RE: [XEN RFC PATCH 07/40] xen/arm: use !CONFIG_NUMA to keep fake NUMA API

[Wei Chen <Wei.Chen@arm.com>]

Hi Julien,

> -----Original Message-----
> From: Julien Grall <julien@xen.org>
> Sent: 2021年8月20日 19:24
> To: Wei Chen <Wei.Chen@arm.com>; xen-devel@lists.xenproject.org;
> sstabellini@kernel.org
> Cc: Bertrand Marquis <Bertrand.Marquis@arm.com>
> Subject: Re: [XEN RFC PATCH 07/40] xen/arm: use !CONFIG_NUMA to keep fake
> NUMA API
> 
> 
> 
> On 20/08/2021 11:24, Wei Chen wrote:
> > Hi Julien,
> 
> Hi Wei,
> 
> >
> >> -----Original Message-----
> >> From: Julien Grall <julien@xen.org>
> >> Sent: 2021年8月20日 16:24
> >> To: Wei Chen <Wei.Chen@arm.com>; xen-devel@lists.xenproject.org;
> >> sstabellini@kernel.org; jbeulich@suse.com
> >> Cc: Bertrand Marquis <Bertrand.Marquis@arm.com>
> >> Subject: Re: [XEN RFC PATCH 07/40] xen/arm: use !CONFIG_NUMA to keep
> fake
> >> NUMA API
> >>
> >>
> >>
> >> On 20/08/2021 03:08, Wei Chen wrote:
> >>> Hi Julien,
> >>
> >> Hi Wei,
> >>
> >>>
> >>>> -----Original Message-----
> >>>> From: Julien Grall <julien@xen.org>
> >>>> Sent: 2021年8月19日 21:34
> >>>> To: Wei Chen <Wei.Chen@arm.com>; xen-devel@lists.xenproject.org;
> >>>> sstabellini@kernel.org; jbeulich@suse.com
> >>>> Cc: Bertrand Marquis <Bertrand.Marquis@arm.com>
> >>>> Subject: Re: [XEN RFC PATCH 07/40] xen/arm: use !CONFIG_NUMA to keep
> >> fake
> >>>> NUMA API
> >>>>
> >>>> Hi Wei,
> >>>>
> >>>> On 11/08/2021 11:23, Wei Chen wrote:
> >>>>> Only Arm64 supports NUMA, the CONFIG_NUMA could not be
> >>>>> enabled for Arm32.
> >>>>
> >>>> What do you mean by "could not be enabled"?
> >>>
> >>> I have not seen any Arm32 hardware support NUMA, so I think
> >>> we don't need to support Arm32 NUMA.
> >>
> >> I understand that there may not be 32-bit platform with NUMA. And
> that's
> >> fine stating that in the commit message. However...
> >>
> >>> In this case, this Kconfig
> >>> option could not be enabled on Arm32.
> >>
> >> ... you continue to say "couldn't be enabled" without clarifying
> whether
> >> this mean that the build will break or this was just not tested because
> >> you don't have any platform.
> >
> > Ok, I understand your concern. Yes, my words would lead to mis-
> understanding.
> > If we make CONFIG_NUMA enabled in Arm32, it need Arm32 to implement some
> > code to support NUMA common code. Otherwise the Arm32 build will failed.
> 
> When I skimmed through the series, most of the code seems to be either
> in common, arm (bitness neutral). So I am not quite too sure why it
> would not build. Do you have more details?
> 

It could not build because I have not tried to enable device_tree_numa
option for Arm32 but enabled NUMA for arm32.

I have tested it again, yes, simple enable device_tree_numa and NUMA
for arm32 can build a image successfully.

So, I think it's OK to enable this on Arm32, and I will do it in next
version. But, can we still keep these FAKE APIs? If user don't want to
enable NUMA they still can make Xen work? And I will remove "could not
enable for Arm32" from commit log.

> > I have not tried to implement those code for Arm32. And I found there is
> > no Arm32 machine support NUMA, so I wanted Arm32 to use fake NUMA API
> > as before.
> >
> >>
> >> To put it differently, the code for NUMA looks bitness neutral. So I
> >> cannot really what what prevent us to potentially use it on Arm 32-bit.
> >>
> >
> > Yes, you're right, it's neutral. But do we really need to add code to an
> > ARCH that it may never use?
> 
> Technically you already added the code because arch/arm/ is common
> between arm32 and arm64. My only ask is to not make the new config
> depends on arm64. If you only build test it that fine because...
> 
> And how can we test this code?
> 
> I don't expect any of the code to be an issue on arm32 as the code
> should mostly be arch neutral.

I mean, we don't have Arm32 NUMA machine to test, I don't know
the code works well on Arm32 NUMA or not. I only can verify them
on non-NUMA arm32, and make sure this code will not break existed
machines.

> 
> Cheers,
> 
> --
> Julien Grall