[openib-general] [PATCH] IB/ipoib: use appropriate path selector

[Michael S. Tsirkin]

Quoting r. Hal Rosenstock <halr at voltaire.com>:
> Subject: Re: [PATCH] IB/ipoib: use appropriate path selector
> 
> On Wed, 2006-09-13 at 18:08, Michael S. Tsirkin wrote:
> > Quoting r. Roland Dreier <rdreier at cisco.com>:
> > > Subject: Re: [PATCH] IB/ipoib: use appropriate path selector
> > > 
> > >     Michael> IPoIB in linux needs 2K MTU. Therefore it must set mtu
> > >     Michael> selector in path record query accordingly.
> > > 
> > > Umm -- why does it need a 2K MTU?  As far as I know it should work
> > > fine with any MTU, assuming the SA sets the MTU of the broadcast
> > > multicast group correctly.
> > 
> > Hmm, you are right, it is just that existing implementations all
> > set that to 2K.
> 
> By default yes. It can be configured.
> 
> > But there is a silent assumption that MTU of any path is >= broadcast
> > multicast group MTU, and this is what I want to fix.
> 
> The spec says:
> "The value (for IB MTU) assigned to the broadcast-GID must not be
> greater than any physical link MTU spanned by the IPoIB subnet".
> so if the broadcast group is improperly setup not to follow this, there
> will be other issues.

Correct. IPoIB uses broadcast group MTU to get the value reported to
Linux. If some link has a lower MTU IPoIB can not use it.

> It doesn't need to be included in the PR request.

I disagree here. If you do not set selector, SA is free to return
a path with lower MTU even though physical link allows higher MTU.
Does it say otherwise somewhere?


-- 
MST