[PATCH v3 4/9] vhost: Add API to get MTU value

[Maxime Coquelin <maxime.coquelin@redhat.com>]

This patch implements the function for the application to
get the MTU value.

rte_vhost_get_mtu() fills the mtu parameter with the MTU value
set in QEMU if VIRTIO_NET_F_MTU has been negotiated and returns 0,
-ENOTSUP otherwise.

The function returns -EAGAIN if Virtio feature negotiation
didn't happened yet.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
---
 lib/librte_vhost/rte_virtio_net.h | 15 +++++++++++++++
 lib/librte_vhost/vhost.c          | 19 +++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/lib/librte_vhost/rte_virtio_net.h b/lib/librte_vhost/rte_virtio_net.h
index 926039c..56829aa 100644
--- a/lib/librte_vhost/rte_virtio_net.h
+++ b/lib/librte_vhost/rte_virtio_net.h
@@ -100,6 +100,21 @@ int rte_vhost_driver_callback_register(struct virtio_net_device_ops const * cons
 int rte_vhost_driver_session_start(void);
 
 /**
+ * Get the MTU value of the device if set in QEMU.
+ *
+ * @param vid
+ *  virtio-net device ID
+ * @param mtu
+ *  The variable to store the MTU value
+ *
+ * @return
+ *  0: success
+ *  -EAGAIN: device not yet started
+ *  -ENOTSUP: device does not support MTU feature
+ */
+int rte_vhost_get_mtu(int vid, uint16_t *mtu);
+
+/**
  * Get the numa node from which the virtio net device's memory
  * is allocated.
  *
diff --git a/lib/librte_vhost/vhost.c b/lib/librte_vhost/vhost.c
index 3974087..80a75e0 100644
--- a/lib/librte_vhost/vhost.c
+++ b/lib/librte_vhost/vhost.c
@@ -313,6 +313,25 @@ vhost_enable_dequeue_zero_copy(int vid)
 }
 
 int
+rte_vhost_get_mtu(int vid, uint16_t *mtu)
+{
+	struct virtio_net *dev = get_device(vid);
+
+	if (!dev)
+		return -ENODEV;
+
+	if (!(dev->flags & VIRTIO_DEV_READY))
+		return -EAGAIN;
+
+	if (!(dev->features & VIRTIO_NET_F_MTU))
+		return -ENOTSUP;
+
+	*mtu = dev->mtu;
+
+	return 0;
+}
+
+int
 rte_vhost_get_numa_node(int vid)
 {
 #ifdef RTE_LIBRTE_VHOST_NUMA
-- 
2.9.3