linux-kernel - Re: [PATCH v2 11/18] arm64: make mrs_s and msr

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Message-ID: <20171116170144.GR10515@gate.crashing.org>
Date:   Thu, 16 Nov 2017 11:01:44 -0600
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Yury Norov <ynorov@...iumnetworks.com>,
        Will Deacon <will.deacon@....com>,
        Alex Matveev <alxmtvv@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Greg Hackmann <ghackmann@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Maxim Kuvyrkov <maxim.kuvyrkov@...aro.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Matthias Kaehlcke <mka@...omium.org>
Subject: Re: [PATCH v2 11/18] arm64: make mrs_s and msr_s macros work with LTO

On Thu, Nov 16, 2017 at 08:46:08AM -0800, Sami Tolvanen wrote:
> On Thu, Nov 16, 2017 at 07:56:50AM -0600, Segher Boessenkool wrote:
> > The compiler is fine, the assembler is fine (and the linker has
> > nothing to do with it).  Your code is not fine.
> 
> Would you care to elaborate? The current code assumes that macros are
> visible in other inline assembly blocks, and LLVM developers seem to
> feel this isn't correct behavior. This patch fixes the current code so
> it works with both assemblers.

If you say e.g.

void f(void)
{
	asm(".macro something\n\t.endm");
}

there is nothing that prevents the compiler from emitting this more
than once.  Expecting things to be emitted in whatever order is a bad
idea, too.

The thing with .purgem can work.  Inelegant, sure, but it can work :-)
Just make sure you do the macro define, the code that uses it, and the
undefine, all in the same inline asm statement.


Segher