Heho, Ok, I just learned more C than I ever wanted to. There is a bit more amiss here (ll from 7d7238c72b983cff5064734349d2d45be9c6282c):
In line 1916 of rtl8139.c we set txdw0; If we calculate the MSS at this point, it is consistently 12 below requested, but generally accurate. The bits that flip re: -12 must happen somewhere in the Linux kernel driver (ll 764 in drivers/net/ethernet/realtek/8139cp.c?); Didn't look there in-depth yet (and do not plan to, maybe one of you has more experience with this?) Given the consistency of this deviation, maybe just doing a +12 might be more straight forward. However, in ll2030ff we reset a couple of status indicators. These overlap with the fields for the MSS, leading to inaccurate values being calculated later on; For example, requesting an MSS of 767 leads to an MSS of 3 being calculated by your patch; Similarly, requesting 1000 leads to 268. At least for the latter I see packets of that size being generated on the wire (which should also not happen, as the MSS should never be below 536; maybe a check could help here to make sure we are not trusting arbitrary values from the driver, esp. given the bobble of sec issues around PMTUD/MSS; Technically, now that MSS is defined earlier, we could also move this closer to the start of TSO large frame handling). Below is also a draft patch following my suggestions (save txdw0, +12, check for <536) and some examples for what I described above, which I can on your last patch. Please note again that this is essentially the first time I do anything in C; Also, I wasn't sure what has less perf impact (save the whole 32bit of txdw0 even though it might not be needed vs. also doing the shift/& even though it might not be needed). Apart from that, my patch seems to work, and the MSS gets set correctly; Someone else testing would be nice, though: # MSS_requested=1320 RTL8139: +++ C+ mode offloaded task TSO IP data 2648 frame data 2668 specified MSS=1320 # MSS_requested=1000 RTL8139: +++ C+ mode offloaded task TSO IP data 2008 frame data 2028 specified MSS=1000 # MSS_requested=600 RTL8139: +++ C+ mode offloaded task TSO IP data 1796 frame data 1816 specified MSS=600 With best regards, Tobias diff --git a/hw/net/rtl8139.c b/hw/net/rtl8139.c index e6643e3c9d..59321460b9 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) @@ -2027,6 +2027,9 @@ static int rtl8139_cplus_transmit_one(RTL8139State *s) s->currCPlusTxDesc = 0; } + /* store unaltered txdw0 for later use in MSS calculation*/ + uint32_t txdw0_save = txdw0; + /* transfer ownership to target */ txdw0 &= ~CP_TX_OWN; @@ -2149,10 +2152,12 @@ static int rtl8139_cplus_transmit_one(RTL8139State *s) goto skip_offload; } - int large_send_mss = (txdw0 >> 16) & CP_TC_LGSEN_MSS_MASK; + /* set large_send_mss from txdw0 before overlapping mss fields were cleared */ + int large_send_mss = ((txdw0_save >> CP_TC_LGSEN_MSS_SHIFT) & + CP_TC_LGSEN_MSS_MASK) + 12; - 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 +2182,13 @@ static int rtl8139_cplus_transmit_one(RTL8139State *s) goto skip_offload; } - /* ETH_MTU = ip header len + tcp header len + payload */ + /* MSS too small? Min MSS = 536 */ + if (tcp_hlen + hlen >= large_send_mss || 535 >= 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, Some examples (with additional DPRINT capturing txdw0/MSS at various places; txdw0_0=ll1923, txdw0_4=ll2029, txdw0_5=ll2039, txdw0_cur=ll2153): MSS_requested=556 +++ txdw0_cur=18000440 txdw0_cur_shift=1800 txdw0_cur_MSS=0; +++ txdw0_0=9a200440 txdw0_0_shift=9a20 txdw0_0_MSS=544; +++ txdw0_1=9a200440 txdw0_1_shift=9a20 txdw0_1_MSS=544; +++ txdw0_2=9a200440 txdw0_2_shift=9a20 txdw0_2_MSS=544; +++ txdw0_3=9a200440 txdw0_3_shift=9a20 txdw0_3_MSS=544; +++ txdw0_4=9a200440 txdw0_4_shift=9a20 txdw0_4_MSS=544; +++ txdw0_5=18000440 txdw0_5_shift=1800 txdw0_5_MSS=0; +++ txdw0_6=18000440 txdw0_6_shift=1800 txdw0_6_MSS=0; +++ txdw0_7=18000440 txdw0_7_shift=1800 txdw0_7_MSS=0; MSS_requested=800 +++ txdw0_0=9b140cab txdw0_0_shift=9b14 txdw0_0_MSS=788; +++ txdw0_1=9b140cab txdw0_1_shift=9b14 txdw0_1_MSS=788; +++ txdw0_2=9b140cab txdw0_2_shift=9b14 txdw0_2_MSS=788; +++ txdw0_3=9b140cab txdw0_3_shift=9b14 txdw0_3_MSS=788; +++ txdw0_4=9b140cab txdw0_4_shift=9b14 txdw0_4_MSS=788; +++ txdw0_5=19040cab txdw0_5_shift=1904 txdw0_5_MSS=260; +++ txdw0_6=19040cab txdw0_6_shift=1904 txdw0_6_MSS=260; +++ txdw0_7=19040cab txdw0_7_shift=1904 txdw0_7_MSS=260; MSS_requested=1050 +++ txdw0_cur=1c0e07bf txdw0_cur_shift=1c0e txdw0_cur_MSS=1038; +++ txdw0_0=9c0e07bf txdw0_0_shift=9c0e txdw0_0_MSS=1038; +++ txdw0_1=9c0e07bf txdw0_1_shift=9c0e txdw0_1_MSS=1038; +++ txdw0_2=9c0e07bf txdw0_2_shift=9c0e txdw0_2_MSS=1038; +++ txdw0_3=9c0e07bf txdw0_3_shift=9c0e txdw0_3_MSS=1038; +++ txdw0_4=9c0e07bf txdw0_4_shift=9c0e txdw0_4_MSS=1038; +++ txdw0_5=1c0e07bf txdw0_5_shift=1c0e txdw0_5_MSS=1038; +++ txdw0_6=1c0e07bf txdw0_6_shift=1c0e txdw0_6_MSS=1038; +++ txdw0_7=1c0e07bf txdw0_7_shift=1c0e txdw0_7_MSS=1038; MSS_requested=1060 +++ txdw0_cur=1c0809ff txdw0_cur_shift=1c08 txdw0_cur_MSS=1032; +++ txdw0_0=9c1809ff txdw0_0_shift=9c18 txdw0_0_MSS=1048; +++ txdw0_1=9c1809ff txdw0_1_shift=9c18 txdw0_1_MSS=1048; +++ txdw0_2=9c1809ff txdw0_2_shift=9c18 txdw0_2_MSS=1048; +++ txdw0_3=9c1809ff txdw0_3_shift=9c18 txdw0_3_MSS=1048; +++ txdw0_4=9c1809ff txdw0_4_shift=9c18 txdw0_4_MSS=1048; +++ txdw0_5=1c0809ff txdw0_5_shift=1c08 txdw0_5_MSS=1032; +++ txdw0_6=1c0809ff txdw0_6_shift=1c08 txdw0_6_MSS=1032; +++ txdw0_7=1c0809ff txdw0_7_shift=1c08 txdw0_7_MSS=1032; MSS_requested=1320 +++ txdw0_cur=1d0c0b37 txdw0_cur_shift=1d0c txdw0_cur_MSS=1292; +++ txdw0_0=9d1c0b37 txdw0_0_shift=9d1c txdw0_0_MSS=1308; +++ txdw0_1=9d1c0b37 txdw0_1_shift=9d1c txdw0_1_MSS=1308; +++ txdw0_2=9d1c0b37 txdw0_2_shift=9d1c txdw0_2_MSS=1308; +++ txdw0_3=9d1c0b37 txdw0_3_shift=9d1c txdw0_3_MSS=1308; +++ txdw0_4=9d1c0b37 txdw0_4_shift=9d1c txdw0_4_MSS=1308; +++ txdw0_5=1d0c0b37 txdw0_5_shift=1d0c txdw0_5_MSS=1292; +++ txdw0_6=1d0c0b37 txdw0_6_shift=1d0c txdw0_6_MSS=1292; +++ txdw0_7=1d0c0b37 txdw0_7_shift=1d0c txdw0_7_MSS=1292;