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

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Message-ID: <41b50e06dab5cb6195b20016c372cdcaa56f1943.camel@infradead.org>
Date: Fri, 23 Jan 2026 17:31:40 -0800
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 Fri, 2026-01-16 at 01:31 -0800, Dongli Zhang wrote:
> 
> With above changes in both QEMU and KVM, a same-host live migration of a 4-vCPU
> VM with approximately 10 seconds of downtime (introduced on purpose) results in
> only about 4 nanoseconds of backward drift in my test environment. We may even
> be able to make more improvement from QEMU to rule out the remaining 4 nanoseconds.

On the same host, even with TSC scaling, there is no excuse for *any*
errors on live migration.

The *offset* of the host → guest TSC should remain precisely the same.
And the calculation of KVM clock from guest TSC should remain precisely the same.

Absolutely *zero* error.

Don't bother with "improvements" which still don't get that right.

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