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

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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@xxxxxxxxxxxxx>
---
 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@xxxxxxxxxx>
+ *
+ * Copyright (C) 2012 Red Hat, Inc.
+ * Author: Asias He <asias@xxxxxxxxxx>
+ *
+ * Copyright (c) 2022 Virtuozzo International GmbH.
+ * Author: Andrey Zhadchenko <andrey.zhadchenko@xxxxxxxxxxxxx>
+ *
+ * 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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