On Fri, Nov 18, 2022 at 12:56 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. > > 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>
Acked-by: Jason Wang <jasow...@redhat.com> Thanks > --- > hw/net/rtl8139.c | 26 ++++++++++++-------------- > 1 file changed, 12 insertions(+), 14 deletions(-) > > diff --git a/hw/net/rtl8139.c b/hw/net/rtl8139.c > index 6dd7a8e6e0..700b1b66b6 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) > @@ -1934,8 +1933,9 @@ static int rtl8139_cplus_transmit_one(RTL8139State *s) > #define CP_TX_LS (1<<28) > /* large send packet flag */ > #define CP_TX_LGSEN (1<<27) > -/* large send MSS mask, bits 16...25 */ > -#define CP_TC_LGSEN_MSS_MASK ((1 << 12) - 1) > +/* large send MSS mask, bits 16...26 */ > +#define CP_TC_LGSEN_MSS_SHIFT 16 > +#define CP_TC_LGSEN_MSS_MASK ((1 << 11) - 1) > > /* 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; > > - 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; > @@ -2180,25 +2181,22 @@ static int rtl8139_cplus_transmit_one(RTL8139State *s) > goto skip_offload; > } > > - /* ETH_MTU = ip header len + tcp header len + payload */ > int tcp_data_len = ip_data_len - tcp_hlen; > - int tcp_chunk_size = ETH_MTU - 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, > - tcp_hlen, tcp_data_len, tcp_chunk_size); > + "data len %d\n", ip_data_len, tcp_hlen, tcp_data_len); > > /* note the cycle below overwrites IP header data, > but restores it from saved_ip_header before sending > packet */ > > int is_last_frame = 0; > > - for (tcp_send_offset = 0; tcp_send_offset < tcp_data_len; > tcp_send_offset += tcp_chunk_size) > + for (tcp_send_offset = 0; tcp_send_offset < tcp_data_len; > tcp_send_offset += large_send_mss) > { > - uint16_t chunk_size = tcp_chunk_size; > + uint16_t chunk_size = large_send_mss; > > /* check if this is the last frame */ > - if (tcp_send_offset + tcp_chunk_size >= tcp_data_len) > + if (tcp_send_offset + large_send_mss >= tcp_data_len) > { > is_last_frame = 1; > chunk_size = tcp_data_len - tcp_send_offset; > @@ -2247,7 +2245,7 @@ static int rtl8139_cplus_transmit_one(RTL8139State *s) > ip->ip_len = cpu_to_be16(hlen + tcp_hlen + chunk_size); > > /* increment IP id for subsequent frames */ > - ip->ip_id = cpu_to_be16(tcp_send_offset/tcp_chunk_size + > be16_to_cpu(ip->ip_id)); > + ip->ip_id = cpu_to_be16(tcp_send_offset/large_send_mss + > be16_to_cpu(ip->ip_id)); > > ip->ip_sum = 0; > ip->ip_sum = ip_checksum(eth_payload_data, hlen); > -- > 2.38.1 >