[PATCH 3/3] virtio-net: graceful fallback to vhost=off for tap netdev

[Yuri Benditovich]

Currently virtio-net silently clears features if they are
not supported by respective vhost. This may create migration
problems in future if vhost features on the source and destination
are different. Implement graceful fallback to no-vhost mode
when some acked features contradict with vhost. The decision is
taken on set_features call and the vhost will be disabled
till next reset (or migration).
Such fallback is currently enabled only for TAP netdev.

Signed-off-by: Yuri Benditovich <yuri.benditovich@daynix.com>
---
 hw/net/virtio-net.c | 58 ++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 50 insertions(+), 8 deletions(-)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 5150f295e8..b353060e63 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -515,6 +515,15 @@ static RxFilterInfo 
*virtio_net_query_rxfilter(NetClientState *nc)
     return info;
 }
 
+static void virtio_net_allow_vhost(VirtIONet *n, bool allow)
+{
+    int i;
+    for (i = 0; i < n->max_queues; i++) {
+        NetClientState *nc = qemu_get_subqueue(n->nic, i)->peer;
+        nc->vhost_net_disabled = !allow;
+    }
+}
+
 static void virtio_net_reset(VirtIODevice *vdev)
 {
     VirtIONet *n = VIRTIO_NET(vdev);
@@ -552,6 +561,7 @@ static void virtio_net_reset(VirtIODevice *vdev)
             assert(!virtio_net_get_subqueue(nc)->async_tx.elem);
         }
     }
+    virtio_net_allow_vhost(n, true);
 }
 
 static void peer_test_vnet_hdr(VirtIONet *n)
@@ -689,6 +699,15 @@ static void virtio_net_set_queues(VirtIONet *n)
     }
 }
 
+static bool can_disable_vhost(VirtIONet *n)
+{
+    NetClientState *peer = qemu_get_queue(n->nic)->peer;
+    if (!get_vhost_net(peer)) {
+        return false;
+    }
+    return !peer || peer->info->type == NET_CLIENT_DRIVER_TAP;
+}
+
 static void virtio_net_set_multiqueue(VirtIONet *n, int multiqueue);
 
 static uint64_t virtio_net_get_features(VirtIODevice *vdev, uint64_t features,
@@ -725,14 +744,14 @@ static uint64_t virtio_net_get_features(VirtIODevice 
*vdev, uint64_t features,
         return features;
     }
 
-    virtio_clear_feature(&features, VIRTIO_NET_F_RSS);
-    virtio_clear_feature(&features, VIRTIO_NET_F_HASH_REPORT);
-    features = vhost_net_get_features(get_vhost_net(nc->peer), features);
-    vdev->backend_features = features;
+    vdev->backend_features = vhost_net_get_features(get_vhost_net(nc->peer), 
features);
 
-    if (n->mtu_bypass_backend &&
-            (n->host_features & 1ULL << VIRTIO_NET_F_MTU)) {
-        features |= (1ULL << VIRTIO_NET_F_MTU);
+    if (!can_disable_vhost(n)) {
+        features = vdev->backend_features;
+        if (n->mtu_bypass_backend &&
+                (n->host_features & 1ULL << VIRTIO_NET_F_MTU)) {
+            features |= (1ULL << VIRTIO_NET_F_MTU);
+        }
     }
 
     return features;
@@ -872,10 +891,25 @@ static void failover_add_primary(VirtIONet *n, Error 
**errp)
     error_propagate(errp, err);
 }
 
+static bool check_vhost_features(VirtIONet *n, uint64_t features)
+{
+    NetClientState *nc = qemu_get_queue(n->nic);
+    uint64_t filtered;
+    if (n->rss_data.redirect) {
+        return false;
+    }
+    filtered = vhost_net_get_features(get_vhost_net(nc->peer), features);
+    if (filtered != features) {
+        return false;
+    }
+    return true;
+}
+
 static void virtio_net_set_features(VirtIODevice *vdev, uint64_t features)
 {
     VirtIONet *n = VIRTIO_NET(vdev);
     Error *err = NULL;
+    bool disable_vhost = false;
     int i;
 
     if (n->mtu_bypass_backend &&
@@ -894,13 +928,21 @@ static void virtio_net_set_features(VirtIODevice *vdev, 
uint64_t features)
                                                   VIRTIO_F_VERSION_1),
                                virtio_has_feature(features,
                                                   VIRTIO_NET_F_HASH_REPORT));
-
     n->rsc4_enabled = virtio_has_feature(features, VIRTIO_NET_F_RSC_EXT) &&
         virtio_has_feature(features, VIRTIO_NET_F_GUEST_TSO4);
     n->rsc6_enabled = virtio_has_feature(features, VIRTIO_NET_F_RSC_EXT) &&
         virtio_has_feature(features, VIRTIO_NET_F_GUEST_TSO6);
     n->rss_data.redirect = virtio_has_feature(features, VIRTIO_NET_F_RSS);
 
+    if (can_disable_vhost(n)) {
+        disable_vhost = !check_vhost_features(n, features);
+    }
+    if (disable_vhost) {
+        warn_report("Some of requested features aren't supported by vhost, "
+                    "vhost is turned off till next reset");
+        virtio_net_allow_vhost(n, false);
+    }
+
     if (n->has_vnet_hdr) {
         n->curr_guest_offloads =
             virtio_net_guest_offloads_by_features(features);
-- 
2.17.1