Re: CONFIG_PPC_VAS depends on 64k pages...?

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Hi,

Le 19/11/2020 à 11:58, Will Springer a écrit :
> I learned about the POWER9 gzip accelerator a few months ago when the
> support hit upstream Linux 5.8. However, for some reason the Kconfig
> dictates that VAS depends on a 64k page size, which is problematic as I
> run Void Linux, which uses a 4k-page kernel.
> 
> Some early poking by others indicated there wasn't an obvious page size
> dependency in the code, and suggested I try modifying the config to switch
> it on. I did so, but was stopped by a minor complaint of an "unexpected DT
> configuration" by the VAS code. I wasn't equipped to figure out exactly what
> this meant, even after finding the offending condition, so after writing a
> very drawn-out forum post asking for help, I dropped the subject.
> 
> Fast forward to today, when I was reminded of the whole thing again, and
> decided to debug a bit further. Apparently the VAS platform device
> (derived from the DT node) has 5 resources on my 4k kernel, instead of 4
> (which evidently works for others who have had success on 64k kernels). I
> have no idea what this means in practice (I don't know how to introspect
> it), but after making a tiny patch[1], everything came up smoothly and I
> was doing blazing-fast gzip (de)compression in no time.
> 
> Everything seems to work fine on 4k pages. So, what's up? Are there
> pitfalls lurking around that I've yet to stumble over? More reasonably,
> I'm curious as to why the feature supposedly depends on 64k pages, or if
> there's anything else I should be concerned about.
> 

Maybe ask Sukadev who did the implementation and is maintaining it ?

> I do have to say I'm quite satisfied with the results of the NX
> accelerator, though. Being able to shuffle data to a RaptorCS box over gigE
> and get compressed data back faster than most software gzip could ever
> hope to achieve is no small feat, let alone the instantaneous results locally.
> :)
> 
> Cheers,
> Will Springer [she/her]
> 
> [1]: https://github.com/Skirmisher/void-packages/blob/vas-4k-pages/srcpkgs/linux5.9/patches/ppc-vas-on-4k.patch
> 


Christophe