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

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Message-ID: <4a3be390fe559de0bd5c61d24853d88f96a6ab6a.camel@infradead.org>
Date: Thu, 04 Sep 2025 14:14:57 +0200
From: David Woodhouse <dwmw2@...radead.org>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paul Durrant <pdurrant@...zon.co.uk>, Fred Griffoul
 <fgriffo@...zon.co.uk>,  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 Thu, 2025-09-04 at 04:59 -0700, Sean Christopherson wrote:
> 
> I thought the original problem being solved was that the _guest_ doesn't know the
> effective TSC frequency?  Userspace can already get the effectively TSC frequency
> via KVM_GET_TSC_KHZ, why do we need another uAPI to provide that?  (Honest question,
> I feel like I'm missing something)

I believe that KVM_GET_TSC_KHZ returns what userspace *asked* for the
TSC frequency to be (vcpu->arch.virtual_tsc_khz), not what it actually
ended up being based on the measured host frequency and the available
scaling granularity (vcpu->hw_tsc_khz).

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