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

[Alistair Delva <adelva@google.com>]

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,