[openib-general] [PATCH for-2.6.18] IB/cma: option to limitMTU to 1K

[Michael S. Tsirkin]

Quoting r. Hal Rosenstock <halr at voltaire.com>:
> Subject: Re: [PATCH for-2.6.18] IB/cma: option to limitMTU to 1K
> 
> On Wed, 2006-09-13 at 17:09, Michael S. Tsirkin wrote:
> > Quoting r. Hal Rosenstock <halr at voltaire.com>:
> > > Subject: Re: [PATCH for-2.6.18] IB/cma: option to limitMTU to 1K
> > > 
> > > On Wed, 2006-09-13 at 15:05, Michael S. Tsirkin wrote:
> > > > Quoting r. Hal Rosenstock <halr at voltaire.com>:
> > > > > > If I understood Michael's comment properly, this will have the side
> > > > > > effect that IPoIB won't work since IPoIB requires 2K MTUs.  As far as
> > > > > > I know, there is no way to specify whether a path is needed for UD vs.
> > > > > > RC in the path query.
> > > > > 
> > > > > I don't know how either. I don't think it can be done (at least
> > > > > currently per the standard).
> > > > 
> > > > We don't really need to know whether path is for RC or UD QP.
> > > > IPoIB needs MTU >= 2K so it should set mtu selector to >= 2K.
> > > 
> > > That's the default and not the minimum MTU (for IPoIB).
> > 
> > How isn't it?
> 
> Look at RFC 4391 as to the requirement.

I'm talking about our implementation not the spec.

> > By default, IPoIB reports 2K MTU to linux.
> > So it will get 2K packets, and since IB swiches
> > can not fragment packets, they will simply get dropped.
> 
> With ifconfig, the MTU can be changed. Fragmentation is at the IP layer
> in the end station stack, not the IB switches.

AFAIK linux won't fragment packets that do not exceed MTU and MSS.

> > I conclude that IPoIB by default requires minimum mtu of 2K.
> > Right?
> 
> Not minimum.
> 
> > And it's not a problem since all HCAs support 2K.
> 
> or more but it could be less per the RFC.

Again, if IPoIB implementation does not need 2K mtu there's
no problem to give it 1K in path. If it wants 2K MTU it must
set selector accordingly.

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MST