On Wed, Nov 16, 2022 at 12:37 AM Stefan Hajnoczi <stefa...@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 <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>
Acked-by: Jason Wang <jasow...@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
>
