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

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Message-ID: <ZRytbih4TUVkEhLC@google.com>
Date:   Tue, 3 Oct 2023 17:10:22 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Dongli Zhang <dongli.zhang@...cle.com>,
        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 Tue, Oct 03, 2023, David Woodhouse wrote:
> On Mon, 2023-10-02 at 17:53 -0700, Sean Christopherson wrote:
> > 
> > This is *very* lightly tested, as in it compiles and doesn't explode, but that's
> > about all I've tested.
> 
> I don't think it's working, if I understand what it's supposed to be
> doing.

...

> When I run xen_shinfo_test, the kvmclock still drifts from the
> "monotonic_raw" I get from kvm_get_time_and_clockread(), even with your
> patch.

It "works", in that it does what I intended it to do.  What I missed, and what's
obvious in hindsight, is that the timekeeping code doesn't *just* update what KVM
uses as the "base" when there "big events" like suspend/resume, the timekeeping
code updates the base clock on every tick.

My hack only smoothed away the delta, it didn't address the time that had gotten
moved into the base.  I though the base would be stable if the host was in a
steady state, but see above...