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

[David Woodhouse]

On Fri, 2025-08-29 at 13:36 -0700, Sean Christopherson wrote:
>  
> > This does mean userspace would have to set the vCPU's TSC frequency and
> > then query the kernel before setting up its CPUID. And in the absence
> > of scaling, this KVM API would report the hardware TSC frequency.
> 
> Reporting the hardware TSC frequency on CPUs without scaling seems all kinds of
> wrong (which another reason I don't like KVM shoving in the state).  Of course,
> reporting the frequency KVM is trying to provide isn't great either, as the guest
> will definitely observe something in between those two.

Yes, on CPUs that don't support TSC scaling, we should not attempt to
advertise a frequency.

Where I said 'in the absence of scaling' I meant modern CPUs but where
the VMM just didn't ask for TSC scaling.

> > I guess the API would have to return -EHARDWARETOOSTUPID if the TSC frequency
> > *isn't* the same across all CPUs and all power states, etc.
> 
> What if KVM advertises the flag in KVM_GET_SUPPORTED_CPUID if and only if the
> TSC will be constant from the guest's perspective?  TSC scaling has been supported
> by AMD and Intel for ~10 years, it doesn't seem at all unreasonable to restrict
> the feature to somewhat modern hardware.  And if userspace or the admin knows
> better than KVM, then userspace can always ignore KVM and report the frequency
> anyways.

I hadn't put it in KVM_GET_SUPPORTED_CPUID; I was following the lead of
the existing Xen leaf support, where *if* userspace provides that leaf,
KVM will dynamically correct the values in it.

The problem is that KVM_GET_SUPPORTED_CPUID is a *system* ioctl on the
bare /dev/kvm device, isn 't it? 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?

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