sim/net: Support to set the MTU of the sim netdevice.

Signed-off-by: liqinhui <liqinhui@xiaomi.com>

arch/sim/Kconfig

@@ -223,6 +223,14 @@ config SIM_NETDEV_VPNKIT
 
 endchoice
 
+config SIM_NETDEV_MTU
+    int "The MTU of Simulated Network Device"
+    default 1500
+    range 1280 65535 if NET_IPv6
+    range 576 65535 if !NET_IPv6
+    ---help---
+        The MTU of the network devices.
+
 config SIM_NETDEV_NUMBER
 	int "Number of Simulated Network Device"
 	default 1

arch/sim/src/sim/posix/sim_tapdev.c

@@ -249,10 +249,20 @@ void sim_tapdev_init(int devidx, void *priv,
       close(tapdevfd);
       return;
     }
-#else
+#endif
+
   memset(&ifr, 0, sizeof(ifr));
   strncpy(ifr.ifr_name, gdevname[devidx], IFNAMSIZ);
-#endif
+  ifr.ifr_mtu = CONFIG_SIM_NETDEV_MTU;
+  ret = ioctl(sockfd, SIOCSIFMTU, &ifr);
+  if (ret < 0)
+    {
+      syslog(LOG_ERR, "TAPDEV: ioctl failed (can't set MTU "
+                      "for %s): %d\n", gdevname[devidx], -ret);
+      close(sockfd);
+      close(tapdevfd);
+      return;
+    }
 
   ret = ioctl(sockfd, SIOCGIFMTU, &ifr);
   close(sockfd);

arch/sim/src/sim/sim_netdriver.c

@@ -71,7 +71,8 @@
 
 #include "sim_internal.h"
 
-#define SIM_NETDEV_BUFSIZE (MAX_NETDEV_PKTSIZE + CONFIG_NET_GUARDSIZE)
+#define SIM_NETDEV_BUFSIZE (CONFIG_SIM_NETDEV_MTU + ETH_HDRLEN + \
+                            CONFIG_NET_GUARDSIZE)
 
 /* We don't know packet length before receiving, so we can only offload it
  * when netpkt's buffer is long enough.