Re: [PATCH] virtio: signal after wrapping packed used_idx

[Michael S. Tsirkin]

On Tue, Nov 30, 2021 at 01:45:10PM +0000, Stefan Hajnoczi wrote:
> Packed Virtqueues wrap used_idx instead of letting it run freely like
> Split Virtqueues do. If the used ring wraps more than once there is no
> way to compare vq->signalled_used and vq->used_idx in
> virtio_packed_should_notify() since they are modulo vq->vring.num.
> 
> This causes the device to stop sending used buffer notifications when
> when virtio_packed_should_notify() is called less than once each time
> around the used ring.
> 
> It is possible to trigger this with virtio-blk's dataplane
> notify_guest_bh() irq coalescing optimization. The call to
> virtio_notify_irqfd() (and virtio_packed_should_notify()) is deferred to
> a BH. If the guest driver is polling it can complete and submit more
> requests before the BH executes, causing the used ring to wrap more than
> once. The result is that the virtio-blk device ceases to raise
> interrupts and I/O hangs.
> 
> Cc: Tiwei Bie <tiwei.bie@intel.com>
> Cc: Jason Wang <jasowang@redhat.com>
> Cc: Michael S. Tsirkin <mst@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

Makes sense.  Fixes tag?

> ---
> Smarter solutions welcome, but I think notifying once per vq->vring.num
> is acceptable.
> ---
>  hw/virtio/virtio.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index ea7c079fb0..f7851c2750 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -885,6 +885,7 @@ static void virtqueue_packed_flush(VirtQueue *vq, 
> unsigned int count)
>      if (vq->used_idx >= vq->vring.num) {
>          vq->used_idx -= vq->vring.num;
>          vq->used_wrap_counter ^= 1;
> +        vq->signalled_used_valid = false;
>      }
>  }
>  
> -- 
> 2.33.1