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Date:   Wed, 21 Oct 2020 10:18:20 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Tonghao Zhang <xiangxia.m.yue@...il.com>,
        kernel test robot <lkp@...el.com>,
        Jason Wang <jasowang@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        virtualization@...ts.linux-foundation.org,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH v3] Revert "virtio-net: ethtool configurable RXCSUM"

On Wed, Oct 21, 2020 at 8:00 AM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> This reverts commit 3618ad2a7c0e78e4258386394d5d5f92a3dbccf8.
>
> When control vq is not negotiated, that commit causes a crash:
>
> [   72.229171] kernel BUG at drivers/net/virtio_net.c:1667!
> [   72.230266] invalid opcode: 0000 [#1] PREEMPT SMP
> [   72.231172] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.9.0-rc8-02934-g3618ad2a7c0e7 #1
> [   72.231172] EIP: virtnet_send_command+0x120/0x140
> [   72.231172] Code: 00 0f 94 c0 8b 7d f0 65 33 3d 14 00 00 00 75 1c 8d 65 f4 5b 5e 5f 5d c3 66 90 be 01 00 00 00 e9 6e ff ff ff 8d b6 00
> +00 00 00 <0f> 0b e8 d9 bb 82 00 eb 17 8d b4 26 00 00 00 00 8d b4 26 00 00 00
> [   72.231172] EAX: 0000000d EBX: f72895c0 ECX: 00000017 EDX: 00000011
> [   72.231172] ESI: f7197800 EDI: ed69bd00 EBP: ed69bcf4 ESP: ed69bc98
> [   72.231172] DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068 EFLAGS: 00010246
> [   72.231172] CR0: 80050033 CR2: 00000000 CR3: 02c84000 CR4: 000406f0
> [   72.231172] Call Trace:
> [   72.231172]  ? __virt_addr_valid+0x45/0x60
> [   72.231172]  ? ___cache_free+0x51f/0x760
> [   72.231172]  ? kobject_uevent_env+0xf4/0x560
> [   72.231172]  virtnet_set_guest_offloads+0x4d/0x80
> [   72.231172]  virtnet_set_features+0x85/0x120
> [   72.231172]  ? virtnet_set_guest_offloads+0x80/0x80
> [   72.231172]  __netdev_update_features+0x27a/0x8e0
> [   72.231172]  ? kobject_uevent+0xa/0x20
> [   72.231172]  ? netdev_register_kobject+0x12c/0x160
> [   72.231172]  register_netdevice+0x4fe/0x740
> [   72.231172]  register_netdev+0x1c/0x40
> [   72.231172]  virtnet_probe+0x728/0xb60
> [   72.231172]  ? _raw_spin_unlock+0x1d/0x40
> [   72.231172]  ? virtio_vdpa_get_status+0x1c/0x20
> [   72.231172]  virtio_dev_probe+0x1c6/0x271
> [   72.231172]  really_probe+0x195/0x2e0
> [   72.231172]  driver_probe_device+0x26/0x60
> [   72.231172]  device_driver_attach+0x49/0x60
> [   72.231172]  __driver_attach+0x46/0xc0
> [   72.231172]  ? device_driver_attach+0x60/0x60
> [   72.231172]  bus_add_driver+0x197/0x1c0
> [   72.231172]  driver_register+0x66/0xc0
> [   72.231172]  register_virtio_driver+0x1b/0x40
> [   72.231172]  virtio_net_driver_init+0x61/0x86
> [   72.231172]  ? veth_init+0x14/0x14
> [   72.231172]  do_one_initcall+0x76/0x2e4
> [   72.231172]  ? rdinit_setup+0x2a/0x2a
> [   72.231172]  do_initcalls+0xb2/0xd5
> [   72.231172]  kernel_init_freeable+0x14f/0x179
> [   72.231172]  ? rest_init+0x100/0x100
> [   72.231172]  kernel_init+0xd/0xe0
> [   72.231172]  ret_from_fork+0x1c/0x30
> [   72.231172] Modules linked in:
> [   72.269563] ---[ end trace a6ebc4afea0e6cb1 ]---
>
> The reason is that virtnet_set_features now calls virtnet_set_guest_offloads
> unconditionally, it used to only call it when there is something
> to configure.

Right. The fix might be to make the call to virtnet_set_guest_offloads
conditional again

-       err = virtnet_set_guest_offloads(vi, offloads);
-       if (err)
-               return err;
+       if (offloads ^ vi->guest_offloads) {
+               err = virtnet_set_guest_offloads(vi, offloads);
+               if (err)
+                       return err;
+       }

Whether the two features in virtnet_set_features are configurable at
all depends on whether they are part of dev->hw_features, which is
conditional on guest offload support:

        if (virtio_has_feature(vdev, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS)) {
                dev->hw_features |= NETIF_F_RXCSUM;
                dev->hw_features |= NETIF_F_LRO;
        }

so I had not expected virtnet_set_features to get called at all unless
this was enabled. Clearly an incorrect assumption.

The conservative solution is a  full revert and resubmitting a correct
new patch for net-next. Thanks for preparing the revert, sorry for the
breakage. Are you sending it for a third time because of the patchwork
quirk?

> If device does not have a control vq, everything breaks.
>
> Looking at this some more, I noticed that it's not really checking the
> hardware too much. E.g.
>
>         if ((dev->features ^ features) & NETIF_F_LRO) {
>                 if (features & NETIF_F_LRO)
>                         offloads |= GUEST_OFFLOAD_LRO_MASK &
>                                     vi->guest_offloads_capable;
>                 else
>                         offloads &= ~GUEST_OFFLOAD_LRO_MASK;
>         }
>
> and
>
>                                 (1ULL << VIRTIO_NET_F_GUEST_TSO6) | \
>                                 (1ULL << VIRTIO_NET_F_GUEST_ECN)  | \
>                                 (1ULL << VIRTIO_NET_F_GUEST_UFO))
>
> But there's no guarantee that e.g. VIRTIO_NET_F_GUEST_TSO6 is set.
>
> If it isn't command should not send it.

It only toggles the subset of this mask that is agreed on at probe and
at that time stored in guest_offloads_capable. As Jason also
mentioned. I think this comment is obsolete.

        for (i = 0; i < ARRAY_SIZE(guest_offloads); i++)
                if (virtio_has_feature(vi->vdev, guest_offloads[i]))
                        set_bit(guest_offloads[i], &vi->guest_offloads);
        vi->guest_offloads_capable = vi->guest_offloads;

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