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Message-ID: <3268e953e14004d1786bf07c76ae52d98d0f8259.camel@infradead.org>
Date: Tue, 02 Sep 2025 19:23:06 +0100
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 Tue, 2025-09-02 at 10:49 -0700, Sean Christopherson wrote:
>
> > So even if a VMM has set the TSC frequency VM-wide with KVM_SET_TSC_KHZ
> > instead of doing it the old per- vCPU way, how can it get the results for a
> > specific VM?
>
> I don't see any need for userspace to query per-VM support. What I'm proposing
> is that KVM advertise the feature if the bare metal TSC is constant and the CPU
> supports TSC scaling. Beyond that, _KVM_ doesn't need to do anything to ensure
> the guest sees a constant frequency, it's userspace's responsibility to provide
> a sane configuration.
>
> And strictly speaking, CPUID is per-CPU, i.e. it's architecturally legal to set
> per-vCPU frequencies and then advertise a different frequency in CPUID for each
> vCPU. That's all but guaranteed to break guests as most/all kernels assume that
> TSC operates at the same frequency on all CPUs, but as above, that's userspace's
> responsibility to not screw up.
Sure, but doesn't that make this whole thing orthogonal to the original
problem being solved? Because userspace still doesn't *know* the actual
effective TSC frequency, whether it's scaled or not.
Or are you suggesting that we add the leaf (with unscaled values) in
KVM_GET_SUPPORTED_CPUID and *also* 'correct' the values if userspace
does pass that leaf to its guests, as I had originally proposed?
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