[Devel] [PATCH RHEL10 COMMIT] vhost-blk: stop fetching descriptors on VHOST_BLK_SET_BACKEND(-1)

[Konstantin Khorenko]

The commit is pushed to "branch-rh10-6.12.0-55.52.1.5.x.vz10-ovz" and will appear at git at bitbucket.org:openvz/vzkernel.git
after rh10-6.12.0-55.52.1.5.29.vz10
------>
commit 2076163d3a76a90daa433aa6bbdc1b97fb560a94
Author: Andrey Drobyshev <andrey.drobyshev at virtuozzo.com>
Date:   Fri May 29 22:35:05 2026 +0300

    vhost-blk: stop fetching descriptors on VHOST_BLK_SET_BACKEND(-1)
    
    Saving or migrating a vhost-blk guest under disk load can fail to load on
    the destination:
    
      qemu-kvm: VQ 0 size 0x100 < last_avail_idx 0xb8ab - used_idx 0xb934
      qemu-kvm: Failed to load vhost-blk:virtio
      qemu-kvm: error while loading state for instance 0x0 of device vhost-blk
      load of migration failed: Operation not permitted
    
    virtio_load() rejects the device because the saved used_idx is ahead of
    last_avail_idx, which is impossible for a coherent vring.
    
    The root cause is that vhost-blk has no "stop fetching" step before the
    device is stopped.  On stop, QEMU's vhost_dev_stop() reads last_avail_idx
    via VHOST_GET_VRING_BASE, but the vhost worker is still running: it keeps
    pulling the avail-ring backlog and completing those requests, advancing
    the guest used->idx past the last_avail_idx that was just sampled.  The
    saved state is therefore incoherent.
    
    vhost-net does not hit this because it detaches the backend
    (VHOST_NET_SET_BACKEND, fd == -1) before VHOST_GET_VRING_BASE, so its
    worker stops fetching.  vhost-blk had no equivalent operation.
    
    Teach VHOST_BLK_SET_BACKEND to treat a negative fd as "stop the device":
    detach the backend from every vq (vhost_blk_handle_guest_kick() bails on a
    NULL backend), drain in-flight requests with vhost_blk_flush(), and release
    the backing file.  After this the worker no longer advances the rings, so
    the subsequent VHOST_GET_VRING_BASE reports a final, coherent
    last_avail_idx.  The unconsumed avail backlog stays in the ring and is
    reprocessed once the device is restarted.  The companion QEMU change issues
    this stop before vhost_dev_stop().
    
    https://virtuozzo.atlassian.net/browse/VSTOR-133464
    Fixes: 40a5928ec730 ("drivers/vhost: vhost-blk accelerator for virtio-blk guests")
    Signed-off-by: Andrey Drobyshev <andrey.drobyshev at virtuozzo.com>
    Reviewed-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
    Reviewed-by: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>
    
    Feature: vhost-blk: in-kernel accelerator for virtio-blk guests
---
 drivers/vhost/blk.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/drivers/vhost/blk.c b/drivers/vhost/blk.c
index 5acdf973ac71a..c66e710ec9105 100644
--- a/drivers/vhost/blk.c
+++ b/drivers/vhost/blk.c
@@ -745,6 +745,24 @@ static long vhost_blk_set_backend(struct vhost_blk *blk, int fd)
 	if (ret)
 		goto out_dev;
 
+	/*
+	 * fd < 0 means "stop the device".  Detach the backend from every vq so
+	 * vhost_blk_handle_guest_kick() stops fetching descriptors, drain the
+	 * in-flight requests, and release the backing file.
+	 */
+	if (fd < 0) {
+		if (!blk->backend) {
+			ret = 0;		/* already stopped */
+			goto out_dev;
+		}
+		vhost_blk_drop_backends(blk);
+		vhost_blk_flush(blk);
+		fput(blk->backend);
+		blk->backend = NULL;
+		ret = 0;
+		goto out_dev;
+	}
+
 	if (blk->backend) {
 		ret = -EBUSY;
 		goto out_dev;