Re: [PATCH] KVM: x86: Pass host's CPUID.16H through to KVM_GET_SUPPORTED_CPUID

[Sean Christopherson <seanjc@google.com>]

On Fri, Sep 23, 2022, Jim Mattson wrote:
> In the default configuration, the guest TSC frequency is the same as
> the host TSC frequency. Similarly, the maximum frequency of the
> virtual CPU is the same as the maximum frequency of the physical
> CPU.

Under the hood, yes, but after the VM is migrated, isn't it possible that the
host frequencies are completely disjoint from the frequencies that are enumerated
to the guest?

> Also, the bus (reference) frequency of the virtual CPU matches
> that of the physical CPU.
> 
> Pass this information directly from host CPUID.16H to guest CPUID.16H
> in KVM_GET_SUPPORTED_CPUID.

What about "solving" this via documentation, same as CPUID.15H?  If the API were
KVM_GET_DEFAULT_CPUID, then enumerating host properties makes sense, but from a
very pedantic point of view, the "supported" frequencies are just about anything.

Somewhat of a moot point, as the leaf comes with "informational only" disclaimer.

  The returned information should not be used for any other purpose as the returned
  information does not accurately correlate to information / counters returned by
  other processor interfaces.