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

[Marcelo Tosatti <mtosatti@redhat.com>]

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).

> What about periodic APIC tick compensation? I suppose the kernel does
> not support this as no common guest makes use of this as clock source,
> right? 

Recent guests use the APIC timer as clock event, but their time keeping 
algorithms are not as susceptible to lost ticks as the ones that use 
PIT/RTC.

> Or the HPET? Once the user space model supports compensation, we
> need to control it as well. Individually?

Ulrich has posted patches for HPET compensation:

http://lists.gnu.org/archive/html/qemu-devel/2011-03/msg01989.html

> I just want to avoid introducing an clumsy interface we then need to
> maintain for a long time.
> 
> Jan

If the option is a qdev property, i don't see what is clumsy about it?