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

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Sep 16, 2025 at 10:37:35AM +0800, Jason Wang wrote:
> 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
>

But this is not what the code comment says:

                     /* Flush batched packets before enabling
                      * virqtueue notification to reduce
                      * unnecssary virtqueue kicks.


So I ask - of we queued more polling, why do we need
to flush batched packets? We might get more in the next
polling round, this is what polling is designed to do.

 
> 
> >
> >
> > > +                     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
> >