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

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Message-ID: <B7AB40CF-165D-448C-963C-787D74BB9042@alien8.de>
Date: Thu, 13 Mar 2025 21:07:20 +0100
From: Borislav Petkov <bp@...en8.de>
To: Uros Bizjak <ubizjak@...il.com>
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 March 13, 2025 8:50:24 PM GMT+01:00, Uros Bizjak <ubizjak@...il.com> wrote:
>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.

Sorry but this doesn't justify this churn. There's nothing quantifyingly palpable here to warrant this.
-- 
Sent from a small device: formatting sucks and brevity is inevitable.