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

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Date:   Mon, 2 Oct 2023 23:06:07 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Dongli Zhang <dongli.zhang@...cle.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Joe Jin <joe.jin@...cle.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com
Subject: Re: [PATCH RFC 1/1] KVM: x86: add param to update master clock
 periodically

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).