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

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Message-ID: <87h61cquww.ffs@tglx>
Date: Thu, 22 May 2025 21:58:55 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Sean Christopherson <seanjc@...gle.com>, Peter Zijlstra
 <peterz@...radead.org>
Cc: 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 at 08:08, Sean Christopherson wrote:
> 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?

Once we have the new infrastructure in place....

> 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.

If there is a full in memory copy of all CPUID leafs, then what needs KVM beyond
reading it from there?

Thanks,

        tglx