Re: [PATCH] Fix the j-core SOC build.

[Dennis Zhou <dennis@kernel.org>]

Hello,

On Wed, Nov 10, 2021 at 07:53:13PM -0500, Rich Felker wrote:
> On Wed, Nov 10, 2021 at 04:33:46PM -0500, Rich Felker wrote:
> > On Sat, Oct 02, 2021 at 10:18:51PM -0400, Rich Felker wrote:
> > > On Sat, Oct 02, 2021 at 02:32:15PM -0500, Rob Landley wrote:
> > > > From: Rob Landley <rob@landley.net>
> > > > 
> > > > Commit b67177ecd956 broke the j-core SOC build with a link failure, because
> > > > mm/percpu.c function pcpu_post_unmap_tlb_flush() calls flush_tlb_kernel_range()
> > > > which is defined under #ifdef CONFIG_MMU.
> > > > 
> > > > Signed-off-by: Rob Landley <rob@landley.net>
> > > > ---
> > > > 
> > > >  arch/sh/kernel/smp.c |    5 +++++
> > > >  1 file changed, 5 insertions(+)
> > > > 
> > > > diff --git a/arch/sh/kernel/smp.c b/arch/sh/kernel/smp.c
> > > > index 65924d9ec245..3ec8f32aad85 100644
> > > > --- a/arch/sh/kernel/smp.c
> > > > +++ b/arch/sh/kernel/smp.c
> > > > @@ -468,4 +468,9 @@ void flush_tlb_one(unsigned long asid, unsigned long vaddr)
> > > >  	local_flush_tlb_one(asid, vaddr);
> > > >  }
> > > > 
> > > > +#else
> > > > +void flush_tlb_kernel_range(unsigned long start, unsigned long end)
> > > > +{
> > > > +	local_flush_tlb_all();
> > > > +}
> > > >  #endif
> > > 
> > > local_flush_tlb_all() is defined in arch/sh/mm/nommu.c as BUG(); so
> > > this is most likely wrong unless it just doesn't get called. I think
> > > there should probably be something at a very general level dummying
> > > out these functions/macros on nommu but I don't know where it should
> > > be.
> > 
> > I've looked into this some more, and while arch/arm does dummy its
> > equivalent functions out on nommu, arch/sh has always had them as
> > BUG(), and indeed it makes some sense to catch erroneous usage.
> > pcpu_post_unmap_tlb_flush should probably have the flush under #ifdef
> > CONFIG_MMU or something.
> > 
> > I've added the author of the commit that broke this to Cc in case he
> > has any thoughts.
> 
> I think we actually have the wrong commit to blame; it looks like
> 93274f1dd6b0 ("percpu: flush tlb in pcpu_reclaim_populated()") added
> the TLB flush. However, this rabbit hole seems to go a lot deeper.
> According to the comments in the files, percpu-km.c, not percpu-vm.c,
> is supposed to be used on nommu archs. However, mm/Kconfig has
> NEED_PER_CPU_KM depending on !SMP rather than !SMP || !MMU, which
> mismatches the comments in the source. So should we be trying to fix
> the Kconfig constraints to use -km? Or fixing -vm to work on nommu
> like it used to?
> 
> Rich

I'm surprised I haven't heard any reports of this until now. Thanks for
ccing me. Fwiw, it's generally good to cc the author / subsystem
maintainer when there is an issue.

I would need to think about it a little bit more, but I think
percpu-km.c was written more in the mindset of !MMU == !SMP. It sounds
like for superh that's not true.

Given that, if sh has no MMU, then it really should be using percpu-km.c
not percpu-vm.c as there is no notion of unpopulated pages for that
configuration. I think we should fix the Kconfig constraints and give
that a shot. I didn't have any known users of !mmu/percpu-km.c, so I
didn't want to make the changes without someone verifying it or needing
it.

Thanks,
Dennis