linux-kernel - Re: [PATCH v2] arm64: Delete the space separator in __emit

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Message-ID: <20200415120752.GD6526@gaia>
Date:   Wed, 15 Apr 2020 13:07:52 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Fangrui Song <maskray@...gle.com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com,
        Ilie Halip <ilie.halip@...il.com>,
        Jian Cai <jiancai@...gle.com>
Subject: Re: [PATCH v2] arm64: Delete the space separator in __emit_inst

On Tue, Apr 14, 2020 at 09:32:55AM -0700, Fangrui Song wrote:
> In assembly, many instances of __emit_inst(x) expand to a directive. In
> a few places __emit_inst(x) is used as an assembler macro argument. For
> example, in arch/arm64/kvm/hyp/entry.S
> 
>   ALTERNATIVE(nop, SET_PSTATE_PAN(1), ARM64_HAS_PAN, CONFIG_ARM64_PAN)
> 
> expands to the following by the C preprocessor:
> 
>   alternative_insn nop, .inst (0xd500401f | ((0) << 16 | (4) << 5) | ((!!1) << 8)), 4, 1
> 
> Both comma and space are separators, with an exception that content
> inside a pair of parentheses/quotes is not split, so the clang
> integrated assembler splits the arguments to:
> 
>    nop, .inst, (0xd500401f | ((0) << 16 | (4) << 5) | ((!!1) << 8)), 4, 1
> 
> GNU as preprocesses the input with do_scrub_chars(). Its arm64 backend
> (along with many other non-x86 backends) sees:
> 
>   alternative_insn nop,.inst(0xd500401f|((0)<<16|(4)<<5)|((!!1)<<8)),4,1
>   # .inst(...) is parsed as one argument
> 
> while its x86 backend sees:
> 
>   alternative_insn nop,.inst (0xd500401f|((0)<<16|(4)<<5)|((!!1)<<8)),4,1
>   # The extra space before '(' makes the whole .inst (...) parsed as two arguments
> 
> The non-x86 backend's behavior is considered unintentional
> (https://sourceware.org/bugzilla/show_bug.cgi?id=25750).
> So drop the space separator inside `.inst (...)` to make the clang
> integrated assembler work.
> 
> Suggested-by: Ilie Halip <ilie.halip@...il.com>
> Signed-off-by: Fangrui Song <maskray@...gle.com>
> Reviewed-by: Mark Rutland <mark.rutland@....com>
> Link: https://github.com/ClangBuiltLinux/linux/issues/939

Queued for 5.7. Thanks.

-- 
Catalin