Re: [PATCH net] virtio_net: CTRL_GUEST_OFFLOADS depends on CTRL_VQ

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

On Tue, Dec 24, 2019 at 10:49:13AM +0800, Jason Wang wrote:
> 
> On 2019/12/24 上午4:21, Alistair Delva wrote:
> > On Mon, Dec 23, 2019 at 12:12 PM Willem de Bruijn
> > <willemdebruijn.kernel@gmail.com> wrote:
> > > On Mon, Dec 23, 2019 at 2:56 PM Willem de Bruijn
> > > <willemdebruijn.kernel@gmail.com> wrote:
> > > > 00fffe0ff0 DR7: 0000000000000400
> > > > > > Call Trace:
> > > > > >   ? preempt_count_add+0x58/0xb0
> > > > > >   ? _raw_spin_lock_irqsave+0x36/0x70
> > > > > >   ? _raw_spin_unlock_irqrestore+0x1a/0x40
> > > > > >   ? __wake_up+0x70/0x190
> > > > > >   virtnet_set_features+0x90/0xf0 [virtio_net]
> > > > > >   __netdev_update_features+0x271/0x980
> > > > > >   ? nlmsg_notify+0x5b/0xa0
> > > > > >   dev_disable_lro+0x2b/0x190
> > > > > >   ? inet_netconf_notify_devconf+0xe2/0x120
> > > > > >   devinet_sysctl_forward+0x176/0x1e0
> > > > > >   proc_sys_call_handler+0x1f0/0x250
> > > > > >   proc_sys_write+0xf/0x20
> > > > > >   __vfs_write+0x3e/0x190
> > > > > >   ? __sb_start_write+0x6d/0xd0
> > > > > >   vfs_write+0xd3/0x190
> > > > > >   ksys_write+0x68/0xd0
> > > > > >   __ia32_sys_write+0x14/0x20
> > > > > >   do_fast_syscall_32+0x86/0xe0
> > > > > >   entry_SYSENTER_compat+0x7c/0x8e
> > > > > > 
> > > > > > A similar crash will likely trigger when enabling XDP.
> > > > > > 
> > > > > > Reported-by: Alistair Delva <adelva@google.com>
> > > > > > Reported-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
> > > > > > Fixes: 3f93522ffab2 ("virtio-net: switch off offloads on demand if possible on XDP set")
> > > > > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > > > > > ---
> > > > > > 
> > > > > > Lightly tested.
> > > > > > 
> > > > > > Alistair, could you please test and confirm that this resolves the
> > > > > > crash for you?
> > > > > This patch doesn't work. The reason is that NETIF_F_LRO is also turned
> > > > > on by TSO4/TSO6, which your patch didn't check for. So it ends up
> > > > > going through the same path and crashing in the same way.
> > > > > 
> > > > >          if (virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_TSO4) ||
> > > > >              virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_TSO6))
> > > > >                  dev->features |= NETIF_F_LRO;
> > > > > 
> > > > > It sounds like this patch is fixing something slightly differently to
> > > > > my patch fixed. virtnet_set_features() doesn't care about
> > > > > GUEST_OFFLOADS, it only tests against NETIF_F_LRO. Even if "offloads"
> > > > > is zero, it will call virtnet_set_guest_offloads(), which triggers the
> > > > > crash.
> > > > 
> > > > Interesting. It's surprising that it is trying to configure a flag
> > > > that is not configurable, i.e., absent from dev->hw_features
> > > > after Michael's change.
> > > > 
> > > > > So either we need to ensure NETIF_F_LRO is never set, or
> > > > LRO might be available, just not configurable. Indeed this was what I
> > > > observed in the past.
> > > dev_disable_lro expects that NETIF_F_LRO is always configurable. Which
> > > I guess is a reasonable assumption, just not necessarily the case in
> > > virtio_net.
> > > 
> > > So I think we need both patches. Correctly mark the feature as fixed
> > > by removing from dev->hw_features and also ignore the request from
> > > dev_disable_lro, which does not check for this.
> > Something like this maybe:
> > 
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 4d7d5434cc5d..0556f42b0fb5 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -2560,6 +2560,9 @@ static int virtnet_set_features(struct net_device *dev,
> >          u64 offloads;
> >          int err;
> > 
> > +       if (!virtio_has_feature(vi->vdev, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS))
> > +               return 0;
> > +
> >          if ((dev->features ^ features) & NETIF_F_LRO) {
> >                  if (vi->xdp_queue_pairs)
> >                          return -EBUSY;
> > @@ -2971,6 +2974,15 @@ static int virtnet_validate(struct virtio_device *vdev)
> >          if (!virtnet_validate_features(vdev))
> >                  return -EINVAL;
> > 
> > +       /* VIRTIO_NET_F_CTRL_GUEST_OFFLOADS does not work without
> > +        * VIRTIO_NET_F_CTRL_VQ. However the virtio spec does not
> > +        * specify that VIRTIO_NET_F_CTRL_GUEST_OFFLOADS depends
> > +        * on VIRTIO_NET_F_CTRL_VQ so devices can set the later but
> > +        * not the former.
> > +        */
> > +       if (!virtio_has_feature(vdev, VIRTIO_NET_F_CTRL_VQ))
> > +               __virtio_clear_bit(vdev, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS);
> > +
> >          if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
> >                  int mtu = virtio_cread16(vdev,
> >                                           offsetof(struct virtio_net_config,
> > 
> 
> We check feature dependency and fail the probe in
> virtnet_validate_features().
> 
> Is it more straightforward to fail the probe there when CTRL_GUEST_OFFLOADS
> was set but CTRL_VQ wasn't?
> 
> Thanks

Expanding on what the comment above says, we can't fail probe
in this configuration without breaking the driver for
spec compliant devices.

-- 
MST