linux-kernel - Re: [PATCH v5 1/5] x86/cpufeatures: Add {required,disabled} feature configs

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Message-ID: <3B96E82B-7AA8-40FF-9401-829D408AA790@zytor.com>
Date: Sat, 15 Feb 2025 06:27:04 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Borislav Petkov <bp@...en8.de>
CC: "Xin Li (Intel)" <xin@...or.com>, linux-kernel@...r.kernel.org,
        linux-perf-users@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
        dave.hansen@...ux.intel.com, x86@...nel.org, will@...nel.org,
        peterz@...radead.org, yury.norov@...il.com, akpm@...ux-foundation.org,
        acme@...nel.org, namhyung@...nel.org, brgerst@...il.com,
        andrew.cooper3@...rix.com, nik.borisov@...e.com
Subject: Re: [PATCH v5 1/5] x86/cpufeatures: Add {required,disabled} feature configs

On February 15, 2025 6:20:17 AM PST, Borislav Petkov <bp@...en8.de> wrote:
>On Sat, Feb 15, 2025 at 02:12:35AM -0800, H. Peter Anvin wrote:
>> CX8 is the name of the CPUID flag, for better or worse. 
>
>Right, so a separate patch:
>
>git grep X86_CMPXCHG64
>arch/x86/Kconfig:135:   select ARCH_USE_CMPXCHG_LOCKREF         if X86_CMPXCHG64
>arch/x86/Kconfig.cpu:369:config X86_CMPXCHG64
>arch/x86/Kconfig.cpu:383:       default "5" if X86_32 && X86_CMPXCHG64
>arch/x86/include/asm/asm-prototypes.h:19:#ifndef CONFIG_X86_CMPXCHG64
>arch/x86/include/asm/atomic64_32.h:51:#ifdef CONFIG_X86_CMPXCHG64
>arch/x86/include/asm/cmpxchg_32.h:72:#ifdef CONFIG_X86_CMPXCHG64
>arch/x86/include/asm/required-features.h:26:#ifdef CONFIG_X86_CMPXCHG64
>arch/x86/lib/Makefile:59:ifneq ($(CONFIG_X86_CMPXCHG64),y)
>arch/x86/lib/cmpxchg8b_emu.S:10:#ifndef CONFIG_X86_CMPXCHG64
>lib/atomic64_test.c:257:#elif defined(CONFIG_X86_CMPXCHG64)
>tools/arch/x86/include/asm/required-features.h:26:#ifdef CONFIG_X86_CMPXCHG64
>
>should be easy.
>
>> It seems to me to be a bit silly to have dummy symbols that mean literally
>> the same thing, but ...
>
>Why if they're confined and encapsulated in a single file:
>arch/x86/Kconfig.cpufeatures ?
>
>Rest of the kernel doesn't need to know whether a feature is required or not,
>right?
>

The point was that that is the *only* use of this particular flag, I believe.