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. > > 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. > > 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. -- Andy Lutomirski AMA Capital Management, LLC -- 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
