Re: [PATCHv6] KVM: x86: Use current rather than snapshotted TSC frequency if it is constant

[Sean Christopherson <seanjc@google.com>]

On Wed, Jun 08, 2022, Anton Romanov wrote:
> 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>
> ---

Merged to kvm/queue, thanks!

https://lore.kernel.org/all/Y4lHxds8pvBhxXFX@google.com