linux-kernel - Re: [PATCH 1/2] ARM: drop unnecessary WASM

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Date:   Sun, 17 Mar 2019 16:44:44 +0100
From:   Stefan Agner <stefan@...er.ch>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Robin Murphy <robin.murphy@....com>,
        Russell King <linux@...linux.org.uk>,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        Mark Rutland <mark.rutland@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Will Deacon <will.deacon@....com>, julien.thierry@....com,
        LKML <linux-kernel@...r.kernel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Kristof Beyls <Kristof.Beyls@....com>,
        Peter Smith <Peter.Smith@....com>
Subject: Re: [PATCH 1/2] ARM: drop unnecessary WASM

On 06.03.2019 01:47, Nick Desaulniers wrote:
> On Tue, Mar 5, 2019 at 3:39 PM Robin Murphy <robin.murphy@....com> wrote:
>>
>> Hi Stefan,
>>
>> On 2019-03-05 10:18 pm, Stefan Agner wrote:
>> > The W macro for generating wide instructions when targeting Thumb-2
>> > is not required for the preload data instructions (pld, pldw) since
>> > they are only available as wide instructions. The GNU assembler seems
>> > to work with or without the .w appended when compiling an Thumb-2
>> > kernel. However, Clang's integrated assembler does not consider the
>> > .w variants as valid instructions:
>> >
>> >    ./arch/arm/include/asm/processor.h:133:5: error: invalid instruction
>> >                            "pldw.w\t%a0 \n"
>> >                             ^
>> >    <inline asm>:2:1: note: instantiated into assembly here
>> >    pldw.w  [r0]
>> >    ^
>> >    1 error generated.
>>
>> Have you filed a bug against Clang for that? Something like "pldwal.w"
> 
> Yes; please.  For each deficiency you find, please file a bug.  We're
> working on identifying what's missing from Clang's integrated
> assembler support.  Given the list of issues, it's easier to estimate
> how much effort is needed, which helps us allocate resources towards
> fixing those issues better.
> 
> It would be good to know if pldw.w is valid under UAL or not.

As far as I understand the Arm ARM it is valid under UAL.

Reported a bug in LLVM's bug tracker:
https://bugs.llvm.org/show_bug.cgi?id=40972

Will send a v2 patch mentioning this is really a work around for LLVM
and link to the bug report.

> 
> Hopefully, the ARM kernel team can stress the importance of assembler
> support for their ISA to their LLVM team.

--
Stefan