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 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?

Yep.

> 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.