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 <[email protected]> Cc: Jason Wang <[email protected]> Cc: Michael S. Tsirkin <[email protected]> Signed-off-by: Stefan Hajnoczi <[email protected]> --- 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
