On Fri, Sep 02, 2016 at 06:37:59PM +0200, Paolo Bonzini wrote: > > > On 02/09/2016 16:51, Roman Kagan wrote: > > On Fri, Sep 02, 2016 at 04:09:42PM +0200, Paolo Bonzini wrote: > >> On 02/09/2016 15:52, Roman Kagan wrote: > >> vs. using a single offset as in the TSC ref page is one nanosecond---and > >> the ref page only has a resolution of 100 ns. > > > > AFAICS it's not a matter of resolution. If one calculation flips from > > value T to T+1 at tsc1, while the other at tsc2, during the window > > between tsc1 and tsc2 we can have monotonicity violation. > > Ok, tried "empirically" (throw numbers in a spreadsheet :)) and indeed > the maximum error is not 1 ns but 100 ns (1 unit in the time reference > count MSR). > > You can get a flip between T/T+1 because the time reference counter may > be more precise with its rounding due to the separation between > tsc_timestamp and system_time. This separation provides some extra > decimal digits to the offset, which the TSC page lacks. For example: > > 51256391 tsc_timestamp > 3311474323 tsc_to_system_mul > 254246 system_time > -1 shift > -195054.1816 offset (computed exactly) > > So the flip happens when the nanoseconds are around 81/82: > > tsc kvmclock refcount TSC page > 51256391 254246 2542 2542 > 51256483 254281 2542 2542 > 51256484 254281 2542 2543 > 51256486 254282 2542 2543 > 51256746 254382 2543 2544 > > I'll change patch 4 to store the parameters and use them when accessing > the time reference counter MSR. I'll still keep the procedure that goes > through kvmclock. It's a bit more involved for the scale, but > vcpu->last_guest_tsc only provides a part of the offset computation; the > other half is vcpu->hv_clock.system_time and it's not stored anywhere.
Erm... It is stored right there, in vcpu->hv_clock.system_time, you can access it just fine when you populate tsc_ref_page values. Am I missing anything? Roman.
