virtio-net: Fix VLAN filter table reset timing

virtio-net: Fix VLAN filter table reset timing

Problem
-------

The expected initial state of the table depends on feature negotiation:

With VIRTIO_NET_F_CTRL_VLAN:
  The table must be empty in accordance with the specification.
Without VIRTIO_NET_F_CTRL_VLAN:
  The table must be filled to permit all VLAN traffic.

Prior to commit 06b636a1e2ad ("virtio-net: do not reset vlan filtering
at set_features"), virtio_net_set_features() always reset the VLAN
table. That commit changed the behavior to skip table reset when
VIRTIO_NET_F_CTRL_VLAN was negotiated, assuming the table would be
properly cleared during device reset and remain stable.

However, this assumption breaks when a driver renegotiates features:
1. Initial negotiation without VIRTIO_NET_F_CTRL_VLAN (table filled)
2. Renegotiation with VIRTIO_NET_F_CTRL_VLAN (table will not be cleared)

The problem was exacerbated by commit 0caed25cd171 ("virtio: Call
set_features during reset"), which triggered virtio_net_set_features()
during device reset, exposing the bug whenever VIRTIO_NET_F_CTRL_VLAN
was negotiated after a device reset.

Solution
--------

Fix the issue by initializing the table when virtio_net_set_features()
is called to change the VIRTIO_NET_F_CTRL_VLAN bit of
vdev->guest_features.

This approach ensures the correct table state regardless of feature
negotiation sequence by performing initialization in
virtio_net_set_features() as QEMU did prior to commit 06b636a1e2ad
("virtio-net: do not reset vlan filtering at set_features").

This change still preserves the goal of the commit, which was to avoid
resetting the table during migration, by checking whether the
VIRTIO_NET_F_CTRL_VLAN bit of vdev->guest_features is being changed;
vdev->guest_features is set before virtio_net_set_features() gets called
during migration.

It also avoids resetting the table when the driver sets a feature
bitmask with no change for the VIRTIO_NET_F_CTRL_VLAN bit, which makes
the operation idempotent and its semantics cleaner.

Additionally, this change ensures the table is initialized after
feature negotiation and before the DRIVER_OK status bit being set for
compatibility with the Linux driver before commit 50c0ada627f5
("virtio-net: fix race between ndo_open() and virtio_device_ready()"),
which did not ensure to set the DRIVER_OK status bit before modifying
the table.

Fixes: 06b636a1e2ad ("virtio-net: do not reset vlan filtering at set_features")
Cc: qemu-stable@nongnu.org
Reported-by: Konstantin Shkolnyy <kshk@linux.ibm.com>
Signed-off-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Tested-by: Konstantin Shkolnyy <kshk@linux.ibm.com>
Tested-by: Lei Yang <leiyang@redhat.com>
Message-Id: <20250727-vlan-v3-1-bbee738619b1@rsg.ci.i.u-tokyo.ac.jp>
Tested-by: Konstantin Shkolnyy <kshk@linux.ibm.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>