[PATCH v4 2/2] virtio-net: prevent offloads reset on migration

[Mikhail Sennikovsky <mikhail.sennikovskii@cloud.ionos.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 preserving the state in saved_guest_offloads field and
pushing out offload initialization to the new post load hook.

Signed-off-by: Mikhail Sennikovsky <mikhail.sennikovskii@cloud.ionos.com>
---
 hw/net/virtio-net.c            | 27 ++++++++++++++++++++++++---
 include/hw/virtio/virtio-net.h |  2 ++
 2 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index b9e1cd7..6adb0fe 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -2330,9 +2330,13 @@ 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);
-    }
+    /*
+     * curr_guest_offloads will be later overwritten by the
+     * virtio_set_features_nocheck call done from the virtio_load.
+     * Here we make sure it is preserved and restored accordingly
+     * in the virtio_net_post_load_virtio callback.
+     */
+    n->saved_guest_offloads = n->curr_guest_offloads;
 
     virtio_net_set_queues(n);
 
@@ -2367,6 +2371,22 @@ static int virtio_net_post_load_device(void *opaque, int version_id)
     return 0;
 }
 
+static int virtio_net_post_load_virtio(VirtIODevice *vdev)
+{
+    VirtIONet *n = VIRTIO_NET(vdev);
+    /*
+     * The actual needed state is now in saved_guest_offloads,
+     * see virtio_net_post_load_device for detail.
+     * Restore it back and apply the desired offloads.
+     */
+    n->curr_guest_offloads = n->saved_guest_offloads;
+    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",
@@ -2909,6 +2929,7 @@ 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;
 }
 
diff --git a/include/hw/virtio/virtio-net.h b/include/hw/virtio/virtio-net.h
index b96f0c6..07a9319 100644
--- a/include/hw/virtio/virtio-net.h
+++ b/include/hw/virtio/virtio-net.h
@@ -182,6 +182,8 @@ struct VirtIONet {
     char *netclient_name;
     char *netclient_type;
     uint64_t curr_guest_offloads;
+    /* used on saved state restore phase to preserve the curr_guest_offloads */
+    uint64_t saved_guest_offloads;
     AnnounceTimer announce_timer;
     bool needs_vnet_hdr_swap;
     bool mtu_bypass_backend;
-- 
2.7.4