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

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Message-ID: <20250517175932.2cd2d678@pumpkin>
Date: Sat, 17 May 2025 17:59:32 +0100
From: David Laight <david.laight.linux@...il.com>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Borislav Petkov <bp@...en8.de>, 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 14:33:50 +0100
Ard Biesheuvel <ardb@...nel.org> wrote:

> 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.

You might then want a Kconfig option to invert the default for the
command line option (and an inverted command line option).

	David