linux-kernel - Re: [PATCH v3 1/7] x86/cpu: Use a new feature flag for 5 level paging

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Message-ID: <20250515095157.GCaCW5PRk1CTPjEtzJ@fat_crate.local>
Date: Thu, 15 May 2025 11:51:57 +0200
From: Borislav Petkov <bp@...en8.de>
To: Ard Biesheuvel <ardb+git@...gle.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org,
	Ard Biesheuvel <ardb@...nel.org>, Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Brian Gerst <brgerst@...il.com>
Subject: Re: [PATCH v3 1/7] x86/cpu: Use a new feature flag for 5 level paging

On Wed, May 14, 2025 at 12:42:44PM +0200, Ard Biesheuvel wrote:
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index f67a93fc9391..d59bee5907e7 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -395,7 +395,7 @@
>  #define X86_FEATURE_AVX512_BITALG	(16*32+12) /* "avx512_bitalg" Support for VPOPCNT[B,W] and VPSHUF-BITQMB instructions */
>  #define X86_FEATURE_TME			(16*32+13) /* "tme" Intel Total Memory Encryption */
>  #define X86_FEATURE_AVX512_VPOPCNTDQ	(16*32+14) /* "avx512_vpopcntdq" POPCNT for vectors of DW/QW */
> -#define X86_FEATURE_LA57		(16*32+16) /* "la57" 5-level page tables */
> +#define X86_FEATURE_LA57		(16*32+16) /* "la57_hw" 5-level page tables */

Is there any real reason to expose that flag in /proc/cpuinfo?

>  #define X86_FEATURE_RDPID		(16*32+22) /* "rdpid" RDPID instruction */
>  #define X86_FEATURE_BUS_LOCK_DETECT	(16*32+24) /* "bus_lock_detect" Bus Lock detect */
>  #define X86_FEATURE_CLDEMOTE		(16*32+25) /* "cldemote" CLDEMOTE instruction */
> @@ -483,6 +483,7 @@
>  #define X86_FEATURE_PREFER_YMM		(21*32+ 8) /* Avoid ZMM registers due to downclocking */
>  #define X86_FEATURE_APX			(21*32+ 9) /* Advanced Performance Extensions */
>  #define X86_FEATURE_INDIRECT_THUNK_ITS	(21*32+10) /* Use thunk for indirect branches in lower half of cacheline */
> +#define X86_FEATURE_5LEVEL_PAGING	(21*32+11) /* "la57" Whether 5 levels of page tables are in use */

Or can we stick to that one?

-- 
Regards/Gruss,
    Boris.

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