linux-kernel - Re: [RFC PATCH v3 19/21] KVM: x86: Avoid periodic KVM clock updates in master clock mode

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Message-ID: <Zr7WfbCBPhoNMukJ@google.com>
Date: Thu, 15 Aug 2024 21:33:01 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: David Woodhouse <dwmw2@...radead.org>
Cc: kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>, 
	Jonathan Corbet <corbet@....net>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, Paul Durrant <paul@....org>, Peter Zijlstra <peterz@...radead.org>, 
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>, 
	Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, 
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, 
	Daniel Bristot de Oliveira <bristot@...hat.com>, Valentin Schneider <vschneid@...hat.com>, Shuah Khan <shuah@...nel.org>, 
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	jalliste@...zon.co.uk, sveith@...zon.de, zide.chen@...el.com, 
	Dongli Zhang <dongli.zhang@...cle.com>, Chenyi Qiang <chenyi.qiang@...el.com>
Subject: Re: [RFC PATCH v3 19/21] KVM: x86: Avoid periodic KVM clock updates
 in master clock mode

On Wed, May 22, 2024, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.co.uk>
> 
> When the KVM clock is in master clock mode, updating the KVM clock is
> pointless.

Please explain why it's pointless.  I almost feel bad asking you to elaborate
since you've written wonderful changelogs for so many other patches, but far too
many of the (historical) kvmclock commits assume the reader is some omniscient
being, and it's definitely contributed to things getting into such a mess state.

> Let the periodic work 'expire', and start it running again
> from kvm_end_pvclock_update() if the master clock mode is ever turned
> off again.