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

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Message-ID: <285e30927dad5736df58aad5d957448e93b2d047.camel@infradead.org>
Date: Thu, 15 Jan 2026 21:13:04 +0000
From: David Woodhouse <dwmw2@...radead.org>
To: Dongli Zhang <dongli.zhang@...cle.com>, kvm@...r.kernel.org
Cc: seanjc@...gle.com, pbonzini@...hat.com, 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

On Thu, 2026-01-15 at 12:37 -0800, Dongli Zhang wrote:
> 
> 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!

Not at all inappropriate; thank you so much for updating it. I've been
meaning to do so but it's never made it back to the top of my list.

I don't believe that the existing KVM_SET_CLOCK is viable though. The
aim is that you should be able to create a new KVM on the same host and
set the kvmclock, and the contents of the pvclock that the new guest
sees should be *identical*. Not just 'close'.

I believe we need Jack's KVM_[GS]ET_CLOCK_GUEST for that to be
feasible, so I'd very much prefer that any resurrection of this series
should include that, even if some of the other patches are dropped for
now.

Thanks again.

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