linux-kernel - Use %a asm operand modifier to obtain %rip-relative addressing

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Date:   Mon, 20 Nov 2023 10:39:08 +0100
From:   Uros Bizjak <ubizjak@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Nadav Amit <namit@...are.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Use %a asm operand modifier to obtain %rip-relative addressing

On Thu, Oct 12, 2023 at 7:10 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Thu, 12 Oct 2023 at 09:55, Uros Bizjak <ubizjak@...il.com> wrote:
> >
> > An example:
>
> Oh, I'm convinced.
>
> The fix seems to be a simple one-liner, ie just
>
> -       asm(__pcpu_op2_##size(op, __percpu_arg(P[var]), "%[val]")       \
> +       asm(__pcpu_op2_##size(op, __percpu_arg(a[var]), "%[val]")       \
>
> and it turns out that we have other places where I think we could use that '%a',
>
> For example, we have things like this:
>
>         asm ("lea sme_cmdline_arg(%%rip), %0"
>              : "=r" (cmdline_arg)
>              : "p" (sme_cmdline_arg));
>
> and I think the only reason we do that ridiculous asm is that the code
> in question really does want that (%rip) encoding. It sounds like this
> could just do
>
>         asm ("lea %a1, %0"
>              : "=r" (cmdline_arg)
>              : "p" (sme_cmdline_arg));
>
> instead. Once again, I claim ignorance of the operand modifiers as the
> reason for these kinds of things.

I have looked a bit in the above code. From the compiler PoV, the
above can be written as:

         asm ("lea %1, %0"
              : "=r" (cmdline_arg)
              : "m" (sme_cmdline_arg));

and it will always result in %rip-relative asm address:

#APP
# 585 "arch/x86/mm/mem_encrypt_identity.c" 1
    lea sme_cmdline_arg(%rip), %rsi
# 0 "" 2
#NO_APP

Uros.