Re: iommu/io-pgtable: depend on !GENERIC_ATOMIC64 when using COMPILE_TEST with LPAE

[Will Deacon]

Hi Geert,

On Mon, Sep 25, 2017 at 09:16:22AM +0200, Geert Uytterhoeven wrote:
> On Wed, Jul 12, 2017 at 7:16 PM, Linux Kernel Mailing List
> <linux-kernel@xxxxxxxxxxxxxxx> wrote:
> > Web:        https://git.kernel.org/torvalds/c/c1004803b40596c1aabbbc78a6b1b33e4dfd96c6
> > Commit:     c1004803b40596c1aabbbc78a6b1b33e4dfd96c6
> > Parent:     58188afeb727e0f73706f1460707bd3ba6ccc221
> > Refname:    refs/heads/master
> > Author:     Will Deacon <will.deacon@xxxxxxx>
> > AuthorDate: Fri Jun 23 11:45:57 2017 +0100
> > Committer:  Will Deacon <will.deacon@xxxxxxx>
> > CommitDate: Fri Jun 23 17:58:02 2017 +0100
> >
> >     iommu/io-pgtable: depend on !GENERIC_ATOMIC64 when using COMPILE_TEST with LPAE
> >
> >     The LPAE/ARMv8 page table format relies on the ability to read and write
> >     64-bit page table entries in an atomic fashion. With the move to a lockless
> >     implementation, we also need support for cmpxchg64 to resolve races when
> >     installing table entries concurrently.
> >
> >     Unfortunately, not all architectures support cmpxchg64, so the code can
> >     fail to compiler when building for these architectures using COMPILE_TEST.
> >     Rather than disable COMPILE_TEST altogether, instead check that
> >     GENERIC_ATOMIC64 is not selected, which is a reasonable indication that
> >     the architecture has support for 64-bit cmpxchg.
> >
> >     Reported-by: kbuild test robot <fengguang.wu@xxxxxxxxx>
> >     Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
> > ---
> >  drivers/iommu/Kconfig | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
> > index 6ee3a25ae731..c88cfa7522b2 100644
> > --- a/drivers/iommu/Kconfig
> > +++ b/drivers/iommu/Kconfig
> > @@ -23,7 +23,7 @@ config IOMMU_IO_PGTABLE
> >  config IOMMU_IO_PGTABLE_LPAE
> >         bool "ARMv7/v8 Long Descriptor Format"
> >         select IOMMU_IO_PGTABLE
> > -       depends on HAS_DMA && (ARM || ARM64 || COMPILE_TEST)
> > +       depends on HAS_DMA && (ARM || ARM64 || (COMPILE_TEST && !GENERIC_ATOMIC64))
> >         help
> >           Enable support for the ARM long descriptor pagetable format.
> >           This allocator supports 4K/2M/1G, 16K/32M and 64K/512M page
> 
> I can't find where this patch was submitted and discussed, so I'm replying
> to this email. On which architectures did it fail to compile?

It was in response to a report from the kbuild test robot on m32r, but
looking back I now see that it didn't go to the lists for some reason. I've
included the report at the end of this email so you can have a look.

> cmpxchg64() is defined by include/asm-generic/cmpxchg.h, so I fail to
> see what's the relation with GENERIC_ATOMIC64, which is related to
> lib/atomic64.c instead.

Yeah, it's a bit of a hack, but we're basically relying on architectures
that don't select GENERIC_ATOMIC64 providing their own cmpxchg64
implementation, which seems to be the case.

> E.g. on m68k, which uses GENERIC_ATOMIC64, it compile-tested fine before.

FWIW, the lock-based atomics wouldn't be sufficient at runtime, but I
appreciate that we're only talking about COMPILE_TEST here.

> Perhaps there's another (SMP vs UP?) dependency, as
> include/asm-generic/cmpxchg.h cannot be used on SMP?
> Should it be COMPILE_TEST && (!GENERIC_ATOMIC64 || !SMP)?

I don't see how that helps. Are you seeing build failures on a non-SMP
arch?

Will

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