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

[Sean Christopherson <seanjc@google.com>]

On Tue, Sep 02, 2025, David Woodhouse wrote:
> 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.

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)

> 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?

The effective guest TSC frequency should be whatever is reported in KVM_GET_TSC_KHZ
when done on a vCPU, modulo temporarily skewed results without hardware scaling.
If that doesn't hold true, we should fix that.