Re: What time is it kvm-clock?

["Radim Krčmář" <rkrcmar@redhat.com>]

2016-02-24 09:35-0800, Peter Hornyack:
> On Tue, Feb 23, 2016 at 7:57 PM, Marcelo Tosatti <mtosatti@redhat.com> 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.
?

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 ...

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.