vhost/vsock: re-scan TX virtqueue on device start

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>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Message-ID: <20260720102241.371610-4-andrey.drobyshev@virtuozzo.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

0 files changed, 0 insertions, 0 deletions