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

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



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>


Daniel
-- 
|: http://berrange.com      -o-    http://www.flickr.com/photos/dberrange/ :|
|: http://libvirt.org              -o-             http://virt-manager.org :|
|: http://autobuild.org       -o-         http://search.cpan.org/~danberr/ :|
|: http://entangle-photo.org       -o-       http://live.gnome.org/gtk-vnc :|
--
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


[Index of Archives]     [KVM ARM]     [KVM ia64]     [KVM ppc]     [Virtualization Tools]     [Spice Development]     [Libvirt]     [Libvirt Users]     [Linux USB Devel]     [Linux Audio Users]     [Yosemite Questions]     [Linux Kernel]     [Linux SCSI]     [XFree86]
  Powered by Linux