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>

2 files changed, 2 insertions, 0 deletions

diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
index b0830ba..a62992c 100644
--- a/include/hw/virtio/vhost.h
+++ b/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;
 };
diff --git a/include/net/vhost_net.h b/include/net/vhost_net.h
index eb26ed9..8f4fddf 100644
--- a/include/net/vhost_net.h
+++ b/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;