linux-kernel - Re: [PATCH 2/2] KVM: selftests: Add KVM/PV clock selftest to prove timer drift correction

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Message-ID: <65FF4D51-05A8-42E0-9D07-6E42913CC75E@infradead.org>
Date: Fri, 19 Apr 2024 19:43:43 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: "Chen, Zide" <zide.chen@...el.com>, Jack Allister <jalliste@...zon.com>,
 Paolo Bonzini <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>,
 Sean Christopherson <seanjc@...gle.com>,
 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>,
 Shuah Khan <shuah@...nel.org>
CC: Paul Durrant <paul@....org>, kvm@...r.kernel.org, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 2/2] KVM: selftests: Add KVM/PV clock selftest to prove timer drift correction

On 19 April 2024 19:40:06 BST, David Woodhouse <dwmw2@...radead.org> wrote:
>On 19 April 2024 18:13:16 BST, "Chen, Zide" <zide.chen@...el.com> wrote:
>>I'm wondering what's the underling theory that we definitely can achieve
>>±1ns accuracy? I tested it on a Sapphire Rapids @2100MHz TSC frequency,
>>and I can see delta_corrected=2 in ~2% cases.
>
>Hm. Thanks for testing!
>
>So the KVM clock is based on the guest TSC. Given a delta between the guest TSC T and some reference point in time R, the KVM clock is expressed as a(T-R)+r, where little r is the value of the KVM clock when the guest TSC was R, and (a) is the rate of the guest TSC.
>
>When set the clock with KVM_SET_CLOCK_GUEST, we are changing the values of R and r to a new point in time. Call the new ones Q and q respectively.
>
>But we calculate precisely (within 1ns at least) what the KVM clock would have been with the *old* formula, and adjust our new offset (q) so that at our new reference TSC value Q, the formulae give exactly the same result.
>
>And because the *rates* are the same, they should continue to give the same results, ±1ns.
>
>Or such *was* my theory, at least. 
>
>Would be interesting to see it disproven with actual numbers for the old+new pvclock structs, so I can understand where the logic goes wrong.
>
>Were you using frequency scaling?
>

Oh, also please could you test the updated version I posted yesterday, from https://git.infradead.org/?p=users/dwmw2/linux.git;a=shortlog;h=refs/heads/clocks