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

Jason Wang
2025-09-16T02:37:50Z

On Tue, Sep 16, 2025 at 12:03 AM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> 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.
> >
> > 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>
> > ---
> > 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.
> > */
> > + vhost_tx_batch(net, nvq, sock, &msg);
>
> So why don't we do this in the "else" branch"? If we are busy polling
> then we are not enabling kicks, so is there a reason to flush?

It should be functional equivalent:

do {
if (head == vq->num) {
vhost_tx_batch();
if (unlikely(busyloop_intr)) {
vhost_poll_queue()
} else if () {
vhost_disable_notify(&net->dev, vq);
continue;
}
return;
}

vs

do {
if (head == vq->num) {
if (unlikely(busyloop_intr)) {
vhost_poll_queue()
} else if () {
vhost_tx_batch();
vhost_disable_notify(&net->dev, vq);
continue;
}
break;
}

vhost_tx_batch();
return;

Thanks

>
>
> > + 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
>