On 23/01/2017 19:44, Richard Cochran wrote: >> device clock |sample1P,deviceclock| |sample2P,deviceclock| >> ------------------------------------------------------------- >> realtime clock |sample1P,realtimeclock| |sample2P,realtimeclock| > > Are |sample1P,deviceclock| and |sample1P,realtimeclock| taken at the > same instant in time? > > If not, then calling that PTP_SYS_OFFSET_PRECISE is misleading. Yes, of course. This was added for the e1000e drivers first, but chrony isn't using it yet. In the case of KVM, the same host TSC value is used to produce the host clock and (converted to guest TSC) the guest clock. If you just implement getclock64 the PTP_SYS_OFFSET output: device clock | |sample2| |sample4| |sample6| ... ------------------------------------------------------------- realtime clock |sample1| |sample3| |sample5| has a very large distance between samples on the same line (about 1 us), and I think it is too noisy for userspace to make sense of the output. So, on one hand chrony only uses the mean of realtime clock samples, in an attempt to produce precise cross timestamps. On the other hand, even though KVM could produce those natively, chrony does not support PTP_SYS_OFFSET_PRECISE. Marcelo's patch then produces fake realtime clock samples that, however, let chrony derive the cross timestamps that KVM produced in the first place. The outcome is really great accuracy compared to previous versions of the patch, often just +/- 2 or 3 nanoseconds. Paolo > Still don't get what you are doing here...
