Re: [PATCH for-7.2 v3 3/3] rtl8139: honor large send MSS value

[Philippe Mathieu-Daudé]

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 <jasowang@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 <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 | 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 <philmd@linaro.org>