netdev - Re: [RFC PATCH 4/4] virtio-net: sleep instead of busy waiting for cvq command

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Date:   Fri, 23 Dec 2022 11:03:59 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Eugenio Perez Martin <eperezma@...hat.com>
Cc:     mst@...hat.com, davem@...emloft.net, edumazet@...gle.com,
        kuba@...nel.org, pabeni@...hat.com,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, maxime.coquelin@...hat.com,
        alvaro.karsz@...id-run.com
Subject: Re: [RFC PATCH 4/4] virtio-net: sleep instead of busy waiting for cvq command

On Thu, Dec 22, 2022 at 5:19 PM Eugenio Perez Martin
<eperezma@...hat.com> wrote:
>
> On Thu, Dec 22, 2022 at 7:05 AM Jason Wang <jasowang@...hat.com> wrote:
> >
> > We used to busy waiting on the cvq command this tends to be
> > problematic since:
> >
> > 1) CPU could wait for ever on a buggy/malicous device
> > 2) There's no wait to terminate the process that triggers the cvq
> >    command
> >
> > So this patch switch to use sleep with a timeout (1s) instead of busy
> > polling for the cvq command forever. This gives the scheduler a breath
> > and can let the process can respond to a signal.
> >
> > Signed-off-by: Jason Wang <jasowang@...hat.com>
> > ---
> >  drivers/net/virtio_net.c | 15 ++++++++-------
> >  1 file changed, 8 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 8225496ccb1e..69173049371f 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -405,6 +405,7 @@ static void disable_rx_mode_work(struct virtnet_info *vi)
> >         vi->rx_mode_work_enabled = false;
> >         spin_unlock_bh(&vi->rx_mode_lock);
> >
> > +       virtqueue_wake_up(vi->cvq);
> >         flush_work(&vi->rx_mode_work);
> >  }
> >
> > @@ -1497,6 +1498,11 @@ static bool try_fill_recv(struct virtnet_info *vi, struct receive_queue *rq,
> >         return !oom;
> >  }
> >
> > +static void virtnet_cvq_done(struct virtqueue *cvq)
> > +{
> > +       virtqueue_wake_up(cvq);
> > +}
> > +
> >  static void skb_recv_done(struct virtqueue *rvq)
> >  {
> >         struct virtnet_info *vi = rvq->vdev->priv;
> > @@ -2024,12 +2030,7 @@ static bool virtnet_send_command(struct virtnet_info *vi, u8 class, u8 cmd,
> >         if (unlikely(!virtqueue_kick(vi->cvq)))
> >                 return vi->ctrl->status == VIRTIO_NET_OK;
> >
> > -       /* Spin for a response, the kick causes an ioport write, trapping
> > -        * into the hypervisor, so the request should be handled immediately.
> > -        */
> > -       while (!virtqueue_get_buf(vi->cvq, &tmp) &&
> > -              !virtqueue_is_broken(vi->cvq))
> > -               cpu_relax();
> > +       virtqueue_wait_for_used(vi->cvq, &tmp);
> >
> >         return vi->ctrl->status == VIRTIO_NET_OK;
> >  }
> > @@ -3524,7 +3525,7 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
> >
> >         /* Parameters for control virtqueue, if any */
> >         if (vi->has_cvq) {
> > -               callbacks[total_vqs - 1] = NULL;
> > +               callbacks[total_vqs - 1] = virtnet_cvq_done;
>
> If we're using CVQ callback, what is the actual use of the timeout?

Because we can't sleep forever since locks could be held like RTNL_LOCK.

>
> I'd say there is no right choice neither in the right timeout value
> nor in the action to take.

In the next version, I tend to put BAD_RING() to prevent future requests.

> Why not simply trigger the cmd and do all
> the changes at command return?

I don't get this, sorry.

>
> I suspect the reason is that it complicates the code. For example,
> having the possibility of many in flight commands, races between their
> completion, etc.

Actually the cvq command was serialized through RTNL_LOCK, so we don't
need to worry about this.

In the next version I can add ASSERT_RTNL().

Thanks

> The virtio standard does not even cover unordered
> used commands if I'm not wrong.
>
> Is there any other fundamental reason?
>
> Thanks!
>
> >                 names[total_vqs - 1] = "control";
> >         }
> >
> > --
> > 2.25.1
> >
>