linux-kernel - Re: [RFC PATCH v2 2/6] x86/cpu: Use a new feature flag for 5 level paging

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Message-ID: <7uh3pi23cdd5r2t6ln5p2z2htgmzo5b6omlhb6vyddobcbqqnt@nyujbhsnpioh>
Date: Wed, 14 May 2025 10:32:21 +0300
From: "Kirill A. Shutemov" <kirill@...temov.name>
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>
Subject: Re: [RFC PATCH v2 2/6] x86/cpu: Use a new feature flag for 5 level
 paging

On Tue, May 13, 2025 at 01:12:00PM +0200, Ard Biesheuvel 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.

Unfortunately, there's already userspace that use la57 flag in
/proc/cpuinfo as indication that 5-level paging is active. :/

See va_high_addr_switch.sh in kernel selftests for instance.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov