[openib-general] Re: Re: [PATCH] change Mellanox SDP workaround toa moduleparameter

[Dror Goldenberg]

> From: openib-general-bounces at openib.org 
> [mailto:openib-general-bounces at openib.org] On Behalf Of Hal Rosenstock
> Sent: Thursday, February 16, 2006 1:13 PM
> 
> On Thu, 2006-02-16 at 02:54, Michael S. Tsirkin wrote:
> > Quoting r. Hal Rosenstock <halr at voltaire.com>:
> > > Subject: Re: Re: [PATCH] change Mellanox SDP workaround to a 
> > > moduleparameter
> > > 
> > > On Wed, 2006-02-15 at 19:03, Roland Dreier wrote:
> > > 
> > > > I guess the question is what to do when a Tavor (with the 
> > > > performance bug that makes a 1K MTU faster) connects to someone 
> > > > else.
> > > 
> > > Isn't it the other way 'round (when something with a larger MTU 
> > > connects to Tavor) ?
> > 
> > Right. I wish we had an MTU field in the REP packet, but we dont.
> 
> Yes, that would be better IMO too. Not sure why it wasn't 
> done that way. Guess you could file an erratum on this.
> 
> -- Hal

The SWG defined a generic mechanism which uses REJ to indicate that 
the passive side does not accept a certain REQ fields, and allows the
passive 
side to indicate an alternative value. Indirection is also supported
through the
same protocol. It also allows the active side, following the REJ, to use
an 
alternate value, other than the one suggested by the passive side, i.e.
passive
side only has a veto capability. This is the mechanism and the short
theory 
behind it. Unfortunately it's a bit inefficient in terms of performance
because of
the ping pong of messages. Solving just the MTU might not be a good
enough 
argument. The approach should be to enable the active side to specify a
set
of acceptable parameters for each one of the REQ fields, and then let
the passive
side to choose. This may change the CM packets all over and will
introduce new
problems. I don't think that there's a good chance of just adding a
solution for
just one of the fields. Anyway, you can still try and propose this to
IBTA, I tried it
once already :)