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

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Message-ID: <CAMj1kXGY0VYLdKiveeznH7+8AVD7Uq4PGEhr=+pcT-Nw3WWvgg@mail.gmail.com>
Date: Thu, 15 May 2025 14:33:50 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Borislav Petkov <bp@...en8.de>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, Shivank Garg <shivankg@....com>, linux-kernel@...r.kernel.org, 
	x86@...nel.org, Ingo Molnar <mingo@...nel.org>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Brian Gerst <brgerst@...il.com>, 
	"Rao, Bharata Bhasker" <bharata@....com>
Subject: Re: [PATCH v3 1/7] x86/cpu: Use a new feature flag for 5 level paging

On Thu, 15 May 2025 at 14:11, Borislav Petkov <bp@...en8.de> wrote:
>
> On Wed, May 14, 2025 at 12:42:44PM +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.
>
> Btw, that gunk:
>
> We had started simplifying the whole 5-level crap:
>
> https://lore.kernel.org/all/20240621164406.256314-1-kirill.shutemov@linux.intel.com/
>
> Shivank, I hear the performance issues got resolved in the meantime?
>

It would be interesting to know whether CONFIG_X86_5LEVEL=n deviates
from CONFIG_X86_5LEVEL=y with 'no5lvl' on the command line. If passing
'no5lvl' makes up for the performance hit, then I don't think the
performance issues should stop us from removing this Kconfig symbol.