virtio: signal after wrapping packed used_idx

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>
Message-Id: <20211130134510.267382-1-stefanha@redhat.com>
Fixes: 86044b2 ("virtio: basic packed virtqueue support")
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

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;
     }
 }