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

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Date:   Mon, 26 Dec 2022 11:44:53 +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 Fri, Dec 23, 2022 at 4:05 PM Eugenio Perez Martin
<eperezma@...hat.com> wrote:
>
> On Fri, Dec 23, 2022 at 4:04 AM Jason Wang <jasowang@...hat.com> wrote:
> >
> > 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.
> >
>
> Right, rtnl_lock kind of invalidates it for a general case.
>
> But do all of the commands need to take rtnl_lock? For example I see
> how we could remove it from ctrl_announce,

I think not, it's intended to serialize all cvq commands.

> so lack of ack may not be
> fatal for it.

Then there could be more than one cvq commands sent to the device, the
busy poll logic may not work.

And it's a hint that the device malfunctioned which is something that
the driver should be aware of.

Thanks

> Assuming a buggy device, we can take some cvq commands
> out of this fatal situation.
>
> This series already improves the current situation and my suggestion
> (if it's worth it) can be applied on top of it, so it is not a blocker
> at all.
>
> > >
> > > 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.
> >
>
> It's actually expanding the first point so you already answered it :).
>
> Thanks!
>
> > >
> > > 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
> > > >
> > >
> >
>