Yonghyeon PYUN wrote: > On Wed, Aug 19, 2015 at 08:13:59AM -0400, Rick Macklem wrote: > > Yonghyeon PYUN wrote: > > > On Wed, Aug 19, 2015 at 09:51:44AM +0200, Hans Petter Selasky wrote: > > > > On 08/19/15 09:42, Yonghyeon PYUN wrote: > > > > >On Wed, Aug 19, 2015 at 09:00:52AM +0200, Hans Petter Selasky wrote: > > > > >>On 08/18/15 23:54, Rick Macklem wrote: > > > > >>>Ouch! Yes, I now see that the code that counts the # of mbufs is > > > > >>>before > > > > >>>the > > > > >>>code that adds the tcp/ip header mbuf. > > > > >>> > > > > >>>In my opinion, this should be fixed by setting if_hw_tsomaxsegcount > > > > >>>to > > > > >>>whatever > > > > >>>the driver provides - 1. It is not the driver's responsibility to > > > > >>>know > > > > >>>if > > > > >>>a tcp/ip > > > > >>>header mbuf will be added and is a lot less confusing that expecting > > > > >>>the > > > > >>>driver > > > > >>>author to know to subtract one. (I had mistakenly thought that > > > > >>>tcp_output() had > > > > >>>added the tc/ip header mbuf before the loop that counts mbufs in the > > > > >>>list. > > > > >>>Btw, > > > > >>>this tcp/ip header mbuf also has leading space for the MAC layer > > > > >>>header.) > > > > >>> > > > > >> > > > > >>Hi Rick, > > > > >> > > > > >>Your question is good. With the Mellanox hardware we have separate > > > > >>so-called inline data space for the TCP/IP headers, so if the TCP > > > > >>stack > > > > >>subtracts something, then we would need to add something to the > > > > >>limit, > > > > >>because then the scatter gather list is only used for the data part. > > > > >> > > > > > > > > > >I think all drivers in tree don't subtract 1 for > > > > >if_hw_tsomaxsegcount. Probably touching Mellanox driver would be > > > > >simpler than fixing all other drivers in tree. > > > > > > > > Hi, > > > > > > > > If you change the behaviour don't forget to update and/or add comments > > > > describing it. Maybe the amount of subtraction could be defined by some > > > > macro? Then drivers which inline the headers can subtract it? > > > > > > > > > > I'm also ok with your suggestion. > > > > > > > Your suggestion is fine by me. > > > > > > > > > > > The initial TSO limits were tried to be preserved, and I believe that > > > > TSO limits never accounted for IP/TCP/ETHERNET/VLAN headers! > > > > > > > > > > I guess FreeBSD used to follow MS LSOv1 specification with minor > > > exception in pseudo checksum computation. If I recall correctly the > > > specification says upper stack can generate up to IP_MAXPACKET sized > > > packet. Other L2 headers like ethernet/vlan header size is not > > > included in the packet and it's drivers responsibility to allocate > > > additional DMA buffers/segments for L2 headers. > > > > > Yep. The default for if_hw_tsomax was reduced from IP_MAXPACKET to > > 32 * MCLBYTES - max_ethernet_header_size as a workaround/hack so that > > devices limited to 32 transmit segments would work (ie. the entire packet, > > including MAC header would fit in 32 MCLBYTE clusters). > > This implied that many drivers did end up using m_defrag() to copy the mbuf > > list to one made up of 32 MCLBYTE clusters. > > > > If a driver sets if_hw_tsomaxsegcount correctly, then it can set > > if_hw_tsomax > > to whatever it can handle as the largest TSO packet (without MAC header) > > the > > hardware can handle. If it can handle > IP_MAXPACKET, then it can set it to > > that. > > > > I thought the upper limit was still IP_MAXPACKET. If driver > increase it (i.e. > IP_MAXPACKET, the length field in the IP > header would overflow which in turn may break firewalls and other > packet handling in IPv4/IPv6 code path. I have no idea if a bogus value in the ip_len field of the TSO segment would break something in ip_output() or not. This would need to be checked before anyone configures if_hw_tsomax > IP_MAXPACKET. I didn't think of any effect this would have in ip_output(), I just knew that the hardware would be replacing ip_len when it generated the TCP/IP segments from the TSO segment. As you note, I vaguely recall some hardware being able to handle a TSO segment > IP_MAXPACKET (presumably getting the TSO segment's length some other way).
It would be nice if this was checked, but yes, the comment should specify an upper bound on if_hw_tsomax of IP_MAXPACKET until then. rick > If the limit no longer apply to network stack, that's great. Some > controllers can handle up to 256KB TCP/UDP segmentation and > supporting that feature wouldn't be hard. > _______________________________________________ > freebsd-stable@freebsd.org mailing list > https://lists.freebsd.org/mailman/listinfo/freebsd-stable > To unsubscribe, send any mail to "freebsd-stable-unsubscr...@freebsd.org" > _______________________________________________ freebsd-stable@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/freebsd-stable To unsubscribe, send any mail to "freebsd-stable-unsubscr...@freebsd.org"
