linux-kernel - Re: [PATCH 18/20] x86/smap: Use alternative_io() in smap

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Message-ID: <CAFULd4aY7bb6252YTFdPS4EZWi85Ra2MVr+4hVAP5X5_B8hvyQ@mail.gmail.com>
Date: Sat, 15 Mar 2025 10:09:16 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Peter Zijlstra <peterz@...radead.org>, 
	Borislav Petkov <bp@...en8.de>, "H. Peter Anvin" <hpa@...or.com>, Andrew Cooper <andrew.cooper3@...rix.com>, 
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 18/20] x86/smap: Use alternative_io() in smap_{save,restore}()

On Fri, Mar 14, 2025 at 10:42 PM Josh Poimboeuf <jpoimboe@...nel.org> wrote:
>
> Use the standard alternative_io() interface.
>
> Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
> ---
>  arch/x86/include/asm/smap.h | 18 ++++++++++--------
>  1 file changed, 10 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/include/asm/smap.h b/arch/x86/include/asm/smap.h
> index 60ea21b4c8b7..8b77ddcb37e7 100644
> --- a/arch/x86/include/asm/smap.h
> +++ b/arch/x86/include/asm/smap.h
> @@ -39,20 +39,22 @@ static __always_inline unsigned long smap_save(void)
>  {
>         unsigned long flags;
>
> -       asm volatile ("# smap_save\n\t"
> -                     ALTERNATIVE("",
> -                                 "pushf; pop %[flags]; clac\n\t", X86_FEATURE_SMAP)
> -                     : [flags] "=rm" (flags) : : "memory", "cc");
> +       alternative_io("",
> +                      "pushf; pop %[flags]; clac\n\t", X86_FEATURE_SMAP,

Please merge two lines above (if possible, otherwise put feature
condition in the next line. Please also remove trailing \n\t.

> +                      ARG([flags] "=rm" (flags)),
> +                      ARG(),
> +                      ARG("memory", "cc"));
>
>         return flags;
>  }
>
>  static __always_inline void smap_restore(unsigned long flags)
>  {
> -       asm volatile ("# smap_restore\n\t"
> -                     ALTERNATIVE("",
> -                                 "push %[flags]; popf\n\t", X86_FEATURE_SMAP)
> -                     : : [flags] "g" (flags) : "memory", "cc");
> +       alternative_io("",
> +                      "push %[flags]; popf\n\t", X86_FEATURE_SMAP,

As above.

> +                      ARG(),
> +                      ARG([flags] "g" (flags)),

Uh, a build failure waiting to happen here. Please use "rme" instead
of "g". PUSH doesn't support 64bit immediates ("i") on x86_64 ("g"
expands to "rmi").

Thanks,
Uros.