[PATCH v7 3/6] KVM: x86: Report host tsc and realtime values in KVM_GET_CLOCK

[Marcelo Tosatti]

On Mon, Aug 16, 2021 at 12:11:27AM +0000, Oliver Upton wrote:
> Handling the migration of TSCs correctly is difficult, in part because
> Linux does not provide userspace with the ability to retrieve a (TSC,
> realtime) clock pair for a single instant in time. In lieu of a more
> convenient facility, KVM can report similar information in the kvm_clock
> structure.
> 
> Provide userspace with a host TSC & realtime pair iff the realtime clock
> is based on the TSC. If userspace provides KVM_SET_CLOCK with a valid
> realtime value, advance the KVM clock by the amount of elapsed time. Do
> not step the KVM clock backwards, though, as it is a monotonic
> oscillator.
> 
> Suggested-by: Paolo Bonzini <pbonzini at redhat.com>
> Signed-off-by: Oliver Upton <oupton at google.com>

This is a good idea. Userspace could check if host and destination
clocks are up to a certain difference and not use the feature if
not appropriate.

Is there a qemu patch for it?