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

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Message-ID: <CA+55aFzLhG1L0WXvJiUSWrAN5ghzn4XFAK64wgFoyVAHTHZH-A@mail.gmail.com>
Date:   Fri, 18 May 2018 11:50:12 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Anvin <hpa@...or.com>
Cc:     namit@...are.com, 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

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