linux-kernel - Re: [PATCH] x86/asm: Use asm_inline() instead of asm() in amd_clear

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Message-ID: <CAFULd4ZTkBwFo3nWXNZKXSKiy4dgPoZ8i95nj3UdtQPApKdj3g@mail.gmail.com>
Date: Thu, 13 Mar 2025 20:50:24 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: Borislav Petkov <bp@...en8.de>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...nel.org>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86/asm: Use asm_inline() instead of asm() in amd_clear_divider()

On Thu, Mar 13, 2025 at 8:26 PM Borislav Petkov <bp@...en8.de> wrote:
>
> On March 13, 2025 8:18:09 PM GMT+01:00, Uros Bizjak <ubizjak@...il.com> wrote:
> >Use asm_inline() to instruct the compiler that the size of asm()
> >is the minimum size of one instruction, ignoring how many instructions
> >the compiler thinks it is. ALTERNATIVE macro that expands to several
> >pseudo directives causes instruction length estimate to count
> >more than 20 instructions.
> >
> >bloat-o-meter reports no code size changes.
> >
> >Signed-off-by: Uros Bizjak <ubizjak@...il.com>
> >Cc: Thomas Gleixner <tglx@...utronix.de>
> >Cc: Ingo Molnar <mingo@...nel.org>
> >Cc: Borislav Petkov <bp@...en8.de>
> >Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> >Cc: "H. Peter Anvin" <hpa@...or.com>
> >---
> > arch/x86/include/asm/processor.h | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> >diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> >index 5d2f7e5aff26..06e499ba4fe8 100644
> >--- a/arch/x86/include/asm/processor.h
> >+++ b/arch/x86/include/asm/processor.h
> >@@ -707,7 +707,7 @@ static inline u32 per_cpu_l2c_id(unsigned int cpu)
> >  */
> > static __always_inline void amd_clear_divider(void)
> > {
> >-      asm volatile(ALTERNATIVE("", "div %2\n\t", X86_BUG_DIV0)
> >+      asm_inline volatile(ALTERNATIVE("", "div %2", X86_BUG_DIV0)
> >                    :: "a" (0), "d" (0), "r" (1));
> > }
> >
>
> So there's no point for this one...

Not at its current usage sites, but considering that
amd_clear_divider() and two levels of small functions that include it
( amd_clear_divider(), arch_exit_to_user_mode() and
exit_to_user_mode() ) all need to be decorated with __always_inline
indicates that the issue is not that benign.

It also triggers my search scripts, so I thought I should convert this
one as well and put the issue at rest.

Uros.