On Wed, Feb 24, 2016 at 09:53:23PM +0100, Radim Krčmář wrote: > 2016-02-24 12:24-0800, Andy Lutomirski: > > On Wed, Feb 24, 2016 at 12:17 PM, Radim Krčmář <rkrcmar@xxxxxxxxxx> wrote: > >> 2016-02-24 09:35-0800, Peter Hornyack: > >>> On Tue, Feb 23, 2016 at 7:57 PM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote: > >>>> On Tue, Feb 23, 2016 at 06:31:59PM -0800, Owen Hofmann wrote: > >>>>> Regardless of my opinion, I think that a clear statement of the design > >>>>> goals for kvm-clock (and kvm's implementation of the reference TSC > >>>>> page) would be valuable. > >>>> > >>>> Documentation/virtual/kvm/timekeeping.txt > >>>> > >>> > >>> Hi Marcelo, > >>> > >>> While I appreciate all of the detail in timekeeping.txt, it is not a > >>> very good reference for what kvm-clock is or how it works. kvm-clock > >>> is only mentioned three times in different places throughout that > >>> document, and nowhere is there a very clear statement of what > >>> kvm-clock is supposed to do or how it does it. > >>> > >>> For somebody that does not already have a deep understanding of the > >>> core masterclock code, trying to understand how kvm-clock works is a > >>> real challenge. > >> > >> I agree. Having an overview would be very helpful. > >> > >> Do you find anything incorrect with > >> * kvmclock measures the flow of time. > >> * time in kvmclock flows at the same rate as host's CLOCK_BOOTTIME. > >> ? > > > > If we could supply CLOCK_REALTIME as well and advertise that fact to > > guest userspace (perhaps with a sysctl or similar in the guest to turn > > it on), it would be *awesome*. Guests with access to this feature > > could simply not run ntpd/chronyd. > > I think that pvclock_wall_clock interface is there to do that. > (If pvclock_vcpu_time_info can provide what is claimed above.) > > If pvclock_wall_clock version field matches with pvclock_vcpu_time_info, > then the guest can add those two and get CLOCK_REALTIME. > (Based on observations of angry users, the implementation lacking.) > > >> Maybe it would be better to say "best estimate of real time" instead of > >> "CLOCK_BOOTTIME", so people wouldn't jump to conclusion that > >> CLOCK_BOOTTIME has something to do with kvmclock ... > > > > We still need to define what zero means, if anything. > > I think it's better if only the difference between two reads has a > meaning (the number of nanoseconds that passed). Zero is then an > arbitrary value. > > (If we're talking about system_time.) > > >> Then we could mention migration (why the time becomes imprecise) and > >> finish by explaining the TSC mechanism (that avoids a vmexit on every > >> read) and advantages of masterclock. > > > > We should also explain what masterclock is, aside from being an > > implementation detail. I've read the code and I still don't know. > > Yeah, rewriting the code would be a good deed. Please do so. -- 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
