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

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Message-ID: <54BCC060-1C9B-4BE4-8057-0161E816A9A3@amazon.co.uk>
Date: Fri, 29 Aug 2025 11:08:22 +0000
From: "Durrant, Paul" <pdurrant@...zon.co.uk>
To: David Woodhouse <dwmw2@...radead.org>, Sean Christopherson
	<seanjc@...gle.com>, "Durrant, Paul" <pdurrant@...zon.co.uk>, "Griffoul,
 Fred" <fgriffo@...zon.co.uk>
CC: Colin Percival <cperciva@...snap.com>, 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" <x86@...nel.org>, "H. Peter
 Anvin" <hpa@...or.com>, Vitaly Kuznetsov <vkuznets@...hat.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "Graf (AWS), Alexander" <graf@...zon.de>,
	Ajay Kaher <ajay.kaher@...adcom.com>, Alexey Makhalov
	<alexey.makhalov@...adcom.com>
Subject: Re: [PATCH v2 0/3] Support "generic" CPUID timing leaf as KVM guest and host

On 29/08/2025, 10:51, "David Woodhouse" <dwmw2@...radead.org <mailto:dwmw2@...radead.org>> wrote:
[snip]
> • Declare that we don't care that it's strictly an ABI change, and
> VMMs which used to just populate the leaf and let KVM fill it in
> for Xen guests now *have* to use the new API.
> 
> 
> I'm actually OK with that, even the last one, because I've just noticed
> that KVM is updating the *wrong* Xen leaf. 0x40000x03/2 EAX is supposed
> to be the *host* TSC frequency, and the guest frequency is supposed to
> be in 0x40000x03/0 ECX. And Linux as a Xen guest doesn't even use it
> anyway, AFAICT
> 
> Paul, it was your code originally; are you happy with removing it?

Yes, if it is incorrect then please fix it. I must have become confused whilst reading the original Xen code. 

Cheers,

  Paul