On 17/11/22 17:55, Stefan Hajnoczi wrote:
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:
http://realtek.info/pdf/rtl8139cp.pdf
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.
Jason Wang <jasow...@redhat.com> noticed that the Large-Send MSS value
mask was incorrect so it is adjusted to match the datasheet and Linux
8139cp driver.
This issue was discussed in the past here:
https://lore.kernel.org/all/20161114162505.GD26664@stefanha-x1.localdomain/
Reported-by: Russell King - ARM Linux <li...@armlinux.org.uk>
Reported-by: Tobias Fiebig <tobias+...@fiebig.nl>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1312
Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com>
---
hw/net/rtl8139.c | 26 ++++++++++++--------------
1 file changed, 12 insertions(+), 14 deletions(-)
/* IP checksum offload flag */
#define CP_TX_IPCS (1<<18)
@@ -2152,10 +2152,11 @@ static int rtl8139_cplus_transmit_one(RTL8139State *s)
goto skip_offload;
}
- int large_send_mss = (txdw0 >> 16) & CP_TC_LGSEN_MSS_MASK;
+ int large_send_mss = (txdw0 >> CP_TC_LGSEN_MSS_SHIFT) &
+ CP_TC_LGSEN_MSS_MASK;
Nitpicking/matter of style, the '&' is harder to miss if moved on the
next line just before the mask.
Reviewed-by: Philippe Mathieu-Daudé <phi...@linaro.org>
