Re: [PATCH v5 3/5] vhost/vsock: re-scan TX virtqueue on device start

[Stefano Garzarella <sgarzare@redhat.com>]

On Mon, Jul 20, 2026 at 01:22:39PM +0300, Andrey Drobyshev wrote:
>During QEMU CPR live-update (and VHOST_RESET_OWNER in general) the guest
>keeps running while the host drops and later re-attaches vhost backends.
>If the guest adds a buffer to the TX virtqueue (guest->host) and kicks
>while the backend is temporarily NULL (between vhost_vsock_drop_backends()
>and the next vhost_vsock_start()), then the kick is delivered to the
>vhost worker, handle_tx_kick() sees a NULL backend and returns, and the
>kick signal is consumed.  The buffer is then left in the ring.
>
>Then upon device start vhost_vsock_start() only re-kicks the RX send
>worker, never the TX VQ, so the buffer is processed only if the guest
>happens to kick again.  But if the guest itself is now waiting for data
>from the host, it will never kick TX VQ again, and we end up in a
>deadlock.
>
>The issue itself is pre-existing, but it only manifests during a device
>pause caused by VHOST_RESET_OWNER.  Namely, the deadlock is reproduced
>during active host->guest socat data transfer under multiple consecutive
>CPR live-update's.
>
>To fix this, in vhost_vsock_start(), after kicking the RX send worker, also
>queue the TX vq poll so any buffers the guest enqueued while we were paused
>get scanned.
>
>The VHOST_RESET_OWNER ioctl itself is implemented in the following
>patch, thus this patch is a preparation to support VHOST_RESET_OWNER.
>
>Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
>Reviewed-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
>---
> drivers/vhost/vsock.c | 7 +++++++
> 1 file changed, 7 insertions(+)

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>