linux-kernel - Re: [PATCH 0/3] KVM: x86: Mitigate kvm-clock drift caused by masterclock update

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Message-ID: <cea56100-9c43-4246-912b-234c6cfdc876@oracle.com>
Date: Thu, 15 Jan 2026 12:37:36 -0800
From: Dongli Zhang <dongli.zhang@...cle.com>
To: dwmw2@...radead.org, kvm@...r.kernel.org
Cc: seanjc@...gle.com, pbonzini@...hat.com, dwmw@...zon.co.uk, paul@....org,
        tglx@...nel.org, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        linux-kernel@...r.kernel.org, joe.jin@...cle.com
Subject: Re: [PATCH 0/3] KVM: x86: Mitigate kvm-clock drift caused by
 masterclock update

Hi David,

On 1/15/26 12:22 PM, Dongli Zhang wrote:
> As noted in commit c52ffadc65e2 ("KVM: x86: Don't unnecessarily
> force masterclock update on vCPU hotplug"), each unnecessary
> KVM_REQ_MASTERCLOCK_UPDATE can cause the kvm-clock time to jump.
> 
> Although that commit addressed the kvm-clock drift issue during vCPU
> hotplugl there are still unnecessary KVM_REQ_MASTERCLOCK_UPDATE requests
> during live migration on the target host.
> 
> The patchset below was authored by David Woodhouse. Two of the patches aim
> to avoid unnecessary KVM_REQ_MASTERCLOCK_UPDATE requests.
> 
> [RFC PATCH v3 00/21] Cleaning up the KVM clock mess
> https://lore.kernel.org/all/20240522001817.619072-1-dwmw2@infradead.org/
> 
> [RFC PATCH v3 10/21] KVM: x86: Fix software TSC upscaling in kvm_update_guest_time()
> [RFC PATCH v3 15/21] KVM: x86: Allow KVM master clock mode when TSCs are offset from each other
> 
> The current patchset has three patches.
> 
> PATCH 1 is a partial copy of "[RFC PATCH v3 10/21] KVM: x86: Fix software
> TSC upscaling in kvm_update_guest_time()", as Sean suggested, "Please do
> this in a separate patch. There's no need to squeeze it in here, and this
> change is complex/subtle enough as it is.", and David's authorship is
> preserved.
> 

Please let me know if this is inappropriate and whether I should have
confirmed with you before reusing your code from the patch below, with your
authorship preserved.

[RFC PATCH v3 10/21] KVM: x86: Fix software TSC upscaling in kvm_update_guest_time()
https://lore.kernel.org/all/20240522001817.619072-11-dwmw2@infradead.org/

The objective is to trigger a discussion on whether there is any quick,
short-term solution to mitigate the kvm-clock drift issue. We can also
resurrect your patchset.

I have some other work in QEMU userspace.

[PATCH 1/1] target/i386/kvm: account blackout downtime for kvm-clock and guest TSC
https://lore.kernel.org/qemu-devel/20251009095831.46297-1-dongli.zhang@oracle.com/

The combination of changes in QEMU and this KVM patchset can make kvm-clock
drift during live migration very very trivial.

Thank you very much!

Dongli Zhang