vhost-net: vhost-kernel: introduce vhost_net_virtqueue_restart()

Introduce vhost_net_virtqueue_restart(), which can restart the
specific virtqueue when the vhost net started running before.
If it fails to restart the virtqueue, the device will be stopped.

Here we do not reuse vhost_net_start_one() or vhost_dev_start()
because they work at queue pair level. The mem table and features
do not change, so we can call the vhost_virtqueue_start() to
restart a specific queue.

This patch only considers the case of vhost-kernel, when
NetClientDriver is NET_CLIENT_DRIVER_TAP.

Signed-off-by: Kangjie Xu <kangjie.xu@linux.alibaba.com>
Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20221017092558.111082-11-xuanzhuo@linux.alibaba.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/net/vhost_net-stub.c

@@ -107,3 +107,9 @@ void vhost_net_virtqueue_reset(VirtIODevice *vdev, NetClientState *nc,
 {
 
 }
+
+int vhost_net_virtqueue_restart(VirtIODevice *vdev, NetClientState *nc,
+                                int vq_index)
+{
+    return 0;
+}

hw/net/vhost_net.c

@@ -34,6 +34,7 @@
 #include "standard-headers/linux/virtio_ring.h"
 #include "hw/virtio/vhost.h"
 #include "hw/virtio/virtio-bus.h"
+#include "linux-headers/linux/vhost.h"
 
 
 /* Features supported by host kernel. */
@@ -556,3 +557,55 @@ void vhost_net_virtqueue_reset(VirtIODevice *vdev, NetClientState *nc,
                          net->dev.vqs + idx,
                          net->dev.vq_index + idx);
 }
+
+int vhost_net_virtqueue_restart(VirtIODevice *vdev, NetClientState *nc,
+                                int vq_index)
+{
+    VHostNetState *net = get_vhost_net(nc->peer);
+    const VhostOps *vhost_ops = net->dev.vhost_ops;
+    struct vhost_vring_file file = { };
+    int idx, r;
+
+    if (!net->dev.started) {
+        return -EBUSY;
+    }
+
+    /* should only be called after backend is connected */
+    assert(vhost_ops);
+
+    idx = vhost_ops->vhost_get_vq_index(&net->dev, vq_index);
+
+    r = vhost_virtqueue_start(&net->dev,
+                              vdev,
+                              net->dev.vqs + idx,
+                              net->dev.vq_index + idx);
+    if (r < 0) {
+        goto err_start;
+    }
+
+    if (net->nc->info->type == NET_CLIENT_DRIVER_TAP) {
+        file.index = idx;
+        file.fd = net->backend;
+        r = vhost_net_set_backend(&net->dev, &file);
+        if (r < 0) {
+            r = -errno;
+            goto err_start;
+        }
+    }
+
+    return 0;
+
+err_start:
+    error_report("Error when restarting the queue.");
+
+    if (net->nc->info->type == NET_CLIENT_DRIVER_TAP) {
+        file.fd = VHOST_FILE_UNBIND;
+        file.index = idx;
+        int r = vhost_net_set_backend(&net->dev, &file);
+        assert(r >= 0);
+    }
+
+    vhost_dev_stop(&net->dev, vdev);
+
+    return r;
+}

include/net/vhost_net.h

@@ -50,4 +50,6 @@ int vhost_net_set_mtu(struct vhost_net *net, uint16_t mtu);
 
 void vhost_net_virtqueue_reset(VirtIODevice *vdev, NetClientState *nc,
                                int vq_index);
+int vhost_net_virtqueue_restart(VirtIODevice *vdev, NetClientState *nc,
+                                int vq_index);
 #endif