linux-kernel - Re: [PATCH 1/2] KVM: x86: Add KVM_[GS]ET_CLOCK

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Date: Wed, 10 Apr 2024 10:08:29 +0000
From: "Allister, Jack" <jalliste@...zon.co.uk>
To: "dongli.zhang@...cle.com" <dongli.zhang@...cle.com>
CC: "dwmw2@...radead.org" <dwmw2@...radead.org>, "corbet@....net"
	<corbet@....net>, "seanjc@...gle.com" <seanjc@...gle.com>, "bp@...en8.de"
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	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Allister,
 Jack" <jalliste@...zon.co.uk>, "linux-doc@...r.kernel.org"
	<linux-doc@...r.kernel.org>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
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Subject: Re: [PATCH 1/2] KVM: x86: Add KVM_[GS]ET_CLOCK_GUEST for KVM clock
 drift fixup

> Would the above rely not only on TSC clocksource, but also
> ka->use_master_clock==true?

Yes this should only be in the case of master clock. Extra verification
to check this for both GET/SET should be present.

> What will happen if the vCPU=0 (e.g., BSP) thread is racing with here
> to update the vcpu->arch.hv_clock?

> In addition to live migration, can the user call this API any time
> during the VM is running (to fix the clock drift)? Therefore, any
> requirement to protect the update of kvmclock_offset from racing?

This should occur when none of the vCPUs are running, in the context of
a live-migration/update this would be after deserialization before the
resume. I may have been unintentionally misleading here describing some
of the problem space. There isn't a "drift" per say when a VM is
running and the PVTI stays constant. It's more the relationship between
the TSC & PV clock changes due to inaccuracy when KVM decides to
generate that information, e.g on a live-update/migration KVM will
perform the update. The KVM_REQ_MASTERCLOCK_UPDATE is just an example
as to how to simulate/trigger this.

I've posted a V2 which hopefully addresses some of above and makes it
clearer overall the aim behind the patches.