linux-kernel - Re: iommu/io-pgtable: depend on !GENERIC_ATOMIC64 when using COMPILE

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Message-ID: <CAMuHMdVSR1fcDPgruPPDFAXQe_yVXHJFE-sCR3U8MNYOAUzfKQ@mail.gmail.com>
Date:   Mon, 25 Sep 2017 09:16:22 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Will Deacon <will.deacon@....com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        iommu@...ts.linux-foundation.org
Subject: Re: iommu/io-pgtable: depend on !GENERIC_ATOMIC64 when using
 COMPILE_TEST with LPAE

Hi Will,

On Wed, Jul 12, 2017 at 7:16 PM, Linux Kernel Mailing List
<linux-kernel@...r.kernel.org> wrote:
> Web:        https://git.kernel.org/torvalds/c/c1004803b40596c1aabbbc78a6b1b33e4dfd96c6
> Commit:     c1004803b40596c1aabbbc78a6b1b33e4dfd96c6
> Parent:     58188afeb727e0f73706f1460707bd3ba6ccc221
> Refname:    refs/heads/master
> Author:     Will Deacon <will.deacon@....com>
> AuthorDate: Fri Jun 23 11:45:57 2017 +0100
> Committer:  Will Deacon <will.deacon@....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@...el.com>
>     Signed-off-by: Will Deacon <will.deacon@....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?

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.

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

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)?

Thanks!

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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