On 4/19/2024 12:34 PM, David Woodhouse wrote:
> On 19 April 2024 18:13:16 BST, "Chen, Zide" <[email protected]> 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().
