Re: [PATCH v1 2/2] virtio-net: virtio_net_flush_tx() check for per-queue reset

[Jason Wang <jasowang@redhat.com>]

On Sun, Jan 29, 2023 at 3:44 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
>
> On Sun, 29 Jan 2023 14:23:21 +0800, Jason Wang <jasowang@redhat.com> wrote:
> > On Sun, Jan 29, 2023 at 10:52 AM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> > >
> > > Check whether it is per-queue reset state in virtio_net_flush_tx().
> > >
> > > Before per-queue reset, we need to recover async tx resources. At this
> > > time, virtio_net_flush_tx() is called, but we should not try to send
> > > new packets, so virtio_net_flush_tx() should check the current
> > > per-queue reset state.
> > >
> > > Fixes: 7dc6be52 ("virtio-net: support queue reset")
> > > Fixes: https://gitlab.com/qemu-project/qemu/-/issues/1451
> > > Reported-by: Alexander Bulekov <alxndr@bu.edu>
> > > Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > > ---
> > >  hw/net/virtio-net.c | 3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> > > index 3ae909041a..fba6451a50 100644
> > > --- a/hw/net/virtio-net.c
> > > +++ b/hw/net/virtio-net.c
> > > @@ -2627,7 +2627,8 @@ static int32_t virtio_net_flush_tx(VirtIONetQueue *q)
> > >      VirtQueueElement *elem;
> > >      int32_t num_packets = 0;
> > >      int queue_index = vq2q(virtio_get_queue_index(q->tx_vq));
> > > -    if (!(vdev->status & VIRTIO_CONFIG_S_DRIVER_OK)) {
> > > +    if (!(vdev->status & VIRTIO_CONFIG_S_DRIVER_OK) ||
> > > +        virtio_queue_reset_state(q->tx_vq)) {
> >
> > We have other places that check DRIVER_OK do we need to check queue
> > reset as well?
>
> I checked it again. I still think that the location of other checking DRIVER_OK
> does not need to check the queue reset.

For example, if we don't disable can_receive() when the queue is
reset, it means rx may go for virtio_net_receive_rcu(). It means the
Qemu is still trying to process the traffic from the network backend
like tap which may waste cpu cycles.

I think the correct way is to return false when the queue is reset in
can_receive(), then the backend poll will be disabled (e.g TAP). When
the queue is enabled again, qemu_flush_queued_packets() will wake up
the backend polling.

Having had time to check other places but it would be better to
mention why it doesn't need a check in the changelog.

Thanks

>
> Thanks.
>
>
> >
> > E.g:
> > virtio_net_can_receive()
> > virtio_net_tx_{timer|bh}()
> >
> > Thanks
> >
> > >          return num_packets;
> > >      }
> > >
> > > --
> > > 2.32.0.3.g01195cf9f
> > >
> >
>