[Devel] [PATCH VZ10 2/2] vhost/vsock: re-scan TX virtqueue on device start

[Pavel Tikhomirov]

On 6/4/26 17:46, 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 deadlock is reproduced during active host->guest socat data transfer
> under multiple consecutive qemu-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.
> 
> https://virtuozzo.atlassian.net/browse/VSTOR-131956
> https://virtuozzo.atlassian.net/browse/VSTOR-101116

Fixes: e541d8e1cf515 ("vhost/vsock: add VHOST_RESET_OWNER ioctl")

right?

> 
> Signed-off-by: Andrey Drobyshev <andrey.drobyshev at virtuozzo.com>
> 
> Feature: vhost-vsock: VHOST_RESET_OWNER ioctl
> ---
>  drivers/vhost/vsock.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
> index 35f65b58108d..9aee012ababe 100644
> --- a/drivers/vhost/vsock.c
> +++ b/drivers/vhost/vsock.c
> @@ -623,6 +623,12 @@ static int vhost_vsock_start(struct vhost_vsock *vsock)
>  	 */
>  	vhost_vq_work_queue(&vsock->vqs[VSOCK_VQ_RX], &vsock->send_pkt_work);
>  
> +	/*
> +	 * Some packets might've also been queued in TX VQ.  Re-scan it here,
> +	 * mirroring the RX send-worker kick above.
> +	 */
> +	vhost_poll_queue(&vsock->vqs[VSOCK_VQ_TX].poll);
> +
>  	mutex_unlock(&vsock->dev.mutex);
>  	return 0;
>  

-- 
Best regards, Pavel Tikhomirov
Senior Software Developer, Virtuozzo.