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Date:   Sun, 5 Jan 2020 08:22:07 -0500
From:   "Michael S. Tsirkin" <>
Cc:     Alistair Delva <>,
        Willem de Bruijn <>,
        Jason Wang <>,
        "David S. Miller" <>,,
Subject: [PATCH v2] virtio_net: CTRL_GUEST_OFFLOADS depends on CTRL_VQ

The only way for guest to control offloads (as enabled by
VIRTIO_NET_F_CTRL_GUEST_OFFLOADS) is by sending commands
through CTRL_VQ. So it does not make sense to

The spec does not outlaw devices with such a configuration, so we have
to support it. Simply clear VIRTIO_NET_F_CTRL_GUEST_OFFLOADS.
Note that Linux is still crashing if it tries to
change the offloads when there's no control vq.
That needs to be fixed by another patch.

Reported-by: Alistair Delva <>
Reported-by: Willem de Bruijn <>
Fixes: 3f93522ffab2 ("virtio-net: switch off offloads on demand if possible on XDP set")
Signed-off-by: Michael S. Tsirkin <>

Same patch as v1 but update documentation so it's clear it's not
enough to fix the crash.

 drivers/net/virtio_net.c | 9 +++++++++
 1 file changed, 9 insertions(+)

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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