[PATCH v2 2/3] net: add generic get_mtu NetClientInfo callback

[Nariman Sayed <narimansayed28@gmail.com>]

Add a get_mtu callback to NetClientInfo, along with a qemu_get_mtu()
wrapper following the existing qemu_has_ufo()-style pattern, so that
device models can query a netdev backend's host MTU generically
without needing to know or check its backend type.

Wire this up for the tap backend via tap_get_mtu(), which is now
static and only referenced through net_tap_info.get_mtu, matching
the existing pattern used by tap_has_ufo() and similar callbacks.

This allows other backends (e.g. vhost-user, vhost-vdpa) to
implement get_mtu in the future without requiring changes to any
code that calls qemu_get_mtu().

Suggested-by: Eugenio Perez Martin <eperezma@redhat.com>
Signed-off-by: Nariman Sayed <narimansayed28@gmail.com>
---
 include/net/net.h | 3 +++
 net/net.c         | 9 +++++++++
 net/tap.c         | 8 ++++++++
 3 files changed, 20 insertions(+)

diff --git a/include/net/net.h b/include/net/net.h
index 9edfacf827..ed06814644 100644
--- a/include/net/net.h
+++ b/include/net/net.h
@@ -72,6 +72,7 @@ typedef bool (HasVnetHdr)(NetClientState *);
 typedef bool (HasVnetHdrLen)(NetClientState *, int);
 typedef void (SetOffload)(NetClientState *, const NetOffloads *);
 typedef int (GetVnetHdrLen)(NetClientState *);
+typedef int (GetMtu)(NetClientState *);
 typedef void (SetVnetHdrLen)(NetClientState *, int);
 typedef bool (GetVnetHashSupportedTypes)(NetClientState *, uint32_t *);
 typedef int (SetVnetLE)(NetClientState *, bool);
@@ -110,6 +111,7 @@ typedef struct NetClientInfo {
     SetSteeringEBPF *set_steering_ebpf;
     NetCheckPeerType *check_peer_type;
     GetVHostNet *get_vhost_net;
+    GetMtu *get_mtu;
 } NetClientInfo;
 
 struct NetClientState {
@@ -204,6 +206,7 @@ bool qemu_has_uso(NetClientState *nc);
 bool qemu_has_tunnel(NetClientState *nc);
 bool qemu_has_vnet_hdr(NetClientState *nc);
 bool qemu_has_vnet_hdr_len(NetClientState *nc, int len);
+int qemu_get_mtu(NetClientState *nc);
 void qemu_set_offload(NetClientState *nc, const NetOffloads *ol);
 int qemu_get_vnet_hdr_len(NetClientState *nc);
 void qemu_set_vnet_hdr_len(NetClientState *nc, int len);
diff --git a/net/net.c b/net/net.c
index 0a30579ca4..9fb70e412a 100644
--- a/net/net.c
+++ b/net/net.c
@@ -514,6 +514,15 @@ bool qemu_has_ufo(NetClientState *nc)
     return nc->info->has_ufo(nc);
 }
 
+int qemu_get_mtu(NetClientState *nc)
+{
+    if (!nc || !nc->info->get_mtu) {
+        return -1;
+    }
+
+    return nc->info->get_mtu(nc);
+}
+
 bool qemu_has_uso(NetClientState *nc)
 {
     if (!nc || !nc->info->has_uso) {
diff --git a/net/tap.c b/net/tap.c
index 57ffb09885..89cec2accd 100644
--- a/net/tap.c
+++ b/net/tap.c
@@ -379,6 +379,13 @@ int tap_get_fd(NetClientState *nc)
     return s->fd;
 }
 
+static int tap_get_mtu(NetClientState *nc)
+{
+    TAPState *s = DO_UPCAST(TAPState, nc, nc);
+    assert(nc->info->type == NET_CLIENT_DRIVER_TAP);
+    return tap_fd_get_mtu(s->fd);
+}
+
 /*
  * tap_get_vhost_net() can return NULL if a tap net-device backend is
  * created with 'vhost=off' option, 'vhostforce=off' or no vhost or
@@ -411,6 +418,7 @@ static NetClientInfo net_tap_info = {
     .set_vnet_be = tap_set_vnet_be,
     .set_steering_ebpf = tap_set_steering_ebpf,
     .get_vhost_net = tap_get_vhost_net,
+    .get_mtu = tap_get_mtu,
 };
 
 static TAPState *net_tap_fd_init(NetClientState *peer,
-- 
2.34.1