linux-kernel - Re: [PATCH v3 2/5] arm64: vdso32: Detect binutils support for dmb ishld

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Date:   Tue, 1 Oct 2019 14:26:40 +0100
From:   Will Deacon <will@...nel.org>
To:     Vincenzo Frascino <vincenzo.frascino@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        ard.biesheuvel@...aro.org, ndesaulniers@...gle.com,
        catalin.marinas@....com, tglx@...utronix.de
Subject: Re: [PATCH v3 2/5] arm64: vdso32: Detect binutils support for dmb
 ishld

On Thu, Sep 26, 2019 at 10:43:39PM +0100, Vincenzo Frascino wrote:
> As reported by Will Deacon, older versions of binutils that do not
> support certain types of memory barriers can cause build failure of the
> vdso32 library.
> 
> Add a compilation time mechanism that detects if binutils supports those
> instructions and configure the kernel accordingly.
> 
> Cc: Will Deacon <will@...nel.org>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@....com>
> ---
>  arch/arm64/include/asm/vdso/compat_barrier.h | 2 +-
>  arch/arm64/kernel/vdso32/Makefile            | 9 +++++++++
>  2 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/vdso/compat_barrier.h b/arch/arm64/include/asm/vdso/compat_barrier.h
> index fb60a88b5ed4..3fd8fd6d8fc2 100644
> --- a/arch/arm64/include/asm/vdso/compat_barrier.h
> +++ b/arch/arm64/include/asm/vdso/compat_barrier.h
> @@ -20,7 +20,7 @@
>  
>  #define dmb(option) __asm__ __volatile__ ("dmb " #option : : : "memory")
>  
> -#if __LINUX_ARM_ARCH__ >= 8
> +#if __LINUX_ARM_ARCH__ >= 8 && defined(CONFIG_AS_DMB_ISHLD)
>  #define aarch32_smp_mb()	dmb(ish)
>  #define aarch32_smp_rmb()	dmb(ishld)
>  #define aarch32_smp_wmb()	dmb(ishst)
> diff --git a/arch/arm64/kernel/vdso32/Makefile b/arch/arm64/kernel/vdso32/Makefile
> index 19e0d3115ffe..77aa61340374 100644
> --- a/arch/arm64/kernel/vdso32/Makefile
> +++ b/arch/arm64/kernel/vdso32/Makefile
> @@ -15,6 +15,8 @@ cc32-disable-warning = $(call try-run,\
>  	$(COMPATCC) -W$(strip $(1)) -c -x c /dev/null -o "$$TMP",-Wno-$(strip $(1)))
>  cc32-ldoption = $(call try-run,\
>          $(COMPATCC) $(1) -nostdlib -x c /dev/null -o "$$TMP",$(1),$(2))
> +cc32-as-instr = $(call try-run,\
> +	printf "%b\n" "$(1)" | $(COMPATCC) $(VDSO_AFLAGS) -c -x assembler -o "$$TMP" -,$(2),$(3))

It's a shame that we have to duplicate the logic from scripts/Kbuild.include
here. Is there a way to reuse those helpers by temporarily overriding things
like CC and KBUILD_AFLAGS? If not, no bother, but thought I'd better ask.

Will