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

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Message-ID: <2e908322-faa0-4db7-bc14-f87a9d71f577@intel.com>
Date: Fri, 19 Apr 2024 16:53:32 -0700
From: "Chen, Zide" <zide.chen@...el.com>
To: David Woodhouse <dwmw2@...radead.org>, 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 4/19/2024 12:34 PM, David Woodhouse 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. 

Thanks for the explanation.

> 
> 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?

I can see similar ~2% failure ratio w/ or w/o commenting out
configure_scaled_tsc().