linux-kernel - Re: [PATCH 2/6] x86: bug: prevent gcc distortions

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Message-ID: <6A3E90AA-7838-4C2A-B628-908F34902B75@vmware.com>
Date:   Fri, 18 May 2018 19:02:50 +0000
From:   Nadav Amit <namit@...are.com>
To:     "hpa@...or.com" <hpa@...or.com>
CC:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH 2/6] x86: bug: prevent gcc distortions

hpa@...or.com wrote:

> On May 18, 2018 11:50:12 AM PDT, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>> On Fri, May 18, 2018 at 11:34 AM <hpa@...or.com> wrote:
>> 
>>> On May 18, 2018 11:25:32 AM PDT, Linus Torvalds <
>> torvalds@...ux-foundation.org> wrote:
>> 
>>> Unfortunately gcc doesn't guarantee that global assembly inlines will
>> appear at the top of the file.
>> 
>> Yeah. It really would be better to do the "asm version of -inline".
>> 
>> We already do something like that for real *.S files on some
>> architectures
>> (because their assembly really wants it, eg
>> 
>> arch/arm/Makefile:
>> KBUILD_AFLAGS +=$(CFLAGS_ABI) $(AFLAGS_ISA) $(arch-y) $(tune-y)
>> -include
>> asm/unified.h -msoft-float
>> 
>> but I do want to point out that KBUILD_AFLAGS is *not* used for
>> compiler-generated assembly, only for actual *.S files.
>> 
>> Sadly, I don't actually know any way to make gcc call the 'as' phase
>> with
>> particular options. We can use "-Wa,xyzzy" to pass in xyzzy to the
>> assembler, but there is no "-include" option for GNU as afaik.
>> 
>> Can you perhaps define a macro symbol for "--defsym"? Probably not.
>> 
>>                      Linus
> 
> I looked at this thing a long time ago; it's not there, and the best would probably be to get global asm() working properly in gcc.

I can add a -Wa,[filename.s] switch. It works, but sort of undocumented.