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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