linux-kernel - Re: [RESEND PATCH] arm64: fix alternatives with LLVM's integrated assembler

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Date:   Thu, 14 Nov 2019 16:55:44 +0000
From:   Will Deacon <will@...nel.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [RESEND PATCH] arm64: fix alternatives with LLVM's integrated
 assembler

Hi Sami,

Sorry -- I thought I'd already replied to this, but it had actually
slipped through the cracks.

On Thu, Oct 31, 2019 at 12:46:52PM -0700, Sami Tolvanen wrote:
> LLVM's integrated assembler fails with the following error when
> building KVM:
> 
>   <inline asm>:12:6: error: expected absolute expression
>    .if kvm_update_va_mask == 0
>        ^
>   <inline asm>:21:6: error: expected absolute expression
>    .if kvm_update_va_mask == 0
>        ^
>   <inline asm>:24:2: error: unrecognized instruction mnemonic
>           NOT_AN_INSTRUCTION
>           ^
>   LLVM ERROR: Error parsing inline asm
> 
> These errors come from ALTERNATIVE_CB and __ALTERNATIVE_CFG,
> which test for the existence of the callback parameter in inline
> assembly using the following expression:
> 
>   " .if " __stringify(cb) " == 0\n"
> 
> This works with GNU as, but isn't supported by LLVM. This change
> splits __ALTERNATIVE_CFG and ALTINSTR_ENTRY into separate macros
> to fix the LLVM build.

Please could you explain a bit more about the failure and why LLVM's
integrated assembler rejects this? Could we use something like .ifb or
.ifeqs instead?

Thanks,

Will