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RE: [PATCH for-7.2] rtl8139: honor large send MSS value

From: Tobias Fiebig
Subject: RE: [PATCH for-7.2] rtl8139: honor large send MSS value
Date: Wed, 16 Nov 2022 00:36:25 +0100

Just to keep you in the loop; Just applied the patch, but things didn't really 
get better; I am currently doing a 'make clean; make' for good measure (had 
built head first), and will also double-check that there is no accidental use 
of system-qemu libs. 

If that still doesn't show an effect, I'll hold tcpdump to the wire again.

With best regards,

-----Original Message-----
From: Stefan Hajnoczi <stefanha@redhat.com> 
Sent: Tuesday, 15 November 2022 17:37
To: qemu-devel@nongnu.org
Cc: jasowang@redhat.com; qemu-stable@nongnu.org; Stefan Hajnoczi 
<stefanha@redhat.com>; Russell King - ARM Linux <linux@armlinux.org.uk>; Tobias 
Fiebig <tobias+git@fiebig.nl>
Subject: [PATCH for-7.2] rtl8139: honor large send MSS value

The Large-Send Task Offload Tx Descriptor (9.2.1 Transmit) has a Large-Send MSS 
value where the driver specifies the MSS. See the datasheet here:

The code ignores this value and uses a hardcoded MSS of 1500 bytes instead. 
When the MTU is less than 1500 bytes the hardcoded value results in IP 
fragmentation and poor performance.

Use the Large-Send MSS value to correctly size Large-Send packets.

This issue was discussed in the past here:

Reported-by: Russell King - ARM Linux <linux@armlinux.org.uk>
Reported-by: Tobias Fiebig <tobias+git@fiebig.nl>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1312
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
 hw/net/rtl8139.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

Tobias: Please test this fix. Thanks!

diff --git a/hw/net/rtl8139.c b/hw/net/rtl8139.c index e6643e3c9d..ecc4dcb07f 
--- a/hw/net/rtl8139.c
+++ b/hw/net/rtl8139.c
@@ -77,7 +77,6 @@
     ( ( input ) & ( size - 1 )  )
 #define ETHER_TYPE_LEN 2
-#define ETH_MTU     1500
 #define VLAN_TCI_LEN 2
 #define VLAN_HLEN (ETHER_TYPE_LEN + VLAN_TCI_LEN) @@ -2151,8 +2150,8 @@ static 
int rtl8139_cplus_transmit_one(RTL8139State *s)
                 int large_send_mss = (txdw0 >> 16) & CP_TC_LGSEN_MSS_MASK;
-                DPRINTF("+++ C+ mode offloaded task TSO MTU=%d IP data %d "
-                    "frame data %d specified MSS=%d\n", ETH_MTU,
+                DPRINTF("+++ C+ mode offloaded task TSO IP data %d "
+                    "frame data %d specified MSS=%d\n",
                     ip_data_len, saved_size - ETH_HLEN, large_send_mss);
                 int tcp_send_offset = 0; @@ -2177,9 +2176,13 @@ static int 
rtl8139_cplus_transmit_one(RTL8139State *s)
                     goto skip_offload;
-                /* ETH_MTU = ip header len + tcp header len + payload */
+                /* MSS too small? */
+                if (tcp_hlen + hlen >= large_send_mss) {
+                    goto skip_offload;
+                }
                 int tcp_data_len = ip_data_len - tcp_hlen;
-                int tcp_chunk_size = ETH_MTU - hlen - tcp_hlen;
+                int tcp_chunk_size = large_send_mss - hlen - tcp_hlen;
                 DPRINTF("+++ C+ mode TSO IP data len %d TCP hlen %d TCP "
                     "data len %d TCP chunk size %d\n", ip_data_len,

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