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

[Jason Wang <jasowang@redhat.com>]

On Wed, Nov 16, 2022 at 12:37 AM Stefan Hajnoczi <stefanha@redhat.com> 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.
>
> 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>

Acked-by: Jason Wang <jasowang@redhat.com>

Thanks

> ---
>  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 100644
> --- 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,
> --
> 2.38.1
>