linux-kernel - Re: [PATCH v4 1/6] x86/cpu: Use a new feature flag for 5 level paging

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Message-ID: <20250519131944.GCaCsv8A71vn21AB1W@fat_crate.local>
Date: Mon, 19 May 2025 15:19:44 +0200
From: Borislav Petkov <bp@...en8.de>
To: Ingo Molnar <mingo@...nel.org>,
	"Ahmed S. Darwish" <darwi@...utronix.de>,
	Thomas Gleixner <tglx@...utronix.de>
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>,
	Brian Gerst <brgerst@...il.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>
Subject: Re: [PATCH v4 1/6] x86/cpu: Use a new feature flag for 5 level paging

On Mon, May 19, 2025 at 03:08:56PM +0200, Ingo Molnar wrote:
> The second best thing we can do is to have a sane, constant LA57 flag 
> for the hardware capability, and introduce a synthethic flag that is 
> set conditionally (X86_FEATURE_5LEVEL_PAGING) - which is how it should 
> have been done originally, and to maintain compatibility, expose the 
> synthethic flag in /proc/cpuinfo as 'la57' to maintain the ABI.

- we don't expose every CPUID flag in /proc/cpuinfo for obvious reasons:

  Documentation/arch/x86/cpuinfo.rst

- if you want to mirror CPUID *capability* flags with X86_FEATURE flags *and*
  use the same alternatives infrastructure to test *enabled* feature flags,
  then you almost always must define *two* flags - a capability one or an
  enabled one. I don't think we want that.

And since we're dealing with ancient infrastructure which has grown warts over
the years, we all - x86 maintainers - need to decide here how we should go
forward. I have raised these questions multiple times but we have never
discussed it properly.

Also, Ahmed and tglx are working on a unified CPUID view where you can test
capability.  Which means, what is enabled can be used solely by the
X86_FEATURE flags but I haven't looked at his set yet.

So it is high time we sit down and hammer out the rules for the feature flags
as apparently what we have now is a total mess.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette