netdev - Re: [PATCH net] virtio_net: CTRL_GUEST_OFFLOADS depends on CTRL

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Date:   Mon, 23 Dec 2019 15:12:20 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Alistair Delva <adelva@...gle.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Jason Wang <jasowang@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        virtualization@...ts.linux-foundation.org,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net] virtio_net: CTRL_GUEST_OFFLOADS depends on CTRL_VQ

On Mon, Dec 23, 2019 at 2:56 PM Willem de Bruijn
<willemdebruijn.kernel@...il.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@...gle.com>
> > > Reported-by: Willem de Bruijn <willemdebruijn.kernel@...il.com>
> > > Fixes: 3f93522ffab2 ("virtio-net: switch off offloads on demand if possible on XDP set")
> > > Signed-off-by: Michael S. Tsirkin <mst@...hat.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.