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

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Message-ID: <aC896zTw_z7Bx6I2@google.com>
Date: Thu, 22 May 2025 08:08:27 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>, 
	"Ahmed S. Darwish" <darwi@...utronix.de>, 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 Thu, May 22, 2025, Peter Zijlstra wrote:
> On Wed, May 21, 2025 at 05:23:37PM +0200, Thomas Gleixner wrote:
> 
> >    4) Drivers having access to CPUID is just wrong. We've had issues
> >       with that in the past because drivers evaluated CPUID themself and
> >       missed that the core code had stuff disabled.
> 
> I had this patch that read the module instructions and failed loading if
> they used 'fancy' instructions. Do you want me to revive that?

Unless you want to grant exceptions, that's not going to fly for KVM.  KVM makes
heavy use of CPUID, the consumption/output of which is firmly entrenched in KVM's
ABI.