linux-kernel - Re: [RFC PATCH v2 4/6] x86/boot: Set 5-level paging CPU cap before entering C code

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Message-ID: <aCRWSpS8B-B26AB6@gmail.com>
Date: Wed, 14 May 2025 10:37:30 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org,
	x86@...nel.org, Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC PATCH v2 4/6] x86/boot: Set 5-level paging CPU cap before
 entering C code

* Ard Biesheuvel <ardb@...nel.org> wrote:

> > > -     if (IS_ENABLED(CONFIG_X86_5LEVEL) && (native_read_cr4() & X86_CR4_LA57))
> > > -             setup_force_cpu_cap(X86_FEATURE_5LEVEL_PAGING);
> 
> Note that at this point, we'll likely still have to force clear the 
> original X86_FEATURE_LA57 bit, to address the issue that Kirill 
> raised that user space is now likely to conflate the "la57" cpuinfo 
> string with 5-level paging being in use.

No, I think the general outcome of your series is fine and clean in 
terms of kernel-internal logic, and I wouldn't mess up that clarity 
with user ABI quirks: and we can solve the /proc/cpuinfo ABI 
compatibility requirement by exposing X86_FEATURE_5LEVEL_PAGING as 
'la57', and renaming X86_FEATURE_LA57 to X86_FEATURE_LA57_HW and 
exposing it as a (new) la57_hw flag, or so.

Thanks,

	Ingo