Messages in this thread | | | From | Geert Uytterhoeven <> | Date | Mon, 25 Sep 2017 17:37:46 +0200 | Subject | Re: iommu/io-pgtable: depend on !GENERIC_ATOMIC64 when using COMPILE_TEST with LPAE |
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Hi Will,
On Mon, Sep 25, 2017 at 5:21 PM, Will Deacon <will.deacon@arm.com> wrote: > 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@vger.kernel.org> wrote: >> > Web: https://git.kernel.org/torvalds/c/c1004803b40596c1aabbbc78a6b1b33e4dfd96c6 >> > Commit: c1004803b40596c1aabbbc78a6b1b33e4dfd96c6 >> > Parent: 58188afeb727e0f73706f1460707bd3ba6ccc221 >> > Refname: refs/heads/master >> > Author: Will Deacon <will.deacon@arm.com> >> > AuthorDate: Fri Jun 23 11:45:57 2017 +0100 >> > Committer: Will Deacon <will.deacon@arm.com> >> > 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@intel.com> >> > Signed-off-by: Will Deacon <will.deacon@arm.com> >> > --- >> > 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.
Thanks!
>> 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.
Correct.
>> 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?
I'm not seeing build failures. I see reduced build coverage on m68k due to this change.
So m32r fails to provide a definition for cmpxchg64(), which has nothing to do with GENERIC_ATOMIC64. Probably m32r just needs the same treatment as in commit feb20ec2bb859d1d ("m68k: Add missing cmpxchg64() if CONFIG_RMW_INSNS=y")
Worse, people are now adding more depends on !GENERIC_ATOMIC64 for various IOMMU drivers, to fix up "selects IOMMU_IO_PGTABLE_LPAE which has unmet direct dependencies" warnings :-(
Gr{oetje,eeting}s,
Geert
-- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds
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