Re: [PATCH RFC 1/1] x86/paravirt: introduce param to disable pv sched_clock

[Sean Christopherson]

On Thu, Oct 19, 2023, David Woodhouse wrote:
> On Thu, 2023-10-19 at 08:40 -0700, Sean Christopherson wrote:
> > > If for some 'historical reasons' we can't revoke features we can always
> > > introduce a new PV feature bit saying that TSC is preferred.
> 
> Don't we already have one? It's the PVCLOCK_TSC_STABLE_BIT. Why would a
> guest ever use kvmclock if the PVCLOCK_TSC_STABLE_BIT is set?
>
> The *point* in the kvmclock is that the hypervisor can mess with the
> epoch/scaling to try to compensate for TSC brokenness as the host
> scales/sleeps/etc.
> 
> And the *problem* with the kvmclock is that it does just that, even
> when the host TSC hasn't done anything wrong and the kvmclock shouldn't
> have changed at all.
> 
> If the PVCLOCK_TSC_STABLE_BIT is set, a guest should just use the guest
> TSC directly without looking to the kvmclock for adjusting it.
> 
> No?

No :-)

PVCLOCK_TSC_STABLE_BIT doesn't provide the guarantees that are needed to use the
raw TSC directly.  It's close, but there is at least one situation where using TSC
directly even when the TSC is stable is bad idea: when hardware doesn't support TSC
scaling and the guest virtual TSC is running at a higher frequency than the hardware
TSC.  The guest doesn't have to worry about the TSC going backwards, but using the
TSC directly would cause the guest's time calculations to be inaccurate.

And PVCLOCK_TSC_STABLE_BIT is also much more dynamic as it's tied to a given
generation/sequence.  E.g. if KVM stops using its masterclock for whatever reason,
then kvm_guest_time_update() will effectively clear PVCLOCK_TSC_STABLE_BIT and the
guest-side __pvclock_clocksource_read() will be forced to do a bit of extra work
to ensure the clock is monotonically increasing.