linux-kernel - Re: [RFC PATCH 3/3] x86/boot: Use alternatives based selector for 5-level paging constants

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Message-ID: <CAHk-=whrcutH0LcMmaeJxxMeYpw2R5T80yoQAXbrRHNSTCB0Pw@mail.gmail.com>
Date: Tue, 6 May 2025 09:23:13 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Ard Biesheuvel <ardb+git@...gle.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, 
	Ard Biesheuvel <ardb@...nel.org>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC PATCH 3/3] x86/boot: Use alternatives based selector for
 5-level paging constants

On Tue, 6 May 2025 at 08:49, Ard Biesheuvel <ardb+git@...gle.com> wrote:
>
> +       asm_inline(ALTERNATIVE_TERNARY("jmp 6f; 8:", %c[feat], "movl %[yes], %[ret]", "")
> +               "       .pushsection .altinstr_aux,\"ax\"       \n"
> +               "6:     pushfq                                  \n"
> +               "       testb   $1, %a[l5en]                    \n"
> +               "       jz      7f                              \n"
> +               "       movl    %[yes], %[ret]                  \n"
> +               "7:     popfq                                   \n"
> +               "       jmp     8b                              \n"
> +               "       .popsection                             \n"
> +               : [ret]  "+rm" (ret)
> +               : [feat] "i" (X86_FEATURE_LA57),
> +                 [yes]  "i" (yes),
> +                 [l5en] "i" (&__pgtable_l5_enabled));

I really detest these things. I don't think it's worth the complexity.
Is there really such a hot path somewhere that we should do something
like this?

If we can't use some existing infrastructure for this, we should just
keep it simple. Not write ten lines of specialized inline asm.

             Linus