On 19 April 2024 19:40:06 BST, David Woodhouse <dwmw2@xxxxxxxxxxxxx> wrote: >On 19 April 2024 18:13:16 BST, "Chen, Zide" <zide.chen@xxxxxxxxx> 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
