Re: [PATCH v2] virtio_ring: don't update event idx on get_buf

[Michael S. Tsirkin]

On Sat, Mar 25, 2023 at 06:56:33PM +0800, Albert Huang wrote:
> in virtio_net, if we disable the napi_tx, when we triger a tx interrupt,
> the vq->event_triggered will be set to true. It will no longer be set to
> false. Unless we explicitly call virtqueue_enable_cb_delayed or
> virtqueue_enable_cb_prepare.
> 
> If we disable the napi_tx, it will only be called when the tx ring
> buffer is relatively small.
> 
> Because event_triggered is true. Therefore, VRING_AVAIL_F_NO_INTERRUPT or
> VRING_PACKED_EVENT_FLAG_DISABLE will not be set. So we update
> vring_used_event(&vq->split.vring) or vq->packed.vring.driver->off_wrap
> every time we call virtqueue_get_buf_ctx. This will bring more interruptions.
> 
> To summarize:
> 1) event_triggered was set to true in vring_interrupt()
> 2) after this nothing will happen for virtqueue_disable_cb() so
>    VRING_AVAIL_F_NO_INTERRUPT is not set in avail_flags_shadow
> 3) virtqueue_get_buf_ctx_split() will still think the cb is enabled
>    then it tries to publish new event
> 
> To fix, if event_triggered is set to true, do not update
> vring_used_event(&vq->split.vring) or vq->packed.vring.driver->off_wrap
> 
> Tested with iperf:
> iperf3 tcp stream:
> vm1 -----------------> vm2
> vm2 just receives tcp data stream from vm1, and sends the ack to vm1,
> there are many tx interrupts in vm2.
> but without event_triggered there are just a few tx interrupts.
> 
> Fixes: 8d622d21d248 ("virtio: fix up virtio_disable_cb")
> Signed-off-by: Albert Huang <huangjie.albert@xxxxxxxxxxxxx>
> Message-Id: <20230321085953.24949-1-huangjie.albert@xxxxxxxxxxxxx>
> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>

dropped for now due to breakage of 9p.
Pls post v3 with the fix you suggested (moving the check
of vq->event_triggered).

> ---
>  drivers/virtio/virtio_ring.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index cbeeea1b0439..1c36fa477966 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -914,7 +914,8 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
>  	/* If we expect an interrupt for the next entry, tell host
>  	 * by writing event index and flush out the write before
>  	 * the read in the next get_buf call. */
> -	if (!(vq->split.avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT))
> +	if (unlikely(!(vq->split.avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT) &&
> +		     !vq->event_triggered))
>  		virtio_store_mb(vq->weak_barriers,
>  				&vring_used_event(&vq->split.vring),
>  				cpu_to_virtio16(_vq->vdev, vq->last_used_idx));
> @@ -1744,7 +1745,8 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
>  	 * by writing event index and flush out the write before
>  	 * the read in the next get_buf call.
>  	 */
> -	if (vq->packed.event_flags_shadow == VRING_PACKED_EVENT_FLAG_DESC)
> +	if (unlikely(vq->packed.event_flags_shadow == VRING_PACKED_EVENT_FLAG_DESC &&
> +		     !vq->event_triggered))
>  		virtio_store_mb(vq->weak_barriers,
>  				&vq->packed.vring.driver->off_wrap,
>  				cpu_to_le16(vq->last_used_idx));
> -- 
> 2.37.0 (Apple Git-136)