[PATCH v3] virtio-net: prevent offloads reset on migration

[Mikhail Sennikovsky]

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 making the virtio_net_set_features not reset
the curr_guest_offloads in case the VM runstate is RUN_STATE_INMIGRATE

Signed-off-by: Mikhail Sennikovsky <address@hidden>
---
 hw/net/virtio-net.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index b9e1cd7..49be172 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -767,7 +767,14 @@ static void virtio_net_set_features(VirtIODevice *vdev, 
uint64_t features)
     n->rsc6_enabled = virtio_has_feature(features, VIRTIO_NET_F_RSC_EXT) &&
         virtio_has_feature(features, VIRTIO_NET_F_GUEST_TSO6);
 
-    if (n->has_vnet_hdr) {
+    /*
+     * In case of RUN_STATE_INMIGRATE the virtio_net_set_features
+     * is called as part of VM state restore process.
+     * At this stage we do not want the curr_guest_offloads to be reset,
+     * i.e. want to preserve them in the same state as was set
+     * by the guest on the source machine.
+     */
+    if (n->has_vnet_hdr && !runstate_check(RUN_STATE_INMIGRATE)) {
         n->curr_guest_offloads =
             virtio_net_guest_offloads_by_features(features);
         virtio_net_apply_guest_offloads(n);
-- 
2.7.4