linux-kernel - Re: [PATCH] ARM64: Fix compiling with GCC 6 and Atomics enabled

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Message-ID: <CAKv+Gu9c3+YfeJsf4oiM_rjb0t-2qOsx2ynn=0bGT0N9K1OT8g@mail.gmail.com>
Date:	Thu, 21 Jan 2016 15:32:14 +0100
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	Vladimir Murzin <vladimir.murzin@....com>
Cc:	Will Deacon <will.deacon@....com>,
	Andrew Pinski <apinski@...ium.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ARM64: Fix compiling with GCC 6 and Atomics enabled

On 21 January 2016 at 13:32, Vladimir Murzin <vladimir.murzin@....com> wrote:
> On 21/12/15 13:37, Will Deacon wrote:
>> On Mon, Dec 21, 2015 at 01:58:30PM +0100, Ard Biesheuvel wrote:
>>> On 21 December 2015 at 13:51, Will Deacon <will.deacon@....com> wrote:
>>>> On Mon, Dec 21, 2015 at 01:46:22PM +0100, Ard Biesheuvel wrote:
>>>>> On 21 December 2015 at 13:38, Will Deacon <will.deacon@....com> wrote:
>>>>>> On Fri, Dec 18, 2015 at 08:17:35PM -0800, Andrew Pinski wrote:
>>>>>>> The problem here is that GCC 6 and above emits .arch now
>>>>>>> for each function so now the global .arch_extension has
>>>>>>> no effect.  This fixes the problem by putting
>>>>>>> .arch_extension inside ARM64_LSE_ATOMIC_INSN so
>>>>>>> it is enabled for each place where LSE is used.
>>>>>>
>>>>>> Hmm, this is going to affect arch/arm/ much more heavily than arch/arm64.
>>>>>> .arch_extension is used for virt, mp and sec over there, and it may be
>>>>>> tricky to isolate the actual instruction usage (at least, virt looks
>>>>>> lost in kvm/arm.c).
>>>>>>
>>>>>> Why can't gas have an option to accept all instruction encodings that it
>>>>>> knows about, inspite of any .arch directives?
>>>>>>
>>>>>
>>>>> Modern GAS supports things like -march=armv7-a+mp+sec+virt, so it
>>>>> probably makes sense to pass that on the command line when building
>>>>> for v7 (or +sec only for v6) if the assembler is found to support it
>>>>> at build time.
>>>>
>>>> Does that override a more restrictive .arch directive emitted by the
>>>> compiler?
>>>>
>>>
>>> It seems to be additive: -march=armv7-a+mp+sec allows a .S file
>>> containing a virt arch_extension + both hvc and smc instructions to be
>>> assembled.
>>
>> The problem I'm seeing is if I have something like:
>>
>>   .arch_extension lse
>>
>> before something like:
>>
>>   .cpu cortex-a57+fp+simd+crc
>>
>> -or-
>>
>>   .arch armv8-a+fp+simd+crc
>>
>> then I can no longer assemble lse instructions. So the .cpu/.arch
>> directive is undoing the .arch_extension. We can fix this by following
>> Andrew's suggestion to have .arch_extension before every point of use,
>> but the whole thing would be much simpler if we could just tell gas to
>> assemble harder.
>>
>> Maybe we just need to construct the mother of all -march options based
>> on build-time checks in the Makefile?
>>
>
> Since I've been bitten by this I'm curious what was conclusion on this
> topic?
>

None that I'm aware of