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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 27 Dec 2022 01:58:22 -0500

On Tue, Dec 27, 2022 at 12:33:53PM +0800, Jason Wang wrote:
> On Tue, Dec 27, 2022 at 10:25 AM Xuan Zhuo <[email-masked]> wrote:
> >
> > On Mon, 26 Dec 2022 15:49:08 +0800, Jason Wang <[email-masked]> 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 virtqueue_wait_for_used() to sleep with a
> > > timeout (1s) instead of busy polling for the cvq command forever. This
> >
> > I don't think that a fixed 1S is a good choice.
> 
> Well, it could be tweaked to be a little bit longer.
> 
> One way, as discussed, is to let the device advertise a timeout then
> the driver can validate if it's valid and use that timeout. But it
> needs extension to the spec.

Controlling timeout from device is a good idea, e.g. hardware devices
would benefit from a shorter timeout, hypervisor devices from a longer
timeout or no timeout.

> 
> > Some of the DPUs are very
> > lazy for cvq handle.
> 
> Such design needs to be revisited, cvq (control path) should have a
> better priority or QOS than datapath.

Spec says nothing about this, so driver can't assume this either.

> > In particular, we will also directly break the device.
> 
> It's kind of hardening for malicious devices.

ATM no amount of hardening can prevent a malicious hypervisor from
blocking the guest. Recovering when a hardware device is broken would be
nice but I think if we do bother then we should try harder to recover,
such as by driving device reset.

Also, does your patch break surprise removal? There's no callback
in this case ATM.

> >
> > I think it is necessary to add a Virtio-Net parameter to allow users to define
> > this timeout by themselves. Although I don't think this is a good way.
> 
> Very hard and unfriendly to the end users.
> 
> Thanks
> 
> >
> > Thanks.
> >
> >
> > > gives the scheduler a breath and can let the process can respond to
> > > asignal. If the device doesn't respond in the timeout, break the
> > > device.
> > >
> > > Signed-off-by: Jason Wang <[email-masked]>
> > > ---
> > > Changes since V1:
> > > - break the device when timeout
> > > - get buffer manually since the virtio core check more_used() instead
> > > ---
> > >  drivers/net/virtio_net.c | 24 ++++++++++++++++--------
> > >  1 file changed, 16 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > index efd9dd55828b..6a2ea64cfcb5 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;
> > > @@ -1984,6 +1990,8 @@ static int virtnet_tx_resize(struct virtnet_info *vi,
> > >       return err;
> > >  }
> > >
> > > +static int virtnet_close(struct net_device *dev);
> > > +
> > >  /*
> > >   * Send command via the control virtqueue and check status.  Commands
> > >   * supported by the hypervisor, as indicated by feature bits, should
> > > @@ -2026,14 +2034,14 @@ 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();
> > > +     if (virtqueue_wait_for_used(vi->cvq)) {
> > > +             virtqueue_get_buf(vi->cvq, &tmp);
> > > +             return vi->ctrl->status == VIRTIO_NET_OK;
> > > +     }
> > >
> > > -     return vi->ctrl->status == VIRTIO_NET_OK;
> > > +     netdev_err(vi->dev, "CVQ command timeout, break the virtio device.");
> > > +     virtio_break_device(vi->vdev);
> > > +     return VIRTIO_NET_ERR;
> > >  }
> > >
> > >  static int virtnet_set_mac_address(struct net_device *dev, void *p)
> > > @@ -3526,7 +3534,7 @@ 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;
> > >               names[total_vqs - 1] = "control";
> > >       }
> > >
> > > --
> > > 2.25.1
> > >
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> >