Date:   Sun, 22 Dec 2019 16:12:24 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Alistair Delva <adelva@...gle.com>,
        Network Development <netdev@...r.kernel.org>,
        stable <stable@...r.kernel.org>,
        Jason Wang <jasowang@...hat.com>,
        "David S . Miller" <davem@...emloft.net>, kernel-team@...roid.com,
        virtualization@...ts.linux-foundation.org,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net] virtio-net: Skip set_features on non-cvq devices

On Sun, Dec 22, 2019 at 10:54:23AM -0500, Willem de Bruijn wrote:
> On Sun, Dec 22, 2019 at 9:57 AM Michael S. Tsirkin <mst@...hat.com> wrote:
> >
> > On Sun, Dec 22, 2019 at 09:21:43AM -0500, Willem de Bruijn wrote:
> > > On Sun, Dec 22, 2019 at 8:11 AM Michael S. Tsirkin <mst@...hat.com> wrote:
> > > >
> > > > On Fri, Dec 20, 2019 at 10:08:41PM -0500, Willem de Bruijn wrote:
> > > > > On Fri, Dec 20, 2019 at 4:22 PM Alistair Delva <adelva@...gle.com> wrote:
> > > > > >
> > > > > > On devices without control virtqueue support, such as the virtio_net
> > > > > > implementation in crosvm[1], attempting to configure LRO will panic the
> > > > > > kernel:
> > > > > >
> > > > > > kernel BUG at drivers/net/virtio_net.c:1591!
> > > > > > invalid opcode: 0000 [#1] PREEMPT SMP PTI
> > > > > > CPU: 1 PID: 483 Comm: Binder:330_1 Not tainted 5.4.5-01326-g19463e9acaac #1
> > > > > > Hardware name: ChromiumOS crosvm, BIOS 0
> > > > > > RIP: 0010:virtnet_send_command+0x15d/0x170 [virtio_net]
> > > > > > Code: d8 00 00 00 80 78 02 00 0f 94 c0 65 48 8b 0c 25 28 00 00 00 48 3b 4c 24 70 75 11 48 8d 65 d8 5b 41 5c 41 5d 41 5e 41 5f 5d c3 <0f> 0b e8 ec a4 12 c8 66 90 66 2e 0f 1f 84 00 00 00 00 00 55 48 89
> > > > > > RSP: 0018:ffffb97940e7bb50 EFLAGS: 00010246
> > > > > > RAX: ffffffffc0596020 RBX: ffffa0e1fc8ea840 RCX: 0000000000000017
> > > > > > RDX: ffffffffc0596110 RSI: 0000000000000011 RDI: 000000000000000d
> > > > > > RBP: ffffb97940e7bbf8 R08: ffffa0e1fc8ea0b0 R09: ffffa0e1fc8ea0b0
> > > > > > R10: ffffffffffffffff R11: ffffffffc0590940 R12: 0000000000000005
> > > > > > R13: ffffa0e1ffad2c00 R14: ffffb97940e7bc08 R15: 0000000000000000
> > > > > > FS:  0000000000000000(0000) GS:ffffa0e1fd100000(006b) knlGS:00000000e5ef7494
> > > > > > CS:  0010 DS: 002b ES: 002b CR0: 0000000080050033
> > > > > > CR2: 00000000e5eeb82c CR3: 0000000079b06001 CR4: 0000000000360ee0
> > > > > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > > > > DR3: 0000000000000000 DR6: 00000000fffe0ff0 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
> > > > > >
> > > > > > This happens because virtio_set_features() does not check the presence
> > > > > > of the control virtqueue feature, which is sanity checked by a BUG_ON
> > > > > > in virtnet_send_command().
> > > > > >
> > > > > > Fix this by skipping any feature processing if the control virtqueue is
> > > > > > missing. This should be OK for any future feature that is added, as
> > > > > > presumably all of them would require control virtqueue support to notify
> > > > > > the endpoint that offload etc. should begin.
> > > > > >
> > > > > > [1] https://chromium.googlesource.com/chromiumos/platform/crosvm/
> > > > > >
> > > > > > Fixes: a02e8964eaf9 ("virtio-net: ethtool configurable LRO")
> > > > > > Cc: stable@...r.kernel.org [4.20+]
> > > > > > Cc: Michael S. Tsirkin <mst@...hat.com>
> > > > > > Cc: Jason Wang <jasowang@...hat.com>
> > > > > > Cc: David S. Miller <davem@...emloft.net>
> > > > > > Cc: kernel-team@...roid.com
> > > > > > Cc: virtualization@...ts.linux-foundation.org
> > > > > > Cc: linux-kernel@...r.kernel.org
> > > > > > Signed-off-by: Alistair Delva <adelva@...gle.com>
> > > > >
> > > > > Thanks for debugging this, Alistair.
> > > > >
> > > > > > ---
> > > > > >  drivers/net/virtio_net.c | 3 +++
> > > > > >  1 file changed, 3 insertions(+)
> > > > > >
> > > > > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > > > > index 4d7d5434cc5d..709bcd34e485 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 (!vi->has_cvq)
> > > > > > +               return 0;
> > > > > > +
> > > > >
> > > > > Instead of checking for this in virtnet_set_features, how about we
> > > > > make configurability contingent on cvq in virtnet_probe:
> > > > >
> > > > > -       if (virtio_has_feature(vdev, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS))
> > > > > +       if (virtio_has_feature(vdev, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS) &&
> > > > > +           virtio_has_feature(vdev, VIRTIO_NET_F_CTRL_VQ))
> > > > >                 dev->hw_features |= NETIF_F_LRO;
> > > > >
> > > > > Based on this logic a little below in the same function
> > > > >
> > > > >         if (virtio_has_feature(vdev, VIRTIO_NET_F_CTRL_VQ))
> > > > >                 vi->has_cvq = true;
> > > >
> > > >
> > > > This would be a regression on old hypervisors which didn't have
> > > > CTL VQ - suddenly they will lose offloads.
> > >
> > > dev->features still correctly displays whether offloads are enabled.
> > > Removing it from dev->hw_features just renders it non-configurable.
> >
> > Oh you are right. I confused it with dev->features.
> >
> > > Note that before the patch that is being fixed the offloads were
> > > enabled, but ethtool would show them as off.
> >
> > So the bug is in spec, it should have said
> > VIRTIO_NET_F_CTRL_GUEST_OFFLOADS depends on VIRTIO_NET_F_CTRL_VQ, but we
> > missed that part. We can and I guess should add this as a recommendation
> > but it's too late to make it a MUST.
> >
> > Meanwhile I would say it's cleanest to work around
> > this in virtnet_validate by clearing VIRTIO_NET_F_CTRL_GUEST_OFFLOADS
> > if VIRTIO_NET_F_CTRL_VQ is off, with a big comment explaining
> > it's a spec bug.
> 
> Wouldn't that cause precisely the regression you were concerned about?

Not sure how do you mean.  VIRTIO_NET_F_CTRL_GUEST_OFFLOADS simply can't
work without a ctrl vq. What's the point of keeping it on?

> Workloads may now depend on LRO for cycle efficiency. Reverting to
> behavior before this patch (though now displaying the offload state
> correctly) is more conservative in that regard.

Do you see a problem with the following (untested):

Signed-off-by: Michael S. Tsirkin <mst@...hat.com>


diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 4d7d5434cc5d..7b8805b47f0d 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -2971,6 +2971,15 @@ 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. Unfortunately spec forgot to
+	 * 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,

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