Patchwork net/socket: remove hardcoded packet size in favor of new mtu parameter

login
register
mail settings
Submitter Nguyễn Thái Ngọc Duy
Date April 22, 2011, 12:59 p.m.
Message ID <1303477168-4663-1-git-send-email-pclouds@gmail.com>
Download mbox | patch
Permalink /patch/92530/
State New
Headers show

Comments

Nguyễn Thái Ngọc Duy - April 22, 2011, 12:59 p.m.
Also mention the default value 4096.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 free new buffers in net_socket_cleanup()

 net.c           |    4 ++++
 net/socket.c    |   52 ++++++++++++++++++++++++++++++++++++----------------
 qemu-options.hx |   11 ++++++-----
 3 files changed, 46 insertions(+), 21 deletions(-)
Stefan Hajnoczi - April 23, 2011, 8:30 a.m.
2011/4/22 Nguyễn Thái Ngọc Duy <pclouds@gmail.com>:
> Also mention the default value 4096.
>
> Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
> ---
>  free new buffers in net_socket_cleanup()
>
>  net.c           |    4 ++++
>  net/socket.c    |   52 ++++++++++++++++++++++++++++++++++++----------------
>  qemu-options.hx |   11 ++++++-----
>  3 files changed, 46 insertions(+), 21 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Michael S. Tsirkin - May 15, 2011, 8:17 p.m.
On Fri, Apr 22, 2011 at 07:59:28PM +0700, Nguyễn Thái Ngọc Duy wrote:
> Also mention the default value 4096.
> 
> Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>

IS the option really necessary? Why not just allocate 128K or so and be
done with it? That should be big enough even with GSO etc.

> ---
>  free new buffers in net_socket_cleanup()
> 
>  net.c           |    4 ++++
>  net/socket.c    |   52 ++++++++++++++++++++++++++++++++++++----------------
>  qemu-options.hx |   11 ++++++-----
>  3 files changed, 46 insertions(+), 21 deletions(-)
> 
> diff --git a/net.c b/net.c
> index 8d6a555..6746bc7 100644
> --- a/net.c
> +++ b/net.c
> @@ -1001,6 +1001,10 @@ static const struct {
>                  .name = "localaddr",
>                  .type = QEMU_OPT_STRING,
>                  .help = "source address for multicast packets",
> +            }, {
> +                .name = "mtu",
> +                .type = QEMU_OPT_NUMBER,
> +                .help = "MTU size",
>              },
>              { /* end of list */ }
>          },
> diff --git a/net/socket.c b/net/socket.c
> index 7337f4f..e932064 100644
> --- a/net/socket.c
> +++ b/net/socket.c
> @@ -38,7 +38,8 @@ typedef struct NetSocketState {
>      int state; /* 0 = getting length, 1 = getting data */
>      unsigned int index;
>      unsigned int packet_len;
> -    uint8_t buf[4096];
> +    unsigned int mtu;
> +    uint8_t *buf, *buf1;
>      struct sockaddr_in dgram_dst; /* contains inet host and port destination iff connectionless (SOCK_DGRAM) */
>  } NetSocketState;
>  
> @@ -47,6 +48,7 @@ typedef struct NetSocketListenState {
>      char *model;
>      char *name;
>      int fd;
> +    unsigned int mtu;
>  } NetSocketListenState;
>  
>  /* XXX: we consider we can send the whole packet without blocking */
> @@ -73,10 +75,10 @@ static void net_socket_send(void *opaque)
>      NetSocketState *s = opaque;
>      int size, err;
>      unsigned l;
> -    uint8_t buf1[4096];
> +    uint8_t *buf1 = s->buf1;
>      const uint8_t *buf;
>  
> -    size = recv(s->fd, (void *)buf1, sizeof(buf1), 0);
> +    size = recv(s->fd, (void *)buf1, s->mtu, 0);
>      if (size < 0) {
>          err = socket_error();
>          if (err != EWOULDBLOCK)
> @@ -111,7 +113,7 @@ static void net_socket_send(void *opaque)
>              l = s->packet_len - s->index;
>              if (l > size)
>                  l = size;
> -            if (s->index + l <= sizeof(s->buf)) {
> +            if (s->index + l <= s->mtu) {
>                  memcpy(s->buf + s->index, buf, l);
>              } else {
>                  fprintf(stderr, "serious error: oversized packet received,"
> @@ -138,7 +140,7 @@ static void net_socket_send_dgram(void *opaque)
>      NetSocketState *s = opaque;
>      int size;
>  
> -    size = recv(s->fd, (void *)s->buf, sizeof(s->buf), 0);
> +    size = recv(s->fd, (void *)s->buf, s->mtu, 0);
>      if (size < 0)
>          return;
>      if (size == 0) {
> @@ -228,6 +230,8 @@ static void net_socket_cleanup(VLANClientState *nc)
>      NetSocketState *s = DO_UPCAST(NetSocketState, nc, nc);
>      qemu_set_fd_handler(s->fd, NULL, NULL, NULL);
>      close(s->fd);
> +    qemu_free(s->buf);
> +    qemu_free(s->buf1);
>  }
>  
>  static NetClientInfo net_dgram_socket_info = {
> @@ -238,6 +242,7 @@ static NetClientInfo net_dgram_socket_info = {
>  };
>  
>  static NetSocketState *net_socket_fd_init_dgram(VLANState *vlan,
> +                                                unsigned int mtu,
>                                                  const char *model,
>                                                  const char *name,
>                                                  int fd, int is_connected)
> @@ -288,6 +293,10 @@ static NetSocketState *net_socket_fd_init_dgram(VLANState *vlan,
>  
>      s = DO_UPCAST(NetSocketState, nc, nc);
>  
> +    s->mtu = mtu;
> +    s->buf = qemu_malloc(s->mtu);
> +    s->buf1 = qemu_malloc(s->mtu);
> +
>      s->fd = fd;
>  
>      qemu_set_fd_handler(s->fd, net_socket_send_dgram, NULL, s);
> @@ -312,6 +321,7 @@ static NetClientInfo net_socket_info = {
>  };
>  
>  static NetSocketState *net_socket_fd_init_stream(VLANState *vlan,
> +                                                 unsigned int mtu,
>                                                   const char *model,
>                                                   const char *name,
>                                                   int fd, int is_connected)
> @@ -325,6 +335,10 @@ static NetSocketState *net_socket_fd_init_stream(VLANState *vlan,
>  
>      s = DO_UPCAST(NetSocketState, nc, nc);
>  
> +    s->mtu = mtu;
> +    s->buf = qemu_malloc(s->mtu);
> +    s->buf1 = qemu_malloc(s->mtu);
> +
>      s->fd = fd;
>  
>      if (is_connected) {
> @@ -335,7 +349,7 @@ static NetSocketState *net_socket_fd_init_stream(VLANState *vlan,
>      return s;
>  }
>  
> -static NetSocketState *net_socket_fd_init(VLANState *vlan,
> +static NetSocketState *net_socket_fd_init(VLANState *vlan, unsigned int mtu,
>                                            const char *model, const char *name,
>                                            int fd, int is_connected)
>  {
> @@ -348,13 +362,13 @@ static NetSocketState *net_socket_fd_init(VLANState *vlan,
>      }
>      switch(so_type) {
>      case SOCK_DGRAM:
> -        return net_socket_fd_init_dgram(vlan, model, name, fd, is_connected);
> +        return net_socket_fd_init_dgram(vlan, mtu, model, name, fd, is_connected);
>      case SOCK_STREAM:
> -        return net_socket_fd_init_stream(vlan, model, name, fd, is_connected);
> +        return net_socket_fd_init_stream(vlan, mtu, model, name, fd, is_connected);
>      default:
>          /* who knows ... this could be a eg. a pty, do warn and continue as stream */
>          fprintf(stderr, "qemu: warning: socket type=%d for fd=%d is not SOCK_DGRAM or SOCK_STREAM\n", so_type, fd);
> -        return net_socket_fd_init_stream(vlan, model, name, fd, is_connected);
> +        return net_socket_fd_init_stream(vlan, mtu, model, name, fd, is_connected);
>      }
>      return NULL;
>  }
> @@ -376,7 +390,7 @@ static void net_socket_accept(void *opaque)
>              break;
>          }
>      }
> -    s1 = net_socket_fd_init(s->vlan, s->model, s->name, fd, 1);
> +    s1 = net_socket_fd_init(s->vlan, s->mtu, s->model, s->name, fd, 1);
>      if (!s1) {
>          closesocket(fd);
>      } else {
> @@ -387,6 +401,7 @@ static void net_socket_accept(void *opaque)
>  }
>  
>  static int net_socket_listen_init(VLANState *vlan,
> +                                  unsigned int mtu,
>                                    const char *model,
>                                    const char *name,
>                                    const char *host_str)
> @@ -425,11 +440,13 @@ static int net_socket_listen_init(VLANState *vlan,
>      s->model = qemu_strdup(model);
>      s->name = name ? qemu_strdup(name) : NULL;
>      s->fd = fd;
> +    s->mtu = mtu;
>      qemu_set_fd_handler(fd, net_socket_accept, NULL, s);
>      return 0;
>  }
>  
>  static int net_socket_connect_init(VLANState *vlan,
> +                                   unsigned int mtu,
>                                     const char *model,
>                                     const char *name,
>                                     const char *host_str)
> @@ -470,7 +487,7 @@ static int net_socket_connect_init(VLANState *vlan,
>              break;
>          }
>      }
> -    s = net_socket_fd_init(vlan, model, name, fd, connected);
> +    s = net_socket_fd_init(vlan, mtu, model, name, fd, connected);
>      if (!s)
>          return -1;
>      snprintf(s->nc.info_str, sizeof(s->nc.info_str),
> @@ -480,6 +497,7 @@ static int net_socket_connect_init(VLANState *vlan,
>  }
>  
>  static int net_socket_mcast_init(VLANState *vlan,
> +                                 unsigned int mtu,
>                                   const char *model,
>                                   const char *name,
>                                   const char *host_str,
> @@ -505,7 +523,7 @@ static int net_socket_mcast_init(VLANState *vlan,
>      if (fd < 0)
>  	return -1;
>  
> -    s = net_socket_fd_init(vlan, model, name, fd, 0);
> +    s = net_socket_fd_init(vlan, mtu, model, name, fd, 0);
>      if (!s)
>          return -1;
>  
> @@ -523,6 +541,8 @@ int net_init_socket(QemuOpts *opts,
>                      const char *name,
>                      VLANState *vlan)
>  {
> +    unsigned int mtu = qemu_opt_get_number(opts, "mtu", 4096);
> +
>      if (qemu_opt_get(opts, "fd")) {
>          int fd;
>  
> @@ -539,7 +559,7 @@ int net_init_socket(QemuOpts *opts,
>              return -1;
>          }
>  
> -        if (!net_socket_fd_init(vlan, "socket", name, fd, 1)) {
> +        if (!net_socket_fd_init(vlan, mtu, "socket", name, fd, 1)) {
>              close(fd);
>              return -1;
>          }
> @@ -556,7 +576,7 @@ int net_init_socket(QemuOpts *opts,
>  
>          listen = qemu_opt_get(opts, "listen");
>  
> -        if (net_socket_listen_init(vlan, "socket", name, listen) == -1) {
> +        if (net_socket_listen_init(vlan, mtu, "socket", name, listen) == -1) {
>              return -1;
>          }
>      } else if (qemu_opt_get(opts, "connect")) {
> @@ -572,7 +592,7 @@ int net_init_socket(QemuOpts *opts,
>  
>          connect = qemu_opt_get(opts, "connect");
>  
> -        if (net_socket_connect_init(vlan, "socket", name, connect) == -1) {
> +        if (net_socket_connect_init(vlan, mtu, "socket", name, connect) == -1) {
>              return -1;
>          }
>      } else if (qemu_opt_get(opts, "mcast")) {
> @@ -588,7 +608,7 @@ int net_init_socket(QemuOpts *opts,
>          mcast = qemu_opt_get(opts, "mcast");
>          localaddr = qemu_opt_get(opts, "localaddr");
>  
> -        if (net_socket_mcast_init(vlan, "socket", name, mcast, localaddr) == -1) {
> +        if (net_socket_mcast_init(vlan, mtu, "socket", name, mcast, localaddr) == -1) {
>              return -1;
>          }
>      } else {
> diff --git a/qemu-options.hx b/qemu-options.hx
> index 66fffe3..4d5af96 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -1095,9 +1095,9 @@ DEF("net", HAS_ARG, QEMU_OPTION_net,
>      "                use vhostforce=on to force vhost on for non-MSIX virtio guests\n"
>      "                use 'vhostfd=h' to connect to an already opened vhost net device\n"
>  #endif
> -    "-net socket[,vlan=n][,name=str][,fd=h][,listen=[host]:port][,connect=host:port]\n"
> +    "-net socket[,vlan=n][,name=str][,fd=h][,listen=[host]:port][,connect=host:port][,mtu=mtu]\n"
>      "                connect the vlan 'n' to another VLAN using a socket connection\n"
> -    "-net socket[,vlan=n][,name=str][,fd=h][,mcast=maddr:port[,localaddr=addr]]\n"
> +    "-net socket[,vlan=n][,name=str][,fd=h][,mcast=maddr:port[,localaddr=addr]][,mtu=mtu]\n"
>      "                connect the vlan 'n' to multicast maddr and port\n"
>      "                use 'localaddr=addr' to specify the host address to send packets from\n"
>  #ifdef CONFIG_VDE
> @@ -1273,14 +1273,14 @@ qemu linux.img -net nic,vlan=0 -net tap,vlan=0,ifname=tap0 \
>                 -net nic,vlan=1 -net tap,vlan=1,ifname=tap1
>  @end example
>  
> -@item -net socket[,vlan=@var{n}][,name=@var{name}][,fd=@var{h}] [,listen=[@var{host}]:@var{port}][,connect=@var{host}:@var{port}]
> +@item -net socket[,vlan=@var{n}][,name=@var{name}][,fd=@var{h}] [,listen=[@var{host}]:@var{port}][,connect=@var{host}:@var{port}][,mtu=@var{mtu}]
>  
>  Connect the VLAN @var{n} to a remote VLAN in another QEMU virtual
>  machine using a TCP socket connection. If @option{listen} is
>  specified, QEMU waits for incoming connections on @var{port}
>  (@var{host} is optional). @option{connect} is used to connect to
>  another QEMU instance using the @option{listen} option. @option{fd}=@var{h}
> -specifies an already opened TCP socket.
> +specifies an already opened TCP socket. MTU is 4096 by default.
>  
>  Example:
>  @example
> @@ -1293,11 +1293,12 @@ qemu linux.img -net nic,macaddr=52:54:00:12:34:57 \
>                 -net socket,connect=127.0.0.1:1234
>  @end example
>  
> -@item -net socket[,vlan=@var{n}][,name=@var{name}][,fd=@var{h}][,mcast=@var{maddr}:@var{port}[,localaddr=@var{addr}]]
> +@item -net socket[,vlan=@var{n}][,name=@var{name}][,fd=@var{h}][,mcast=@var{maddr}:@var{port}[,localaddr=@var{addr}]][,mtu=@var{mtu}]
>  
>  Create a VLAN @var{n} shared with another QEMU virtual
>  machines using a UDP multicast socket, effectively making a bus for
>  every QEMU with same multicast address @var{maddr} and @var{port}.
> +MTU is 4096 by default.
>  NOTES:
>  @enumerate
>  @item
> -- 
> 1.7.4.74.g639db
>
Nguyễn Thái Ngọc Duy - May 16, 2011, 1:31 a.m.
2011/5/16 Michael S. Tsirkin <mst@redhat.com>:
> On Fri, Apr 22, 2011 at 07:59:28PM +0700, Nguyễn Thái Ngọc Duy wrote:
>> Also mention the default value 4096.
>>
>> Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
>
> IS the option really necessary? Why not just allocate 128K or so and be
> done with it? That should be big enough even with GSO etc.

If it's big enough for practically everything, then yes, that way
would be simpler.

Patch

diff --git a/net.c b/net.c
index 8d6a555..6746bc7 100644
--- a/net.c
+++ b/net.c
@@ -1001,6 +1001,10 @@  static const struct {
                 .name = "localaddr",
                 .type = QEMU_OPT_STRING,
                 .help = "source address for multicast packets",
+            }, {
+                .name = "mtu",
+                .type = QEMU_OPT_NUMBER,
+                .help = "MTU size",
             },
             { /* end of list */ }
         },
diff --git a/net/socket.c b/net/socket.c
index 7337f4f..e932064 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -38,7 +38,8 @@  typedef struct NetSocketState {
     int state; /* 0 = getting length, 1 = getting data */
     unsigned int index;
     unsigned int packet_len;
-    uint8_t buf[4096];
+    unsigned int mtu;
+    uint8_t *buf, *buf1;
     struct sockaddr_in dgram_dst; /* contains inet host and port destination iff connectionless (SOCK_DGRAM) */
 } NetSocketState;
 
@@ -47,6 +48,7 @@  typedef struct NetSocketListenState {
     char *model;
     char *name;
     int fd;
+    unsigned int mtu;
 } NetSocketListenState;
 
 /* XXX: we consider we can send the whole packet without blocking */
@@ -73,10 +75,10 @@  static void net_socket_send(void *opaque)
     NetSocketState *s = opaque;
     int size, err;
     unsigned l;
-    uint8_t buf1[4096];
+    uint8_t *buf1 = s->buf1;
     const uint8_t *buf;
 
-    size = recv(s->fd, (void *)buf1, sizeof(buf1), 0);
+    size = recv(s->fd, (void *)buf1, s->mtu, 0);
     if (size < 0) {
         err = socket_error();
         if (err != EWOULDBLOCK)
@@ -111,7 +113,7 @@  static void net_socket_send(void *opaque)
             l = s->packet_len - s->index;
             if (l > size)
                 l = size;
-            if (s->index + l <= sizeof(s->buf)) {
+            if (s->index + l <= s->mtu) {
                 memcpy(s->buf + s->index, buf, l);
             } else {
                 fprintf(stderr, "serious error: oversized packet received,"
@@ -138,7 +140,7 @@  static void net_socket_send_dgram(void *opaque)
     NetSocketState *s = opaque;
     int size;
 
-    size = recv(s->fd, (void *)s->buf, sizeof(s->buf), 0);
+    size = recv(s->fd, (void *)s->buf, s->mtu, 0);
     if (size < 0)
         return;
     if (size == 0) {
@@ -228,6 +230,8 @@  static void net_socket_cleanup(VLANClientState *nc)
     NetSocketState *s = DO_UPCAST(NetSocketState, nc, nc);
     qemu_set_fd_handler(s->fd, NULL, NULL, NULL);
     close(s->fd);
+    qemu_free(s->buf);
+    qemu_free(s->buf1);
 }
 
 static NetClientInfo net_dgram_socket_info = {
@@ -238,6 +242,7 @@  static NetClientInfo net_dgram_socket_info = {
 };
 
 static NetSocketState *net_socket_fd_init_dgram(VLANState *vlan,
+                                                unsigned int mtu,
                                                 const char *model,
                                                 const char *name,
                                                 int fd, int is_connected)
@@ -288,6 +293,10 @@  static NetSocketState *net_socket_fd_init_dgram(VLANState *vlan,
 
     s = DO_UPCAST(NetSocketState, nc, nc);
 
+    s->mtu = mtu;
+    s->buf = qemu_malloc(s->mtu);
+    s->buf1 = qemu_malloc(s->mtu);
+
     s->fd = fd;
 
     qemu_set_fd_handler(s->fd, net_socket_send_dgram, NULL, s);
@@ -312,6 +321,7 @@  static NetClientInfo net_socket_info = {
 };
 
 static NetSocketState *net_socket_fd_init_stream(VLANState *vlan,
+                                                 unsigned int mtu,
                                                  const char *model,
                                                  const char *name,
                                                  int fd, int is_connected)
@@ -325,6 +335,10 @@  static NetSocketState *net_socket_fd_init_stream(VLANState *vlan,
 
     s = DO_UPCAST(NetSocketState, nc, nc);
 
+    s->mtu = mtu;
+    s->buf = qemu_malloc(s->mtu);
+    s->buf1 = qemu_malloc(s->mtu);
+
     s->fd = fd;
 
     if (is_connected) {
@@ -335,7 +349,7 @@  static NetSocketState *net_socket_fd_init_stream(VLANState *vlan,
     return s;
 }
 
-static NetSocketState *net_socket_fd_init(VLANState *vlan,
+static NetSocketState *net_socket_fd_init(VLANState *vlan, unsigned int mtu,
                                           const char *model, const char *name,
                                           int fd, int is_connected)
 {
@@ -348,13 +362,13 @@  static NetSocketState *net_socket_fd_init(VLANState *vlan,
     }
     switch(so_type) {
     case SOCK_DGRAM:
-        return net_socket_fd_init_dgram(vlan, model, name, fd, is_connected);
+        return net_socket_fd_init_dgram(vlan, mtu, model, name, fd, is_connected);
     case SOCK_STREAM:
-        return net_socket_fd_init_stream(vlan, model, name, fd, is_connected);
+        return net_socket_fd_init_stream(vlan, mtu, model, name, fd, is_connected);
     default:
         /* who knows ... this could be a eg. a pty, do warn and continue as stream */
         fprintf(stderr, "qemu: warning: socket type=%d for fd=%d is not SOCK_DGRAM or SOCK_STREAM\n", so_type, fd);
-        return net_socket_fd_init_stream(vlan, model, name, fd, is_connected);
+        return net_socket_fd_init_stream(vlan, mtu, model, name, fd, is_connected);
     }
     return NULL;
 }
@@ -376,7 +390,7 @@  static void net_socket_accept(void *opaque)
             break;
         }
     }
-    s1 = net_socket_fd_init(s->vlan, s->model, s->name, fd, 1);
+    s1 = net_socket_fd_init(s->vlan, s->mtu, s->model, s->name, fd, 1);
     if (!s1) {
         closesocket(fd);
     } else {
@@ -387,6 +401,7 @@  static void net_socket_accept(void *opaque)
 }
 
 static int net_socket_listen_init(VLANState *vlan,
+                                  unsigned int mtu,
                                   const char *model,
                                   const char *name,
                                   const char *host_str)
@@ -425,11 +440,13 @@  static int net_socket_listen_init(VLANState *vlan,
     s->model = qemu_strdup(model);
     s->name = name ? qemu_strdup(name) : NULL;
     s->fd = fd;
+    s->mtu = mtu;
     qemu_set_fd_handler(fd, net_socket_accept, NULL, s);
     return 0;
 }
 
 static int net_socket_connect_init(VLANState *vlan,
+                                   unsigned int mtu,
                                    const char *model,
                                    const char *name,
                                    const char *host_str)
@@ -470,7 +487,7 @@  static int net_socket_connect_init(VLANState *vlan,
             break;
         }
     }
-    s = net_socket_fd_init(vlan, model, name, fd, connected);
+    s = net_socket_fd_init(vlan, mtu, model, name, fd, connected);
     if (!s)
         return -1;
     snprintf(s->nc.info_str, sizeof(s->nc.info_str),
@@ -480,6 +497,7 @@  static int net_socket_connect_init(VLANState *vlan,
 }
 
 static int net_socket_mcast_init(VLANState *vlan,
+                                 unsigned int mtu,
                                  const char *model,
                                  const char *name,
                                  const char *host_str,
@@ -505,7 +523,7 @@  static int net_socket_mcast_init(VLANState *vlan,
     if (fd < 0)
 	return -1;
 
-    s = net_socket_fd_init(vlan, model, name, fd, 0);
+    s = net_socket_fd_init(vlan, mtu, model, name, fd, 0);
     if (!s)
         return -1;
 
@@ -523,6 +541,8 @@  int net_init_socket(QemuOpts *opts,
                     const char *name,
                     VLANState *vlan)
 {
+    unsigned int mtu = qemu_opt_get_number(opts, "mtu", 4096);
+
     if (qemu_opt_get(opts, "fd")) {
         int fd;
 
@@ -539,7 +559,7 @@  int net_init_socket(QemuOpts *opts,
             return -1;
         }
 
-        if (!net_socket_fd_init(vlan, "socket", name, fd, 1)) {
+        if (!net_socket_fd_init(vlan, mtu, "socket", name, fd, 1)) {
             close(fd);
             return -1;
         }
@@ -556,7 +576,7 @@  int net_init_socket(QemuOpts *opts,
 
         listen = qemu_opt_get(opts, "listen");
 
-        if (net_socket_listen_init(vlan, "socket", name, listen) == -1) {
+        if (net_socket_listen_init(vlan, mtu, "socket", name, listen) == -1) {
             return -1;
         }
     } else if (qemu_opt_get(opts, "connect")) {
@@ -572,7 +592,7 @@  int net_init_socket(QemuOpts *opts,
 
         connect = qemu_opt_get(opts, "connect");
 
-        if (net_socket_connect_init(vlan, "socket", name, connect) == -1) {
+        if (net_socket_connect_init(vlan, mtu, "socket", name, connect) == -1) {
             return -1;
         }
     } else if (qemu_opt_get(opts, "mcast")) {
@@ -588,7 +608,7 @@  int net_init_socket(QemuOpts *opts,
         mcast = qemu_opt_get(opts, "mcast");
         localaddr = qemu_opt_get(opts, "localaddr");
 
-        if (net_socket_mcast_init(vlan, "socket", name, mcast, localaddr) == -1) {
+        if (net_socket_mcast_init(vlan, mtu, "socket", name, mcast, localaddr) == -1) {
             return -1;
         }
     } else {
diff --git a/qemu-options.hx b/qemu-options.hx
index 66fffe3..4d5af96 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -1095,9 +1095,9 @@  DEF("net", HAS_ARG, QEMU_OPTION_net,
     "                use vhostforce=on to force vhost on for non-MSIX virtio guests\n"
     "                use 'vhostfd=h' to connect to an already opened vhost net device\n"
 #endif
-    "-net socket[,vlan=n][,name=str][,fd=h][,listen=[host]:port][,connect=host:port]\n"
+    "-net socket[,vlan=n][,name=str][,fd=h][,listen=[host]:port][,connect=host:port][,mtu=mtu]\n"
     "                connect the vlan 'n' to another VLAN using a socket connection\n"
-    "-net socket[,vlan=n][,name=str][,fd=h][,mcast=maddr:port[,localaddr=addr]]\n"
+    "-net socket[,vlan=n][,name=str][,fd=h][,mcast=maddr:port[,localaddr=addr]][,mtu=mtu]\n"
     "                connect the vlan 'n' to multicast maddr and port\n"
     "                use 'localaddr=addr' to specify the host address to send packets from\n"
 #ifdef CONFIG_VDE
@@ -1273,14 +1273,14 @@  qemu linux.img -net nic,vlan=0 -net tap,vlan=0,ifname=tap0 \
                -net nic,vlan=1 -net tap,vlan=1,ifname=tap1
 @end example
 
-@item -net socket[,vlan=@var{n}][,name=@var{name}][,fd=@var{h}] [,listen=[@var{host}]:@var{port}][,connect=@var{host}:@var{port}]
+@item -net socket[,vlan=@var{n}][,name=@var{name}][,fd=@var{h}] [,listen=[@var{host}]:@var{port}][,connect=@var{host}:@var{port}][,mtu=@var{mtu}]
 
 Connect the VLAN @var{n} to a remote VLAN in another QEMU virtual
 machine using a TCP socket connection. If @option{listen} is
 specified, QEMU waits for incoming connections on @var{port}
 (@var{host} is optional). @option{connect} is used to connect to
 another QEMU instance using the @option{listen} option. @option{fd}=@var{h}
-specifies an already opened TCP socket.
+specifies an already opened TCP socket. MTU is 4096 by default.
 
 Example:
 @example
@@ -1293,11 +1293,12 @@  qemu linux.img -net nic,macaddr=52:54:00:12:34:57 \
                -net socket,connect=127.0.0.1:1234
 @end example
 
-@item -net socket[,vlan=@var{n}][,name=@var{name}][,fd=@var{h}][,mcast=@var{maddr}:@var{port}[,localaddr=@var{addr}]]
+@item -net socket[,vlan=@var{n}][,name=@var{name}][,fd=@var{h}][,mcast=@var{maddr}:@var{port}[,localaddr=@var{addr}]][,mtu=@var{mtu}]
 
 Create a VLAN @var{n} shared with another QEMU virtual
 machines using a UDP multicast socket, effectively making a bus for
 every QEMU with same multicast address @var{maddr} and @var{port}.
+MTU is 4096 by default.
 NOTES:
 @enumerate
 @item