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

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Message-ID: <B0BAC00B-2FF4-40C4-9034-68EAB51341B1@zytor.com>
Date: Sat, 15 Mar 2025 12:08:02 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Uros Bizjak <ubizjak@...il.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
CC: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH] x86/asm: Use alternative_input() in amd_clear_divider()

On March 14, 2025 1:14:38 AM PDT, Uros Bizjak <ubizjak@...il.com> wrote:
>Use higher-level API to declare assembly with alternatives.
>
>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 | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
>diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
>index 5d2f7e5aff26..b4b5aa46f68b 100644
>--- a/arch/x86/include/asm/processor.h
>+++ b/arch/x86/include/asm/processor.h
>@@ -707,8 +707,9 @@ 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)
>-		     :: "a" (0), "d" (0), "r" (1));
>+	alternative_input("", "div %[den]",
>+			  X86_BUG_DIV0,
>+			  "a" (0), "d" (0), [den] "r" (1));
> }
> 
> extern void amd_check_microcode(void);

I realize I'm probably in the minority, but I find these "higher-level APIs" for alternatives much harder to read and understand.