Re: [Qemu-devel] qemu-kvm upstreaming: Do we need -no-kvm-pit and -no-kvm-pit-reinjection semantics?

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2012-01-20 12:45, Daniel P. Berrange wrote:
> On Fri, Jan 20, 2012 at 12:13:48PM +0100, Jan Kiszka wrote:
>> On 2012-01-20 11:25, Daniel P. Berrange wrote:
>>> On Fri, Jan 20, 2012 at 11:22:27AM +0100, Jan Kiszka wrote:
>>>> On 2012-01-20 11:14, Marcelo Tosatti wrote:
>>>>> On Thu, Jan 19, 2012 at 07:01:44PM +0100, Jan Kiszka wrote:
>>>>>> On 2012-01-19 18:53, Marcelo Tosatti wrote:
>>>>>>>> What problems does it cause, and in which scenarios? Can't they be
>>>>>>>> fixed?
>>>>>>>
>>>>>>> If the guest compensates for lost ticks, and KVM reinjects them, guest
>>>>>>> time advances faster then it should, to the extent where NTP fails to
>>>>>>> correct it. This is the case with RHEL4.
>>>>>>>
>>>>>>> But for example v2.4 kernel (or Windows with non-acpi HAL) do not
>>>>>>> compensate. In that case you want KVM to reinject.
>>>>>>>
>>>>>>> I don't know of any other way to fix this.
>>>>>>
>>>>>> OK, i see. The old unsolved problem of guessing what is being executed.
>>>>>>
>>>>>> Then the next question is how and where to control this. Conceptually,
>>>>>> there should rather be a global switch say "compensate for lost ticks of
>>>>>> periodic timers: yes/no" - instead of a per-timer knob. Didn't we
>>>>>> discussed something like this before?
>>>>>
>>>>> I don't see the advantage of a global control versus per device
>>>>> control (in fact it lowers flexibility).
>>>>
>>>> Usability. Users should not have to care about individual tick-based
>>>> clocks. They care about "my OS requires lost ticks compensation, yes or no".
>>>
>>> FYI, at the libvirt level we model policy against individual timers, for
>>> example:
>>>
>>>   <clock offset="localtime">
>>>     <timer name="rtc" tickpolicy="catchup" track="guest"/>
>>>     <timer name="pit" tickpolicy="delay"/>
>>>   </clock>
>>
>> Are the various modes of tickpolicy fully specified somewhere?
> 
> There are some (not all that great) docs here:
> 
>   http://libvirt.org/formatdomain.html#elementsTime
> 
> The meaning of the 4 policies are:
> 
>       delay: continue to deliver at normal rate

What does this mean? The timer stops ticking until the guest accepts its
ticks again?

>     catchup: deliver at higher rate to catchup
>       merge: ticks merged into 1 single tick
>     discard: all missed ticks are discarded

But those interpretations aren't stated in the docs. That makes it hard
to map them on individual hypervisors - or model proper new hypervisor
interfaces accordingly.

> 
> 
> The original design rationale was here, though beware that some things
> changed between this design & the actual implementation libvirt has:
> 
>   https://www.redhat.com/archives/libvir-list/2010-March/msg00304.html
> 
> Regards,
> Daniel

Given that there is almost no tick compensation in QEMU yet (ignoring
the awful RTC hack for now), this is a good time to establish a useful
generic interface with the advent of the KVM device models.

Jan

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