Re: [PATCH RFC 1/1] KVM: x86: add param to update master clock periodically

[Peter Zijlstra]

On Mon, Oct 02, 2023 at 11:18:50AM -0700, Sean Christopherson wrote:
> +PeterZ
> 
> Thomas and Peter,
> 
> We're trying to address an issue where KVM's paravirt kvmclock drifts from the
> host's TSC-based monotonic raw clock because of historical reasons (at least, AFAICT),
> even when the TSC is constant.  Due to some dubious KVM behavior, KVM may sometimes
> re-sync kvmclock against the host's monotonic raw clock, which causes non-trivial
> jumps in time from the guest's perspective.
> 
> Linux-as-a-guest demotes all paravirt clock sources when the TSC is constant and
> nonstop, and so the goofy KVM behavior isn't likely to affect the guest's clocksource,
> but the guest's sched_clock() implementation keeps using the paravirt clock.
> 
> Irrespective of if/how we fix the KVM host-side mess, using a paravirt clock for
> the scheduler when using a constant, nonstop TSC for the clocksource seems at best
> inefficient, and at worst unnecessarily complex and risky.
> 
> Is there any reason not to prefer native_sched_clock() over whatever paravirt
> clock is present when the TSC is the preferred clocksource? 

I see none, that whole pv_clock thing is horrible crap.

> Assuming the desirable
> thing to do is to use native_sched_clock() in this scenario, do we need a separate
> rating system, or can we simply tie the sched clock selection to the clocksource
> selection, e.g. override the paravirt stuff if the TSC clock has higher priority
> and is chosen?

Yeah, I see no point of another rating system. Just force the thing back
to native (or don't set it to that other thing).