[RFC PATCH v2 3/8] vhost_net: Emulate link state up if backend doesn't expose it

["Eugenio Pérez" <eperezma@redhat.com>]

At this moment this code path is not reached, but vdpa devices can offer
VIRTIO_NET_F_STATUS unconditionally. While the guest must assume that
link is always up by the standard, qemu will set the status bit to 1
always in this case.

This makes little use by itself, but VIRTIO_NET_F_STATUS is needed for
the guest to read status bit VIRTIO_NET_F_GUEST_ANNOUNCE, used by feature
VIRTIO_NET_F_GUEST_ANNOUNCE. So qemu must emulate status feature in case
it needs to emulate the guest announce feature.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
---
 hw/net/vhost_net.c | 27 ++++++++++++++++++++++++++-
 1 file changed, 26 insertions(+), 1 deletion(-)

diff --git a/hw/net/vhost_net.c b/hw/net/vhost_net.c
index d28f8b974b..5660606c1d 100644
--- a/hw/net/vhost_net.c
+++ b/hw/net/vhost_net.c
@@ -117,7 +117,32 @@ uint64_t vhost_net_get_features(struct vhost_net *net, uint64_t features)
 int vhost_net_get_config(struct vhost_net *net,  uint8_t *config,
                          uint32_t config_len)
 {
-    return vhost_dev_get_config(&net->dev, config, config_len, NULL);
+    VirtIODevice *vdev;
+    int r = vhost_dev_get_config(&net->dev, config, config_len, NULL);
+
+    if (unlikely(r != 0)) {
+        return r;
+    }
+
+    if (config_len < endof(struct virtio_net_config, status)) {
+        return 0;
+    }
+
+    /*
+     * TODO: Perform this only if vhost_vdpa.
+     */
+    vdev = net->dev.vdev;
+    if (!vdev) {
+        /* Device is starting */
+        return 0;
+    }
+
+    if ((net->dev.acked_features & BIT_ULL(VIRTIO_NET_F_STATUS)) &&
+        !(net->dev.features & BIT_ULL(VIRTIO_NET_F_STATUS))) {
+        ((struct virtio_net_config *)config)->status |= VIRTIO_NET_S_LINK_UP;
+    }
+
+    return 0;
 }
 int vhost_net_set_config(struct vhost_net *net, const uint8_t *data,
                          uint32_t offset, uint32_t size, uint32_t flags)
-- 
2.31.1