linux-kernel - Re: [PATCH v2 0/3] Support "generic" CPUID timing leaf as KVM guest and host

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Message-ID: <fdcc635f13ddf5c6c2ce3d5376965c81ce4c1b70.camel@infradead.org>
Date: Thu, 21 Aug 2025 22:10:04 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Thomas Gleixner
 <tglx@...utronix.de>,  Ingo Molnar <mingo@...hat.com>, Borislav Petkov
 <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,  x86@...nel.org,
 "H. Peter Anvin" <hpa@...or.com>, Vitaly Kuznetsov <vkuznets@...hat.com>,
 kvm@...r.kernel.org, linux-kernel@...r.kernel.org,  graf@...zon.de, Ajay
 Kaher <ajay.kaher@...adcom.com>, Alexey Makhalov
 <alexey.makhalov@...adcom.com>, Colin Percival <cperciva@...snap.com>
Subject: Re: [PATCH v2 0/3] Support "generic" CPUID timing leaf as KVM guest
 and host

On Thu, 2025-08-21 at 13:48 -0700, Sean Christopherson wrote:
>  
> > I think I'm a lot happier with the explicit CPUID leaf exposed by the
> > hypervisor.
> 
> Why?  If the hypervisor is ultimately the one defining the state, why does it
> matter which CPUID leaf its in?

It matters to the guest. If there's any hypervisor anywhere which
allows the bogus Skylake CPUID contents to show through to a guest, or
which allows the native hardware contents of the 0x15/0x16 leaves to
show even when TSC scaling is in force, then the guest cannot trust
those leaves.

If you tell me that 0x15 is *never* wrong when seen by a KVM guest, and
that it's OK to extend the hardware CPUID support up to 0x15 even on
older CPUs and there'll never be any adverse consequences from weird
assumptions in guest operating systems if we do the latter... well, for
a start, I won't believe you. And even if I do, I won't think it's
worth the risk. Just use a hypervisor leaf :)

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