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: <aCROdV_fIygO8OoM@gmail.com>
Date: Wed, 14 May 2025 10:04:05 +0200
From: Ingo Molnar <mingo@...nel.org>
To: "Kirill A. Shutemov" <kirill@...temov.name>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org,
	x86@...nel.org, Ard Biesheuvel <ardb@...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


* Kirill A. Shutemov <kirill@...temov.name> wrote:

> 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.

Kernel selftests do not really count if that's the only userspace that 
does this - but they indeed increase the likelihood that some external 
userspace uses /proc/cpuinfo in that fashion. Does such external 
user-space code exist?

Thanks,

	Ingo