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

["Tobias Fiebig" <tobias@fiebig.nl>]

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;