk_vq->vq, "Failed to write status\n");
> +		return -EFAULT;
> +	}
> +
> +	return 0;
> +}
> +
> +static void vhost_blk_req_done(struct bio *bio)
> +{
> +	struct vhost_blk_req *req = bio->bi_private;
> +	struct vhost_blk *blk = req->blk;
> +
> +	req->bio_err = blk_status_to_errno(bio->bi_status);
> +
> +	if (atomic_dec_and_test(&req->bio_nr)) {
> +		llist_add(&req->llnode, &req->blk_vq->llhead);
> +		vhost_work_queue(&blk->dev, &req->blk_vq->work);
> +	}
> +
> +	bio_put(bio);
> +}
> +
> +static void vhost_blk_req_umap(struct vhost_blk_req *req)
> +{
> +	struct req_page_list *pl;
> +	int i, j;
> +
> +	if (req->pl) {
> +		for (i = 0; i < req->iov_nr; i++) {
> +			pl = &req->pl[i];
> +
> +			for (j = 0; j < pl->pages_nr; j++) {
> +				if (!req->bi_opf)
> +					set_page_dirty_lock(pl->pages[j]);
> +				put_page(pl->pages[j]);
> +			}
> +		}
> +	}
> +
> +	if (!req->use_inline)
> +		kfree(req->pl);
> +}
> +
> +static int vhost_blk_bio_make_simple(struct vhost_blk_req *req,
> +				     struct block_device *bdev)
> +{
> +	struct bio *bio;
> +
> +	req->use_inline = true;
> +	req->pl = NULL;
> +	req->bio = req->inline_bio;
> +
> +	bio = bio_alloc(GFP_KERNEL, 1);
> +	if (!bio)
> +		return -ENOMEM;
> +
> +	bio->bi_iter.bi_sector = req->sector;
> +	bio_set_dev(bio, bdev);
> +	bio->bi_private = req;
> +	bio->bi_end_io  = vhost_blk_req_done;
> +	bio->bi_opf	= req->bi_opf;
> +	req->bio[0] = bio;
> +
> +	atomic_set(&req->bio_nr, 1);
> +
> +	return 0;
> +}
> +
> +static struct page **vhost_blk_prepare_req(struct vhost_blk_req *req,
> +				 int total_pages, int iov_nr)
> +{
> +	int pl_len, page_len, bio_len;
> +	void *buf;
> +
> +	req->use_inline = false;
> +	pl_len = iov_nr * sizeof(req->pl[0]);
> +	page_len = total_pages * sizeof(struct page *);
> +	bio_len = total_pages * sizeof(struct bio *);
> +
> +	buf = kmalloc(pl_len + page_len + bio_len, GFP_KERNEL);
> +	if (!buf)
> +		return NULL;
> +
> +	req->pl	= buf;
> +	req->bio = buf + pl_len + page_len;
> +
> +	return buf + pl_len;
> +}
> +
> +static int vhost_blk_bio_make(struct vhost_blk_req *req,
> +			      struct block_device *bdev)
> +{
> +	int pages_nr_total, i, j, ret;
> +	struct iovec *iov = req->iov;
> +	int iov_nr = req->iov_nr;
> +	struct page **pages, *page;
> +	struct bio *bio = NULL;
> +	int bio_nr = 0;
> +
> +	if (unlikely(req->bi_opf == REQ_OP_FLUSH))
> +		return vhost_blk_bio_make_simple(req, bdev);
> +
> +	pages_nr_total = 0;
> +	for (i = 0; i < iov_nr; i++)
> +		pages_nr_total += iov_num_pages(&iov[i]);
> +
> +	if (pages_nr_total > NR_INLINE) {
> +		pages = vhost_blk_prepare_req(req, pages_nr_total, iov_nr);
> +		if (!pages)
> +			return -ENOMEM;
> +	} else {
> +		req->use_inline = true;
> +		req->pl = req->inline_pl;
> +		pages = req->inline_page;
> +		req->bio = req->inline_bio;
> +	}
> +
> +	req->iov_nr = 0;
> +	for (i = 0; i < iov_nr; i++) {
> +		int pages_nr = iov_num_pages(&iov[i]);
> +		unsigned long iov_base, iov_len;
> +		struct req_page_list *pl;
> +
> +		iov_base = (unsigned long)iov[i].iov_base;
> +		iov_len  = (unsigned long)iov[i].iov_len;
> +
> +		ret = get_user_pages_fast(iov_base, pages_nr,
> +					  !req->bi_opf, pages);
> +		if (ret != pages_nr)
> +			goto fail;
> +
> +		req->iov_nr++;
> +		pl = &req->pl[i];
> +		pl->pages_nr = pages_nr;
> +		pl->pages = pages;
> +
> +		for (j = 0; j < pages_nr; j++) {
> +			unsigned int off, len;
> +
> +			page = pages[j];
> +			off = iov_base & ~PAGE_MASK;
> +			len = PAGE_SIZE - off;
> +			if (len > iov_len)
> +				len = iov_len;
> +
> +			bio = bio_alloc(GFP_KERNEL, 1);
> +			if (!bio)
> +				goto fail;
> +			bio->bi_iter.bi_sector  = req->sector;
> +			bio_set_dev(bio, bdev);
> +			bio->bi_private = req;
> +			bio->bi_end_io  = vhost_blk_req_done;
> +			bio->bi_opf	= req->bi_opf;
> +			req->bio[bio_nr++] = bio;
> +			bio_add_page(bio, page, len, off);
> +			req->sector	+= len >> 9;
> +			iov_base	+= len;
> +			iov_len		-= len;
> +		}
> +
> +		pages += pages_nr;
> +	}
> +	atomic_set(&req->bio_nr, bio_nr);
> +	return 0;
> +
> +fail:
> +	for (i = 0; i < bio_nr; i++)
> +		bio_put(req->bio[i]);
> +	vhost_blk_req_umap(req);
> +	return -ENOMEM;
> +}
> +
> +static inline void vhost_blk_bio_send(struct vhost_blk_req *req)
> +{
> +	struct blk_plug plug;
> +	int i, bio_nr;
> +
> +	bio_nr = atomic_read(&req->bio_nr);
> +	blk_start_plug(&plug);
> +	for (i = 0; i < bio_nr; i++)
> +		submit_bio(req->bio[i]);
> +
> +	blk_finish_plug(&plug);
> +}
> +
> +static int vhost_blk_req_submit(struct vhost_blk_req *req, struct file *file)
> +{
> +
> +	struct inode *inode = file->f_mapping->host;
> +	struct block_device *bdev = I_BDEV(inode);
> +	int ret;
> +
> +	ret = vhost_blk_bio_make(req, bdev);
> +	if (ret < 0)
> +		return ret;
> +
> +	vhost_blk_bio_send(req);
> +
> +	spin_lock(&req->blk->flush_lock);
> +	req->during_flush = req->blk->during_flush;
> +	atomic_inc(&req->blk->req_inflight[req->during_flush]);
> +	spin_unlock(&req->blk->flush_lock);
> +
> +	return ret;
> +}
> +
> +static int vhost_blk_req_handle(struct vhost_virtqueue *vq,
> +				struct virtio_blk_outhdr *hdr,
> +				u16 head, u16 total_iov_nr,
> +				struct file *file)
> +{
> +	struct vhost_blk *blk = container_of(vq->dev, struct vhost_blk, dev);
> +	struct vhost_blk_vq *blk_vq = container_of(vq, struct vhost_blk_vq, vq);
> +	unsigned char id[VIRTIO_BLK_ID_BYTES];
> +	struct vhost_blk_req *req;
> +	struct iov_iter iter;
> +	int ret, len;
> +	u8 status;
> +
> +	req		= &blk_vq->req[head];
> +	req->blk_vq	= blk_vq;
> +	req->head	= head;
> +	req->blk	= blk;
> +	req->sector	= hdr->sector;
> +	req->iov	= blk_vq->iov;
> +
> +	req->len	= iov_length(vq->iov, total_iov_nr) - sizeof(status);
> +	req->iov_nr	= move_iovec(vq->iov, req->iov, req->len, total_iov_nr,
> +				     ARRAY_SIZE(blk_vq->iov));
> +
> +	move_iovec(vq->iov, req->status, sizeof(status), total_iov_nr, ARRAY_SIZE(req->status));
> +	if (req->iov_nr < 0)
> +		return -EINVAL;
> +
> +	switch (hdr->type) {
> +	case VIRTIO_BLK_T_OUT:
> +		req->bi_opf = REQ_OP_WRITE;
> +		ret = vhost_blk_req_submit(req, file);
> +		break;
> +	case VIRTIO_BLK_T_IN:
> +		req->bi_opf = REQ_OP_READ;
> +		ret = vhost_blk_req_submit(req, file);
> +		break;
> +	case VIRTIO_BLK_T_FLUSH:
> +		req->bi_opf = REQ_OP_FLUSH;
> +		ret = vhost_blk_req_submit(req, file);
> +		break;
> +	case VIRTIO_BLK_T_GET_ID:
> +		ret = snprintf(id, VIRTIO_BLK_ID_BYTES,
> +			       "vhost-blk%d", blk->index);
> +		if (ret < 0)
> +			break;
> +		len = ret;
> +		iov_iter_init(&iter, WRITE, req->iov, req->iov_nr, req->len);
> +		ret = copy_to_iter(id, len, &iter);
> +		status = ret < 0 ? VIRTIO_BLK_S_IOERR : VIRTIO_BLK_S_OK;
> +		ret = vhost_blk_set_status(req, status);
> +		if (ret)
> +			break;
> +		vhost_add_used_and_signal(&blk->dev, vq, head, len);
> +		break;
> +	default:
> +		vq_err(vq, "Unsupported request type %d\n", hdr->type);
> +		status = VIRTIO_BLK_S_UNSUPP;
> +		ret = vhost_blk_set_status(req, status);
> +		if (ret)
> +			break;
> +		vhost_add_used_and_signal(&blk->dev, vq, head, 0);
> +	}
> +
> +	return ret;
> +}
> +
> +static void vhost_blk_handle_guest_kick(struct vhost_work *work)
> +{
> +	struct virtio_blk_outhdr hdr;
> +	struct vhost_blk_vq *blk_vq;
> +	struct vhost_virtqueue *vq;
> +	struct iovec hdr_iovec[VHOST_MAX_METADATA_IOV];
> +	struct vhost_blk *blk;
> +	struct iov_iter iter;
> +	int in, out, ret;
> +	struct file *f;
> +	u16 head;
> +
> +	vq = container_of(work, struct vhost_virtqueue, poll.work);
> +	blk = container_of(vq->dev, struct vhost_blk, dev);
> +	blk_vq = container_of(vq, struct vhost_blk_vq, vq);
> +
> +	f = vhost_vq_get_backend(vq);
> +	if (!f)
> +		return;
> +
> +	vhost_disable_notify(&blk->dev, vq);
> +	for (;;) {
> +		head = vhost_get_vq_desc(vq, vq->iov,
> +					 ARRAY_SIZE(vq->iov),
> +					 &out, &in, NULL, NULL);
> +		if (unlikely(head < 0))
> +			break;
> +
> +		if (unlikely(head == vq->num)) {
> +			if (unlikely(vhost_enable_notify(&blk->dev, vq))) {
> +				vhost_disable_notify(&blk->dev, vq);
> +				continue;
> +			}
> +			break;
> +		}
> +
> +		ret = move_iovec(vq->iov, hdr_iovec, sizeof(hdr), in + out, ARRAY_SIZE(hdr_iovec));
> +		if (ret < 0) {
> +			vq_err(vq, "virtio_blk_hdr is too split!");
> +			vhost_discard_vq_desc(vq, 1);
> +			break;
> +		}
> +
> +		iov_iter_init(&iter, READ, hdr_iovec, ARRAY_SIZE(hdr_iovec), sizeof(hdr));
> +		ret = copy_from_iter(&hdr, sizeof(hdr), &iter);
> +		if (ret != sizeof(hdr)) {
> +			vq_err(vq, "Failed to get block header: read %d bytes instead of %ld!\n",
> +			       ret, sizeof(hdr));
> +			vhost_discard_vq_desc(vq, 1);
> +			break;
> +		}
> +
> +		if (vhost_blk_req_handle(vq, &hdr, head, out + in, f) < 0)
> +			break;
> +
> +		if (!llist_empty(&blk_vq->llhead)) {
> +			vhost_poll_queue(&vq->poll);
> +			break;
> +		}
> +	}
> +}
> +
> +static void vhost_blk_handle_host_kick(struct vhost_work *work)
> +{
> +	struct vhost_blk_vq *blk_vq;
> +	struct vhost_virtqueue *vq;
> +	struct vhost_blk_req *req;
> +	struct llist_node *llnode;
> +	struct vhost_blk *blk = NULL;
> +	bool added, zero;
> +	u8 status;
> +	int ret;
> +
> +	blk_vq = container_of(work, struct vhost_blk_vq, work);
> +	vq = &blk_vq->vq;
> +	llnode = llist_del_all(&blk_vq->llhead);
> +	added = false;
> +	while (llnode) {
> +		req = llist_entry(llnode, struct vhost_blk_req, llnode);
> +		llnode = llist_next(llnode);
> +
> +		if (!blk)
> +			blk = req->blk;
> +
> +		vhost_blk_req_umap(req);
> +
> +		status = req->bio_err == 0 ?  VIRTIO_BLK_S_OK : VIRTIO_BLK_S_IOERR;
> +		ret = vhost_blk_set_status(req, status);
> +		if (unlikely(ret))
> +			continue;
> +
> +		vhost_add_used(vq, req->head, req->len);
> +		added = true;
> +
> +		spin_lock(&req->blk->flush_lock);
> +		zero = atomic_dec_and_test(
> +				&req->blk->req_inflight[req->during_flush]);
> +		if (zero && !req->during_flush)
> +			wake_up(&blk->flush_wait);
> +		spin_unlock(&req->blk->flush_lock);
> +
> +	}
> +
> +	if (likely(added))
> +		vhost_signal(&blk->dev, vq);
> +}
> +
> +static void vhost_blk_flush(struct vhost_blk *blk)
> +{
> +	int i;
> +
> +	spin_lock(&blk->flush_lock);
> +	blk->during_flush = 1;
> +	spin_unlock(&blk->flush_lock);
> +
> +	for (i = 0; i < VHOST_BLK_VQ_MAX; i++)
> +		vhost_poll_flush(&blk->vqs[i].vq.poll);
> +	vhost_work_dev_flush(&blk->dev);
> +	/*
> +	 * Wait until requests fired before the flush to be finished
> +	 * req_inflight[0] is used to track the requests fired before the flush
> +	 * req_inflight[1] is used to track the requests fired during the flush
> +	 */
> +	wait_event(blk->flush_wait, !atomic_read(&blk->req_inflight[0]));
> +
> +	spin_lock(&blk->flush_lock);
> +	blk->during_flush = 0;
> +	spin_unlock(&blk->flush_lock);
> +}
> +
> +static inline void vhost_blk_drop_backends(struct vhost_blk *blk)
> +{
> +	struct vhost_virtqueue *vq;
> +	int i;
> +
> +	for (i = 0; i < VHOST_BLK_VQ_MAX; i++) {
> +		vq = &blk->vqs[i].vq;
> +
> +		mutex_lock(&vq->mutex);
> +		vhost_vq_set_backend(vq, NULL);
> +		mutex_unlock(&vq->mutex);
> +	}
> +}
> +
> +static int vhost_blk_open(struct inode *inode, struct file *file)
> +{
> +	struct vhost_blk *blk;
> +	struct vhost_virtqueue **vqs;
> +	int ret = 0, i = 0;
> +
> +	blk = kvzalloc(sizeof(*blk), GFP_KERNEL);
> +	if (!blk) {
> +		ret = -ENOMEM;
> +		goto out;
> +	}
> +
> +	vqs = kcalloc(VHOST_BLK_VQ_MAX, sizeof(*vqs), GFP_KERNEL);
> +	if (!vqs) {
> +		ret = -ENOMEM;
> +		goto out_blk;
> +	}
> +
> +	for (i = 0; i < VHOST_BLK_VQ_MAX; i++) {
> +		blk->vqs[i].vq.handle_kick = vhost_blk_handle_guest_kick;
> +		vqs[i] = &blk->vqs[i].vq;
> +	}
> +
> +	blk->index = gen++;
> +
> +	atomic_set(&blk->req_inflight[0], 0);
> +	atomic_set(&blk->req_inflight[1], 0);
> +	blk->during_flush = 0;
> +	spin_lock_init(&blk->flush_lock);
> +	init_waitqueue_head(&blk->flush_wait);
> +
> +	vhost_dev_init(&blk->dev, vqs, VHOST_BLK_VQ_MAX, UIO_MAXIOV,
> +		       VHOST_DEV_WEIGHT, VHOST_DEV_PKT_WEIGHT, true, NULL);
> +	file->private_data = blk;
> +
> +	for (i = 0; i < VHOST_BLK_VQ_MAX; i++)
> +		vhost_work_init(&blk->vqs[i].work, vhost_blk_handle_host_kick);
> +
> +	return ret;
> +out_blk:
> +	kvfree(blk);
> +out:
> +	return ret;
> +}
> +
> +static int vhost_blk_release(struct inode *inode, struct file *f)
> +{
> +	struct vhost_blk *blk = f->private_data;
> +	int i;
> +
> +	vhost_blk_drop_backends(blk);
> +	vhost_blk_flush(blk);
> +	vhost_dev_stop(&blk->dev);
> +	if (blk->backend)
> +		fput(blk->backend);
> +	vhost_dev_cleanup(&blk->dev);
> +	for (i = 0; i < VHOST_BLK_VQ_MAX; i++)
> +		kvfree(blk->vqs[i].req);
> +	kfree(blk->dev.vqs);
> +	kvfree(blk);
> +
> +	return 0;
> +}
> +
> +static int vhost_blk_set_features(struct vhost_blk *blk, u64 features)
> +{
> +	struct vhost_virtqueue *vq;
> +	int i;
> +
> +	mutex_lock(&blk->dev.mutex);
> +	if ((features & (1 << VHOST_F_LOG_ALL)) &&
> +	    !vhost_log_access_ok(&blk->dev)) {
> +		mutex_unlock(&blk->dev.mutex);
> +		return -EFAULT;
> +	}
> +
> +	for (i = 0; i < VHOST_BLK_VQ_MAX; i++) {
> +		vq = &blk->vqs[i].vq;
> +		mutex_lock(&vq->mutex);
> +		vq->acked_features = features & (VHOST_BLK_FEATURES);
> +		mutex_unlock(&vq->mutex);
> +	}
> +
> +	vhost_blk_flush(blk);
> +	mutex_unlock(&blk->dev.mutex);
> +
> +	return 0;
> +}
> +
> +static long vhost_blk_set_backend(struct vhost_blk *blk, int fd)
> +{
> +	struct vhost_virtqueue *vq;
> +	struct file *file, *oldfile = NULL;
> +	struct inode *inode;
> +	int ret, i;
> +
> +	mutex_lock(&blk->dev.mutex);
> +	ret = vhost_dev_check_owner(&blk->dev);
> +	if (ret)
> +		goto out_dev;
> +
> +	file = fget(fd);
> +	if (IS_ERR(file)) {
> +		ret = PTR_ERR(file);
> +		goto out_dev;
> +	}
> +
> +	if (file == blk->backend) {
> +		fput(file);
> +		goto out_dev;
> +	}
> +
> +	oldfile = blk->backend;
> +	blk->backend = file;
> +
> +	inode = file->f_mapping->host;
> +	if (!S_ISBLK(inode->i_mode)) {
> +		ret = -EFAULT;
> +		goto out_file;
> +	}
> +
> +	for (i = 0; i < VHOST_BLK_VQ_MAX; i++) {
> +		vq = &blk->vqs[i].vq;
> +		if (!vhost_vq_access_ok(vq)) {
> +			ret = -EFAULT;
> +			goto out_drop;
> +		}
> +
> +		mutex_lock(&vq->mutex);
> +		vhost_vq_set_backend(vq, file);
> +		ret = vhost_vq_init_access(vq);
> +		mutex_unlock(&vq->mutex);
> +	}
> +
> +	if (oldfile) {
> +		vhost_blk_flush(blk);
> +		fput(oldfile);
> +	}
> +
> +	mutex_unlock(&blk->dev.mutex);
> +	return 0;
> +
> +out_drop:
> +	vhost_blk_drop_backends(blk);
> +out_file:
> +	fput(file);
> +	blk->backend = NULL;
> +out_dev:
> +	mutex_unlock(&blk->dev.mutex);
> +	return ret;
> +}
> +
> +static long vhost_blk_reset_owner(struct vhost_blk *blk)
> +{
> +	struct vhost_iotlb *umem;
> +	int err, i;
> +
> +	mutex_lock(&blk->dev.mutex);
> +	err = vhost_dev_check_owner(&blk->dev);
> +	if (err)
> +		goto done;
> +	umem = vhost_dev_reset_owner_prepare();
> +	if (!umem) {
> +		err = -ENOMEM;
> +		goto done;
> +	}
> +	vhost_blk_drop_backends(blk);
> +	if (blk->backend) {
> +		fput(blk->backend);
> +		blk->backend = NULL;
> +	}
> +	vhost_blk_flush(blk);
> +	vhost_dev_stop(&blk->dev);
> +	vhost_dev_reset_owner(&blk->dev, umem);
> +
> +	for (i = 0; i < VHOST_BLK_VQ_MAX; i++) {
> +		kvfree(blk->vqs[i].req);
> +		blk->vqs[i].req = NULL;
> +	}
> +
> +done:
> +	mutex_unlock(&blk->dev.mutex);
> +	return err;
> +}
> +
> +static int vhost_blk_setup(struct vhost_blk *blk, void __user *argp)
> +{
> +	struct vhost_vring_state s;
> +
> +	if (copy_from_user(&s, argp, sizeof(s)))
> +		return -EFAULT;
> +
> +	if (blk->vqs[s.index].req)
> +		return 0;
> +
> +	blk->vqs[s.index].req = kvmalloc(sizeof(struct vhost_blk_req) * s.num, GFP_KERNEL);
> +	if (!blk->vqs[s.index].req)
> +		return -ENOMEM;
> +
> +	return 0;
> +}
> +
> +static long vhost_blk_ioctl(struct file *f, unsigned int ioctl,
> +			    unsigned long arg)
> +{
> +	struct vhost_blk *blk = f->private_data;
> +	void __user *argp = (void __user *)arg;
> +	struct vhost_vring_file backend;
> +	u64 __user *featurep = argp;
> +	u64 features;
> +	int ret;
> +
> +	switch (ioctl) {
> +	case VHOST_BLK_SET_BACKEND:
> +		if (copy_from_user(&backend, argp, sizeof(backend)))
> +			return -EFAULT;
> +		return vhost_blk_set_backend(blk, backend.fd);
> +	case VHOST_GET_FEATURES:
> +		features = VHOST_BLK_FEATURES;
> +		if (copy_to_user(featurep, &features, sizeof(features)))
> +			return -EFAULT;
> +		return 0;
> +	case VHOST_SET_FEATURES:
> +		if (copy_from_user(&features, featurep, sizeof(features)))
> +			return -EFAULT;
> +		if (features & ~VHOST_BLK_FEATURES)
> +			return -EOPNOTSUPP;
> +		return vhost_blk_set_features(blk, features);
> +	case VHOST_RESET_OWNER:
> +		return vhost_blk_reset_owner(blk);
> +	default:
> +		mutex_lock(&blk->dev.mutex);
> +		ret = vhost_dev_ioctl(&blk->dev, ioctl, argp);
> +		if (ret == -ENOIOCTLCMD)
> +			ret = vhost_vring_ioctl(&blk->dev, ioctl, argp);
> +		if (!ret && ioctl == VHOST_SET_VRING_NUM)
> +			ret = vhost_blk_setup(blk, argp);
> +		vhost_blk_flush(blk);
> +		mutex_unlock(&blk->dev.mutex);
> +		return ret;
> +	}
> +}
> +
> +static const struct file_operations vhost_blk_fops = {
> +	.owner          = THIS_MODULE,
> +	.open           = vhost_blk_open,
> +	.release        = vhost_blk_release,
> +	.llseek		= noop_llseek,
> +	.unlocked_ioctl = vhost_blk_ioctl,
> +};
> +
> +static struct miscdevice vhost_blk_misc = {
> +	MISC_DYNAMIC_MINOR,
> +	"vhost-blk",
> +	&vhost_blk_fops,
> +};
> +module_misc_device(vhost_blk_misc);
> +
> +MODULE_VERSION("0.0.1");
> +MODULE_LICENSE("GPL v2");
> +MODULE_AUTHOR("Andrey Zhadchenko");
> +MODULE_DESCRIPTION("Host kernel accelerator for virtio_blk");
> diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
> index cab645d4a645..a54861e9633e 100644
> --- a/include/uapi/linux/vhost.h
> +++ b/include/uapi/linux/vhost.h
> @@ -171,4 +171,8 @@
>  #define VHOST_VDPA_SET_GROUP_ASID	_IOW(VHOST_VIRTIO, 0x7C, \
>  					     struct vhost_vring_state)
>  
> +/* VHOST_BLK specific defines */
> +#define VHOST_BLK_SET_BACKEND		_IOW(VHOST_VIRTIO, 0x90, \
> +					     struct vhost_vring_file)
> +
>  #endif
> -- 
> 2.31.1

Re: [RFC PATCH 1/1] drivers/vhost: vhost-blk accelerator for virtio-blk guests

[Denis V. Lunev via Virtualization]

On 26.07.2022 16:45, Michael S. Tsirkin wrote:
> On Mon, Jul 25, 2022 at 11:27:53PM +0300, Andrey Zhadchenko wrote:
>> Although QEMU virtio is quite fast, there is still some room for
>> improvements. Disk latency can be reduced if we handle virito-blk requests
>> in host kernel istead of passing them to QEMU. The patch adds vhost-blk
>> kernel module to do so.
>>
>> Some test setups:
>> fio --direct=1 --rw=randread  --bs=4k  --ioengine=libaio --iodepth=128
>> QEMU drive options: cache=none
>> filesystem: xfs
>>
>> SSD:
>>                 | randread, IOPS  | randwrite, IOPS |
>> Host           |      95.8k      |      85.3k      |
>> QEMU virtio    |      57.5k      |      79.4k      |
>> QEMU vhost-blk |      95.6k      |      84.3k      |
>>
>> RAMDISK (vq == vcpu):
>>                   | randread, IOPS | randwrite, IOPS |
>> virtio, 1vcpu    |      123k      |      129k       |
>> virtio, 2vcpu    |      253k (??) |      250k (??)  |
>> virtio, 4vcpu    |      158k      |      154k       |
>> vhost-blk, 1vcpu |      110k      |      113k       |
>> vhost-blk, 2vcpu |      247k      |      252k       |
>> vhost-blk, 4vcpu |      576k      |      567k       |
>>
>> Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>
>
> Sounds good to me. What this depends on is whether some userspace
> will actually use it. In the past QEMU rejected support for vhost-blk,
> if this time it fares better then I won't have a problem merging
> the kernel bits.

In general, we are going to use this in production for our
next generation product and thus we would be very glad
to see both parts, i.e. in-kernel and in-QEMU to be merged
to reduce our support costs.

I think that numbers are talking on themselves and this
would be quite beneficial for all parties especially keeping
in mind that QCOW2 is also now supported for this kind
of IO engine.

Den
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: [RFC PATCH 1/1] drivers/vhost: vhost-blk accelerator for virtio-blk guests

[Denis V. Lunev]

On 26.07.2022 16:45, Michael S. Tsirkin wrote:
> On Mon, Jul 25, 2022 at 11:27:53PM +0300, Andrey Zhadchenko wrote:
>> Although QEMU virtio is quite fast, there is still some room for
>> improvements. Disk latency can be reduced if we handle virito-blk requests
>> in host kernel istead of passing them to QEMU. The patch adds vhost-blk
>> kernel module to do so.
>>
>> Some test setups:
>> fio --direct=1 --rw=randread  --bs=4k  --ioengine=libaio --iodepth=128
>> QEMU drive options: cache=none
>> filesystem: xfs
>>
>> SSD:
>>                 | randread, IOPS  | randwrite, IOPS |
>> Host           |      95.8k      |      85.3k      |
>> QEMU virtio    |      57.5k      |      79.4k      |
>> QEMU vhost-blk |      95.6k      |      84.3k      |
>>
>> RAMDISK (vq == vcpu):
>>                   | randread, IOPS | randwrite, IOPS |
>> virtio, 1vcpu    |      123k      |      129k       |
>> virtio, 2vcpu    |      253k (??) |      250k (??)  |
>> virtio, 4vcpu    |      158k      |      154k       |
>> vhost-blk, 1vcpu |      110k      |      113k       |
>> vhost-blk, 2vcpu |      247k      |      252k       |
>> vhost-blk, 4vcpu |      576k      |      567k       |
>>
>> Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>
>
> Sounds good to me. What this depends on is whether some userspace
> will actually use it. In the past QEMU rejected support for vhost-blk,
> if this time it fares better then I won't have a problem merging
> the kernel bits.

In general, we are going to use this in production for our
next generation product and thus we would be very glad
to see both parts, i.e. in-kernel and in-QEMU to be merged
to reduce our support costs.

I think that numbers are talking on themselves and this
would be quite beneficial for all parties especially keeping
in mind that QCOW2 is also now supported for this kind
of IO engine.

Den

Re: [RFC PATCH 0/1] vhost-blk: in-kernel accelerator for virtio-blk guests

[Andrey Zhadchenko]

Corresponding userspace qemu vhost-blk code:
https://lists.nongnu.org/archive/html/qemu-block/2022-07/msg00629.html

On 7/25/22 23:27, Andrey Zhadchenko wrote:
> Although QEMU virtio-blk is quite fast, there is still some room for
> improvements. Disk latency can be reduced if we handle virito-blk requests
> in host kernel so we avoid a lot of syscalls and context switches.
> The idea is quite simple - QEMU gives us block device and we translate
> any incoming virtio requests into bio and push them into bdev.
> The biggest disadvantage of this vhost-blk flavor is raw format.
> Luckily Kirill Thai proposed device mapper driver for QCOW2 format to attach
> files as block devices: https://www.spinics.net/lists/kernel/msg4292965.html
> 
> Also by using kernel modules we can bypass iothread limitation and finaly scale
> block requests with cpus for high-performance devices.
> 
> 
> There have already been several attempts to write vhost-blk:
> 
> Asias' version: https://lkml.org/lkml/2012/12/1/174
> Badari's version: https://lwn.net/Articles/379864/
> Vitaly's https://lwn.net/Articles/770965/
> 
> The main difference between them is API to access backend file. The fastest
> one is Asias's version with bio flavor. It is also the most reviewed and
> have the most features. So his module is partially based on it. Multiple
                          "So this module..."
> virtqueue support was addded, some places reworked.
> 
> test setup and results:
> fio --direct=1 --rw=randread  --bs=4k  --ioengine=libaio --iodepth=128
> QEMU drive options: cache=none
> filesystem: xfs
> 
> SSD:
>                 | randread, IOPS  | randwrite, IOPS |
> Host           |      95.8k	 |	85.3k	   |
> QEMU virtio    |      57.5k	 |	79.4k	   |
> QEMU vhost-blk |      95.6k	 |	84.3k	   |
> 
> RAMDISK (vq == vcpu):
>                   | randread, IOPS | randwrite, IOPS |
> virtio, 1vcpu    |	123k	  |	 129k       |
> virtio, 2vcpu    |	253k (??) |	 250k (??)  |
> virtio, 4vcpu    |	158k	  |	 154k       |
> vhost-blk, 1vcpu |	110k	  |	 113k       |
> vhost-blk, 2vcpu |	247k	  |	 252k       |
> vhost-blk, 4vcpu |	576k	  |	 567k       |
> 
> Major features planned for the next versions:
>   - DISCARD\WRITE_ZEROES support
>   - multiple vhost workers
> 
> Andrey Zhadchenko (1):
>    drivers/vhost: vhost-blk accelerator for virtio-blk guests
> 
>   drivers/vhost/Kconfig      |  12 +
>   drivers/vhost/Makefile     |   3 +
>   drivers/vhost/blk.c        | 831 +++++++++++++++++++++++++++++++++++++
>   include/uapi/linux/vhost.h |   5 +
>   4 files changed, 851 insertions(+)
>   create mode 100644 drivers/vhost/blk.c
>