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

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Message-ID: <aCWSh3P3RgoEkhqO@gmail.com>
Date: Thu, 15 May 2025 09:06:47 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Ard Biesheuvel <ardb+git@...gle.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org,
	Ard Biesheuvel <ardb@...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


* Ard Biesheuvel <ardb+git@...gle.com> wrote:

> From: Ard Biesheuvel <ardb@...nel.org>
> 
> Currently, the LA57 CPU feature flag is taken to mean two different
> things at once:
> - whether the CPU implements the LA57 extension, and is therefore
>   capable of supporting 5 level paging;
> - whether 5 level paging is currently in use.
> 
> This means the LA57 capability of the hardware is hidden when a LA57
> capable CPU is forced to run with 4 levels of paging. It also means the
> the ordinary CPU capability detection code will happily set the LA57
> capability and it needs to be cleared explicitly afterwards to avoid
> inconsistencies.
> 
> Separate the two so that the CPU hardware capability can be identified
> unambigously in all cases.
> 
> To avoid breaking existing users that might assume that 5 level paging
> is being used when the "la57" string is visible in /proc/cpuinfo,
> repurpose that string to mean that 5-level paging is in use, and add a
> new string la57_capable to indicate that the CPU feature is implemented
> by the hardware.

So the new string ended up being "la57_hw", not "la57_capable". :-)

> -#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 */

I fixed the changelog and kept la57_hw.

BTW., I too was considering these variants for the new flag:

	la57_support
	la57_cap
	la57_capable

	  - These are easy to confuse with 5-level paging software 
	    support in the kernel, ie. the name doesn't sufficiently 
	    disambiguate that this flag is about hardware support.

	la57_cpu

	  - While this makes it clear that it's about the CPU, the _cpu 
	    postfix often denotes something related to a specific CPU, 
	    so it's a tiny bit confusing in this context.

... and each had disadvantages, as listed, and "la57_hw" seemed the 
least ambiguous in this context.

Thanks,

	Ingo