On Wed, Jan 18, 2017 at 01:46:58PM +0100, Paolo Bonzini wrote: > On 18/01/2017 13:24, Marcelo Tosatti wrote: > >> Testcase: run a guest and a loop sending SIGUSR1 to vcpu0 (emulating > >> intense interrupts). Follows results: > >> Do you still want to drop it in favour of simplicity? > It's just that it's not obvious why you get better results with biased > host timestamps. What makes the biased host timestamp more precise? > > I'd rather use PTP_SYS_OFFSET_PRECISE instead, but unfortunately chrony > does not support it---but I would still prefer you to support > PTP_SYS_OFFSET_PRECISE as well. Interesting. I wasn't aware that there is a new ioctl for measuring the HW-sys offset. Adding support to chrony shouldn't be difficult. If I understand it correctly, PTP_SYS_OFFSET can be emulated on top of PTP_SYS_OFFSET_PRECISE simply by copying the sys_realtime and device fields to corresponding ts slots. The apparent delay will be zero, but that's ok if the conversion is really accurate. I'm not sure if trying to do that in the opposite direction is a good idea. An application using PTP_SYS_OFFSET_PRECISE may assume the conversion is accurate and not include any delay/dispersion in an estimate of the maximum error, which is needed in NTP for instance. If we know the host timestamp ts[1] is not in the middle between the guests timestamps ts[0] and ts[2], but rather closer to ts[2], why not simply shift ts[1] by (ts[2]-ts[0])/2 ? -- Miroslav Lichvar -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
