[Qemu-devel] [PATCH] net: increase buffer size to accommodate Jumbo frame pkts over netdev UDP socket

[Qemu-devel] [PATCH] net: increase buffer size to accommodate Jumbo frame pkts over netdev UDP socket

[sfeldma]

From: Scott Feldman <sfeldma@cumulusnetworks.com>

Socket buffer sizes were hard-coded to 4K.  Bump this up to 12K to handle typical
MTU=9000 Jumbo frame pkt.  Ran into this limitation when using -netdev UDP sockets
to connect VM-to-VM, where VM interface is configure with MTU=9000.  Using
virtio_net NIC model.  Test is simple: ping -M do -s 8500 <target>.  This test
will attempt to ping with unfragmented packet of given size.  Without patch,
size is limited to < 4K (minus protocol hdrs).  With patch, ping test works
with pkt size up to 9000 (again, minus protocol hdrs).

Signed-off-by: Scott Feldman <sfeldma@cumulusnetworks.com>
---
 net/net.c    |    2 +-
 net/socket.c |    4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/net.c b/net/net.c
index f3d67f8..bab9ced 100644
--- a/net/net.c
+++ b/net/net.c
@@ -497,7 +497,7 @@ ssize_t qemu_send_packet_raw(NetClientState *nc, const uint8_t *buf, int size)
 static ssize_t nc_sendv_compat(NetClientState *nc, const struct iovec *iov,
                                int iovcnt)
 {
-    uint8_t buffer[4096];
+    uint8_t buffer[12*1024];
     size_t offset;
 
     offset = iov_to_buf(iov, iovcnt, 0, buffer, sizeof(buffer));
diff --git a/net/socket.c b/net/socket.c
index 396dc8c..74e0057 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -40,7 +40,7 @@ typedef struct NetSocketState {
     unsigned int index;
     unsigned int packet_len;
     unsigned int send_index;      /* number of bytes sent (only SOCK_STREAM) */
-    uint8_t buf[4096];
+    uint8_t buf[12*1024];
     struct sockaddr_in dgram_dst; /* contains inet host and port destination iff connectionless (SOCK_DGRAM) */
     IOHandler *send_fn;           /* differs between SOCK_STREAM/SOCK_DGRAM */
     bool read_poll;               /* waiting to receive data? */
@@ -146,7 +146,7 @@ static void net_socket_send(void *opaque)
     NetSocketState *s = opaque;
     int size, err;
     unsigned l;
-    uint8_t buf1[4096];
+    uint8_t buf1[12*1024];
     const uint8_t *buf;
 
     size = qemu_recv(s->fd, buf1, sizeof(buf1), 0);
-- 
1.7.2.5

Re: [Qemu-devel] [PATCH] net: increase buffer size to accommodate Jumbo frame pkts over netdev UDP socket

[Stefan Hajnoczi]

On Fri, Mar 15, 2013 at 05:12:38PM -0700, sfeldma@cumulusnetworks.com wrote:
> From: Scott Feldman <sfeldma@cumulusnetworks.com>
> 
> Socket buffer sizes were hard-coded to 4K.  Bump this up to 12K to handle typical
> MTU=9000 Jumbo frame pkt.  Ran into this limitation when using -netdev UDP sockets
> to connect VM-to-VM, where VM interface is configure with MTU=9000.  Using
> virtio_net NIC model.  Test is simple: ping -M do -s 8500 <target>.  This test
> will attempt to ping with unfragmented packet of given size.  Without patch,
> size is limited to < 4K (minus protocol hdrs).  With patch, ping test works
> with pkt size up to 9000 (again, minus protocol hdrs).
> 
> Signed-off-by: Scott Feldman <sfeldma@cumulusnetworks.com>
> ---
>  net/net.c    |    2 +-
>  net/socket.c |    4 ++--
>  2 files changed, 3 insertions(+), 3 deletions(-)

grep reveals that VDE is limited in a similar way.  tap-win32 may be
affected too.

I noticed that Linux tap goes straight for:
net/tap.c:#define TAP_BUFSIZE (4096 + 65536)

The 4096 is reserved for network/virtio headers.

Please move TAP_BUFSIZE to include/net/net.h and rename it to
NET_BUFSIZE.  Then socket, vde, and tap-win32 can use it.

Stefan