linux-kernel - Re: [PATCH] x86/uaccess: fix code generation in put

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Message-ID: <8820745F-E761-42E6-8A70-7B04EE70692C@zytor.com>
Date:   Fri, 23 Oct 2020 13:59:58 -0700
From:   hpa@...or.com
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/uaccess: fix code generation in put_user()

On October 23, 2020 1:55:22 PM PDT, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>Thanks, applied.
>
>On Fri, Oct 23, 2020 at 1:32 PM Rasmus Villemoes
><linux@...musvillemoes.dk> wrote:
>>
>> I'm wondering if one would also need to make __ptr_pu and __ret_pu
>> explicitly "%"_ASM_CX".
>
>No, the "c"/"0" thing is much better, and makes it properly atomic wrt
>the actual asm.
>
>As mentioned to Andy, the "register asm()" thing is not uncommon and
>often useful, but when you can specify the register directly in asm,
>that's certainly simpler and more straightforward and preferred.
>
>              Linus

There is no same reason to mess around with hacks when we are talking about dx:ax, though. We have to do pretty ugly hacks when other register pairs are involved, but "A" is there for a reason. _ASM_AX64 maybe...
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