Re: What time is it kvm-clock?

["Radim Krčmář" <rkrcmar@redhat.com>]

2016-02-24 12:24-0800, Andy Lutomirski:
> On Wed, Feb 24, 2016 at 12:17 PM, Radim Krčmář <rkrcmar@redhat.com> wrote:
>> 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.
>> ?
> 
> 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.