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 | 21 +++++++++++++-------- 1 file changed, 13 insertions(+), 8 deletions(-) v2: - Fix MSS mask [Jason] diff --git a/hw/net/rtl8139.c b/hw/net/rtl8139.c index e6643e3c9d..6c406f39ce 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) @@ -2149,10 +2149,11 @@ 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; @@ -2177,9 +2178,13 @@ 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
