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

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Message-ID: <aKeGBkv6ZjwM6V9T@google.com>
Date: Thu, 21 Aug 2025 13:48:06 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: David Woodhouse <dwmw2@...radead.org>
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, Aug 21, 2025, David Woodhouse wrote:
> On Thu, 2025-08-21 at 12:27 -0700, Sean Christopherson wrote:
> >  
> > > The problem with that is that it's been quite unreliable. The kernel
> > > doesn't trust it even on chips as recent (hah) as Skylake. I'd be
> > > happier to trust what the hypervisor explicitly gives us. But yes, it
> > > should be *one* of the sources of information before we reverse-
> > > calculate it from the pvclock. 
> > 
> > Sorry, by "the VMM use" I mean have the host, e.g. QEMU, explicitly define TSC
> > frequency in CPUID.0x15 and CPU frequency in CPUID.0x16.  And then on the
> > KVM-as-a-guest side of things, trust those leaves when they're available.
> 
> Those leaves are untrustworthy on hardware. Are you suggesting that the
> kernel should trust them when it detects that it's running under KVM,
> on the assumption that KVM will have corrected them? And that KVM will
> be fabricating them even on CPU models which didn't naturally have
> those leaves? And that in the presence of TSC scaling, those leaves
> will show the right values for the guest even on hypervisors running
> today?
> 
> I'll be surprised if that works out well.
> 
> 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?