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? Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux -- 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
