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

From: Vincenzo Frascino <[email-masked]>
Date: Tue, 1 Oct 2019 15:23:41 +0100

On 10/1/19 2:26 PM, Will Deacon wrote:
> 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 <[email-masked]>
>> Cc: Catalin Marinas <[email-masked]>
>> Signed-off-by: Vincenzo Frascino <[email-masked]>
>> ---
>>  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.
> 

I tried to define a rule in scripts/Kbuild.include at the beginning doing what
you are saying but I could not end up with a working solution, hence I ended up
with a dedicated one.

> Will

-- 
Regards,
Vincenzo

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