net: Allow network backends to advertise max TX queue size

This commit refactors how the maximum transmit queue size for
virtio-net devices is determined, making the mechanism more generic
and extensible.

Previously, virtio_net_max_tx_queue_size() contained hardcoded
checks for specific network backend types (vhost-user and
vhost-vdpa) to determine their supported maximum queue size. This
created direct dependencies and would require modifications for
every new backend that supports variable queue sizes.

To improve flexibility, a new max_tx_queue_size field is added
to the vhost_net structure. This allows each network backend
to advertise its supported maximum transmit queue size directly.

The virtio_net_max_tx_queue_size() function now retrieves the max
TX queue size from the vhost_net struct, if available and set.
Otherwise, it defaults to VIRTIO_NET_TX_QUEUE_DEFAULT_SIZE.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

hw/net/vhost_net.c

@@ -245,6 +245,7 @@ struct vhost_net *vhost_net_init(VhostNetOptions *options)
     net->dev.nvqs = options->nvqs;
     net->feature_bits = options->feature_bits;
     net->save_acked_features = options->save_acked_features;
+    net->max_tx_queue_size = options->max_tx_queue_size;
 
     net->dev.max_queues = 1;
     net->dev.vqs = net->vqs;

hw/net/virtio-net.c

@@ -670,22 +670,22 @@ static void virtio_net_set_mrg_rx_bufs(VirtIONet *n, int mergeable_rx_bufs,
 static int virtio_net_max_tx_queue_size(VirtIONet *n)
 {
     NetClientState *peer = n->nic_conf.peers.ncs[0];
+    struct vhost_net *net;
 
-    /*
-     * Backends other than vhost-user or vhost-vdpa don't support max queue
-     * size.
-     */
     if (!peer) {
-        return VIRTIO_NET_TX_QUEUE_DEFAULT_SIZE;
+        goto default_value;
     }
 
-    switch(peer->info->type) {
-    case NET_CLIENT_DRIVER_VHOST_USER:
-    case NET_CLIENT_DRIVER_VHOST_VDPA:
-        return VIRTQUEUE_MAX_SIZE;
-    default:
-        return VIRTIO_NET_TX_QUEUE_DEFAULT_SIZE;
-    };
+    net = get_vhost_net(peer);
+
+    if (!net || !net->max_tx_queue_size) {
+        goto default_value;
+    }
+
+    return net->max_tx_queue_size;
+
+default_value:
+    return VIRTIO_NET_TX_QUEUE_DEFAULT_SIZE;
 }
 
 static int peer_attach(VirtIONet *n, int index)

include/hw/virtio/vhost.h

@@ -145,6 +145,7 @@ struct vhost_net {
     struct vhost_virtqueue vqs[2];
     int backend;
     const int *feature_bits;
+    int max_tx_queue_size;
     SaveAcketFeatures *save_acked_features;
     NetClientState *nc;
 };

include/net/vhost_net.h

@@ -16,6 +16,7 @@ typedef struct VhostNetOptions {
     uint32_t busyloop_timeout;
     unsigned int nvqs;
     const int *feature_bits;
+    int max_tx_queue_size;
     GetAckedFeatures *get_acked_features;
     SaveAcketFeatures *save_acked_features;
     void *opaque;

net/tap.c

@@ -746,6 +746,7 @@ static void net_init_tap_one(const NetdevTapOptions *tap, NetClientState *peer,
         options.feature_bits = kernel_feature_bits;
         options.get_acked_features = NULL;
         options.save_acked_features = NULL;
+        options.max_tx_queue_size = 0;
 
         s->vhost_net = vhost_net_init(&options);
         if (!s->vhost_net) {

net/vhost-user.c

@@ -138,6 +138,7 @@ static int vhost_user_start(int queues, NetClientState *ncs[],
         options.busyloop_timeout = 0;
         options.nvqs = 2;
         options.feature_bits = user_feature_bits;
+        options.max_tx_queue_size = VIRTQUEUE_MAX_SIZE;
         options.get_acked_features = vhost_user_get_acked_features;
         options.save_acked_features = vhost_user_save_acked_features;
 

net/vhost-vdpa.c

@@ -204,6 +204,7 @@ static int vhost_vdpa_add(NetClientState *ncs, void *be,
     options.feature_bits = vdpa_feature_bits;
     options.get_acked_features = NULL;
     options.save_acked_features = NULL;
+    options.max_tx_queue_size = VIRTQUEUE_MAX_SIZE;
 
     net = vhost_net_init(&options);
     if (!net) {