Re: [PATCH net 2/2] vhost-net: correctly flush batched packet before enabling notification

Michael S. Tsirkin
2025-09-12T08:50:42Z

On Fri, Sep 12, 2025 at 04:26:58PM +0800, Jason Wang wrote:
> Commit 8c2e6b26ffe2 ("vhost/net: Defer TX queue re-enable until after
> sendmsg") tries to defer the notification enabling by moving the logic
> out of the loop after the vhost_tx_batch() when nothing new is
> spotted. This will bring side effects as the new logic would be reused
> for several other error conditions.
>
> One example is the IOTLB: when there's an IOTLB miss, get_tx_bufs()
> might return -EAGAIN and exit the loop and see there's still available
> buffers, so it will queue the tx work again until userspace feed the
> IOTLB entry correctly. This will slowdown the tx processing and may
> trigger the TX watchdog in the guest.

It's not that it might.
Pls clarify that it *has been reported* to do exactly that,
and add a link to the report.

> Fixing this by stick the notificaiton enabling logic inside the loop
> when nothing new is spotted and flush the batched before.
>
> Reported-by: Jon Kohler <jon@nutanix.com>
> Cc: stable@vger.kernel.org
> Fixes: 8c2e6b26ffe2 ("vhost/net: Defer TX queue re-enable until after sendmsg")
> Signed-off-by: Jason Wang <jasowang@redhat.com>

So this is mostly a revert, but with
vhost_tx_batch(net, nvq, sock, &msg);
added in to avoid regressing performance.

If you do not want to structure it like this (revert+optimization),
then pls make that clear in the message.

> ---
> drivers/vhost/net.c | 33 +++++++++++++--------------------
> 1 file changed, 13 insertions(+), 20 deletions(-)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 16e39f3ab956..3611b7537932 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -765,11 +765,11 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
> int err;
> int sent_pkts = 0;
> bool sock_can_batch = (sock->sk->sk_sndbuf == INT_MAX);
> - bool busyloop_intr;
> bool in_order = vhost_has_feature(vq, VIRTIO_F_IN_ORDER);
>
> do {
> - busyloop_intr = false;
> + bool busyloop_intr = false;
> +
> if (nvq->done_idx == VHOST_NET_BATCH)
> vhost_tx_batch(net, nvq, sock, &msg);
>
> @@ -780,10 +780,18 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
> break;
> /* Nothing new? Wait for eventfd to tell us they refilled. */
> if (head == vq->num) {
> - /* Kicks are disabled at this point, break loop and
> - * process any remaining batched packets. Queue will
> - * be re-enabled afterwards.
> + /* Flush batched packets before enabling
> + * virqtueue notification to reduce
> + * unnecssary virtqueue kicks.

typos: virtqueue, unnecessary

> */
> + vhost_tx_batch(net, nvq, sock, &msg);
> + if (unlikely(busyloop_intr)) {
> + vhost_poll_queue(&vq->poll);
> + } else if (unlikely(vhost_enable_notify(&net->dev,
> + vq))) {
> + vhost_disable_notify(&net->dev, vq);
> + continue;
> + }
> break;
> }
>
> @@ -839,22 +847,7 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
> ++nvq->done_idx;
> } while (likely(!vhost_exceeds_weight(vq, ++sent_pkts, total_len)));
>
> - /* Kicks are still disabled, dispatch any remaining batched msgs. */
> vhost_tx_batch(net, nvq, sock, &msg);
> -
> - if (unlikely(busyloop_intr))
> - /* If interrupted while doing busy polling, requeue the
> - * handler to be fair handle_rx as well as other tasks
> - * waiting on cpu.
> - */
> - vhost_poll_queue(&vq->poll);
> - else
> - /* All of our work has been completed; however, before
> - * leaving the TX handler, do one last check for work,
> - * and requeue handler if necessary. If there is no work,
> - * queue will be reenabled.
> - */
> - vhost_net_busy_poll_try_queue(net, vq);
> }
>
> static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock)
> --
> 2.34.1