bizarre em + TSO + MSS issue in RELENG_7

[Kip Macy]

On Nov 17, 2007 10:33 AM, Denis Shaposhnikov <dsh at vlink.ru> wrote:
> On Sat, 17 Nov 2007 00:42:54 -0500 (EST)
> Mike Andrews <mandrews at bit0.com> wrote:
>
> > Has anyone run into problems with MSS not being respected when using
> > TSO, specifically on em cards?
>
> Yes, I wrote about this problem on the beginning of 2007, see
>
>     http://tinyurl.com/3e5ak5
>

if_em.c:3502
        /*
         * Payload size per packet w/o any headers.
         * Length of all headers up to payload.
         */
        TXD->tcp_seg_setup.fields.mss = htole16(mp->m_pkthdr.tso_segsz);
        TXD->tcp_seg_setup.fields.hdr_len = hdr_len;


Please print out the value of tso_segsz here. It appears to be being
set correctly. The only thing I can think of is that t_maxopd is not
correct. As tso_segsz is correct here:

        if (tso) {
                m->m_pkthdr.csum_flags = CSUM_TSO;
                m->m_pkthdr.tso_segsz = tp->t_maxopd - optlen;
        }

But maxopd appears to be set correctly in tcp_input:
        mss = min(mss, offer);

        /*
         * maxopd stores the maximum length of data AND options
         * in a segment; maxseg is the amount of data in a normal
         * segment.  We need to store this value (maxopd) apart
         * from maxseg, because now every segment carries options
         * and thus we normally have somewhat less data in segments.
         */
        tp->t_maxopd = mss;