KVM: x86: Use current rather than snapshotted TSC frequency if it is constant

Don't snapshot tsc_khz into per-cpu cpu_tsc_khz if the host TSC is
constant, in which case the actual TSC frequency will never change and thus
capturing TSC during initialization is unnecessary, KVM can simply use
tsc_khz.  This value is snapshotted from
kvm_timer_init->kvmclock_cpu_online->tsc_khz_changed(NULL)

On CPUs with constant TSC, but not a hardware-specified TSC frequency,
snapshotting cpu_tsc_khz and using that to set a VM's target TSC frequency
can lead to VM to think its TSC frequency is not what it actually is if
refining the TSC completes after KVM snapshots tsc_khz.  The actual
frequency never changes, only the kernel's calculation of what that
frequency is changes.

Ideally, KVM would not be able to race with TSC refinement, or would have
a hook into tsc_refine_calibration_work() to get an alert when refinement
is complete.  Avoiding the race altogether isn't practical as refinement
takes a relative eternity; it's deliberately put on a work queue outside of
the normal boot sequence to avoid unnecessarily delaying boot.

Adding a hook is doable, but somewhat gross due to KVM's ability to be
built as a module.  And if the TSC is constant, which is likely the case
for every VMX/SVM-capable CPU produced in the last decade, the race can be
hit if and only if userspace is able to create a VM before TSC refinement
completes; refinement is slow, but not that slow.

For now, punt on a proper fix, as not taking a snapshot can help some uses
cases and not taking a snapshot is arguably correct irrespective of the
race with refinement.

Signed-off-by: Anton Romanov <romanton@google.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20220608183525.1143682-1-romanton@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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