[openib-general] [PATCH] osm: PathRecord prefer 1K MTU for MT23108 devices

[Michael S. Tsirkin]

Quoting r. Or Gerlitz <ogerlitz at voltaire.com>:
> Subject: Re: [PATCH] osm: PathRecord prefer 1K MTU for MT23108 devices
> 
> Eitan Zahavi wrote:
> > The following patch solves an issue with OpenSM preferring largest MTU 
> > for PathRecord/MultiPathRecord for paths going to or from MT23108 (Tavor)
> > devices instead of using a 1K MTU which is best for this device.
> 
> Eitan,
> 
> Isn't the 2K MTU issue with Tavor comes into play only under RC QP?

I don't think so, no. Tavor supports 2K MTU, but it has better performance with
1K MTU than 2K MTU. QP type should not matter.

> more over, doing TAVOR/UD/2K MTU is very common, eg IPoIB.

Correct. And it works with existing SMs.
But ULPs that have specific MTU requirements must set MTU selector
accordingly, otherwise SM is free to select any MTU.

> So does your patch relies on a somehow completing quirk in the host side 
> for UD based ULPs to add some mtu selector which will prevent the SM 
> side quirk to take action?

It's more a bugfix than a quirk.

IPoIB currently has specific MTU requirements but does not set MTU
selector at all, relying on specific SM behaviour. I consider it
a bug in IPoIB and am testing a patch fixing this.

-- 
MST