linux-kernel - Re: [PATCH 19/20] x86/uaccess: Use alternative_io() in __untagged

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Message-ID: <CAFULd4akhhuDDj0H0VS88VNEF1PYgQvySsNHv37XSmVorpzVng@mail.gmail.com>
Date: Sat, 15 Mar 2025 10:12:43 +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 19/20] x86/uaccess: Use alternative_io() in __untagged_addr()

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/uaccess_64.h | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/include/asm/uaccess_64.h b/arch/x86/include/asm/uaccess_64.h
> index c52f0133425b..b507d5fb5443 100644
> --- a/arch/x86/include/asm/uaccess_64.h
> +++ b/arch/x86/include/asm/uaccess_64.h
> @@ -26,10 +26,10 @@ extern unsigned long USER_PTR_MAX;
>   */
>  static inline unsigned long __untagged_addr(unsigned long addr)
>  {
> -       asm (ALTERNATIVE("",
> -                        "and " __percpu_arg([mask]) ", %[addr]", X86_FEATURE_LAM)
> -            : [addr] "+r" (addr)
> -            : [mask] "m" (__my_cpu_var(tlbstate_untag_mask)));
> +       alternative_io("",

No, alternative_io() declares asm as volatile. Please define
alternative_io_nv (or something like that)  that will not use
volatile. You will penalize the above asm unnecessarily with volatile.

Thanks,
Uros.