[RFC 2/2] virtio-net: use post load hook

["Michael S. Tsirkin" <mst@redhat.com>]

Currently offloads disabled by guest via the VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET
command are not preserved on VM migration.
Instead all offloads reported by guest features (via VIRTIO_PCI_GUEST_FEATURES)
get enabled.
What happens is: first the VirtIONet::curr_guest_offloads gets restored
and offloads are getting set correctly:

 #0  qemu_set_offload (nc=0x555556a11400, csum=1, tso4=0, tso6=0, ecn=0, ufo=0) at net/net.c:474
 #1  virtio_net_apply_guest_offloads (n=0x555557701ca0) at hw/net/virtio-net.c:720
 #2  virtio_net_post_load_device (opaque=0x555557701ca0, version_id=11) at hw/net/virtio-net.c:2334
 #3  vmstate_load_state (f=0x5555569dc010, vmsd=0x555556577c80 <vmstate_virtio_net_device>, opaque=0x555557701ca0, version_id=11)
     at migration/vmstate.c:168
 #4  virtio_load (vdev=0x555557701ca0, f=0x5555569dc010, version_id=11) at hw/virtio/virtio.c:2197
 #5  virtio_device_get (f=0x5555569dc010, opaque=0x555557701ca0, size=0, field=0x55555668cd00 <__compound_literal.5>) at hw/virtio/virtio.c:2036
 #6  vmstate_load_state (f=0x5555569dc010, vmsd=0x555556577ce0 <vmstate_virtio_net>, opaque=0x555557701ca0, version_id=11) at migration/vmstate.c:143
 #7  vmstate_load (f=0x5555569dc010, se=0x5555578189e0) at migration/savevm.c:829
 #8  qemu_loadvm_section_start_full (f=0x5555569dc010, mis=0x5555569eee20) at migration/savevm.c:2211
 #9  qemu_loadvm_state_main (f=0x5555569dc010, mis=0x5555569eee20) at migration/savevm.c:2395
 #10 qemu_loadvm_state (f=0x5555569dc010) at migration/savevm.c:2467
 #11 process_incoming_migration_co (opaque=0x0) at migration/migration.c:449

However later on the features are getting restored, and offloads get reset to
everything supported by features:

 #0  qemu_set_offload (nc=0x555556a11400, csum=1, tso4=1, tso6=1, ecn=0, ufo=0) at net/net.c:474
 #1  virtio_net_apply_guest_offloads (n=0x555557701ca0) at hw/net/virtio-net.c:720
 #2  virtio_net_set_features (vdev=0x555557701ca0, features=5104441767) at hw/net/virtio-net.c:773
 #3  virtio_set_features_nocheck (vdev=0x555557701ca0, val=5104441767) at hw/virtio/virtio.c:2052
 #4  virtio_load (vdev=0x555557701ca0, f=0x5555569dc010, version_id=11) at hw/virtio/virtio.c:2220
 #5  virtio_device_get (f=0x5555569dc010, opaque=0x555557701ca0, size=0, field=0x55555668cd00 <__compound_literal.5>) at hw/virtio/virtio.c:2036
 #6  vmstate_load_state (f=0x5555569dc010, vmsd=0x555556577ce0 <vmstate_virtio_net>, opaque=0x555557701ca0, version_id=11) at migration/vmstate.c:143
 #7  vmstate_load (f=0x5555569dc010, se=0x5555578189e0) at migration/savevm.c:829
 #8  qemu_loadvm_section_start_full (f=0x5555569dc010, mis=0x5555569eee20) at migration/savevm.c:2211
 #9  qemu_loadvm_state_main (f=0x5555569dc010, mis=0x5555569eee20) at migration/savevm.c:2395
 #10 qemu_loadvm_state (f=0x5555569dc010) at migration/savevm.c:2467
 #11 process_incoming_migration_co (opaque=0x0) at migration/migration.c:449

Fix this by pushing out offload initialization to the new post load hook.

Reported-by: Mikhail Sennikovsky <mikhail.sennikovskii@cloud.ionos.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 hw/net/virtio-net.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 9f11422337..62fb858e2d 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -2333,10 +2333,6 @@ static int virtio_net_post_load_device(void *opaque, int version_id)
         n->curr_guest_offloads = virtio_net_supported_guest_offloads(n);
     }
 
-    if (peer_has_vnet_hdr(n)) {
-        virtio_net_apply_guest_offloads(n);
-    }
-
     virtio_net_set_queues(n);
 
     /* Find the first multicast entry in the saved MAC filter */
@@ -2370,6 +2366,15 @@ static int virtio_net_post_load_device(void *opaque, int version_id)
     return 0;
 }
 
+static int virtio_net_post_load_virtio(VirtIODevice *vdev)
+{
+    if (peer_has_vnet_hdr(n)) {
+        virtio_net_apply_guest_offloads(n);
+    }
+
+    return 0;
+}
+
 /* tx_waiting field of a VirtIONetQueue */
 static const VMStateDescription vmstate_virtio_net_queue_tx_waiting = {
     .name = "virtio-net-queue-tx_waiting",
@@ -2912,6 +2917,7 @@ static void virtio_net_class_init(ObjectClass *klass, void *data)
     vdc->guest_notifier_mask = virtio_net_guest_notifier_mask;
     vdc->guest_notifier_pending = virtio_net_guest_notifier_pending;
     vdc->legacy_features |= (0x1 << VIRTIO_NET_F_GSO);
+    vdc->post_load = virtio_net_post_load_virtio;
     vdc->vmsd = &vmstate_virtio_net_device;
 }
 
-- 
MST