On Mon, Jan 20, 2014, Marcelo Tosatti wrote about "Re: Monotonic clock with KVM pv-clock": > On Mon, Jan 20, 2014 at 11:56:56AM +0200, Nadav Har'El wrote: > > Hi, > > > > I'm trying to figure out how a guest OS can get a monotonic clock using > > KVM's paravirtual clock. > > > > At first, I thought that the clock I get using KVM_SYSTEM_TIME is a > > monotonic clock, based on the host's monotonic clock. > > It is. However, it is used in conjunction with TSC delta, part > of the structure which is written at KVM_SYSTEM_TIME GPA. See > pvclock_clocksource_read at arch/x86/kernel/pvclock.c. Sure, I'm using the whole protocol for KVM_SYSTEM_TIME. I just wondered if it has a meaningful definition (namely, of being a monotonic clock) when being used not in conjunction with the KVM_WALL_CLOCK protocol. > > 2. What happens when the wall-clock time is set on the host? I was > > hoping that only KVM_WALL_CLOCK changes and KVM_SYSTEM_TIME doesn't, > > but am no longer sure this is actually the case. > > Yes, it is the case. The host clock which backs system_timestamp field > of pvclock structure is > > CLOCK_MONOTONIC > Clock that cannot be set and represents monotonic time > since some unspecified starting point. This clock is not affected by > discontinuous jumps in the system time (e.g., if the system > administrator manually changes the clock), but is affected by the > incremental adjustments per???formed by adjtime(3) and NTP. Excellent. > > > If KVM_SYSTEM_TIME is not a correct way to get a monotonic paravirtual clock > > from KVM, is there a correct way? > > Inside a Linux guest? Can use sched_clock(). > > If not a Linux guest, either implement kvmclock-like driver > (kvm-unit-test contains one). This is actually an OSv guest (http://osv.io/). Until now we were using kvmclock by adding up the wallclock and systemtime protocol, and this resulted with a good wall-time clock (CLOCK_REALTIME), but I really wanted to also have a monotonic clock (CLOCK_MONOTONIC) for the guest. I took a look and how a Linux guest uses the KVM pv clock, and thought that the "system time" part of the kvm clock protocol will be a good monotonic clock, but wasn't sure I was understanding this right. Thanks for the clarifications! Nadav. -- Nadav Har'El | Tuesday, Jan 21 2014, 20 Shevat 5774 nyh@xxxxxxxxxxxxxxxxxxx |----------------------------------------- Phone +972-523-790466, ICQ 13349191 |"The average person thinks he isn't." - http://nadav.harel.org.il |Father Larry Lorenzoni -- 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
