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>

1 files changed, 4 insertions, 3 deletions

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index c4c49b0..6b5b5da 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -929,8 +929,9 @@ static void virtio_net_set_features(VirtIODevice *vdev, uint64_t features)
         vhost_net_save_acked_features(nc->peer);
     }
 
-    if (!virtio_has_feature(features, VIRTIO_NET_F_CTRL_VLAN)) {
-        memset(n->vlans, 0xff, MAX_VLAN >> 3);
+    if (virtio_has_feature(vdev->guest_features ^ features, VIRTIO_NET_F_CTRL_VLAN)) {
+        bool vlan = virtio_has_feature(features, VIRTIO_NET_F_CTRL_VLAN);
+        memset(n->vlans, vlan ? 0 : 0xff, MAX_VLAN >> 3);
     }
 
     if (virtio_has_feature(features, VIRTIO_NET_F_STANDBY)) {
@@ -3942,6 +3943,7 @@ static void virtio_net_device_realize(DeviceState *dev, Error **errp)
     n->mac_table.macs = g_malloc0(MAC_TABLE_ENTRIES * ETH_ALEN);
 
     n->vlans = g_malloc0(MAX_VLAN >> 3);
+    memset(n->vlans, 0xff, MAX_VLAN >> 3);
 
     nc = qemu_get_queue(n->nic);
     nc->rxfilter_notify_enabled = 1;
@@ -4041,7 +4043,6 @@ static void virtio_net_reset(VirtIODevice *vdev)
     memset(n->mac_table.macs, 0, MAC_TABLE_ENTRIES * ETH_ALEN);
     memcpy(&n->mac[0], &n->nic->conf->macaddr, sizeof(n->mac));
     qemu_format_nic_info_str(qemu_get_queue(n->nic), n->mac);
-    memset(n->vlans, 0, MAX_VLAN >> 3);
 
     /* Flush any async TX */
     for (i = 0;  i < n->max_queue_pairs; i++) {