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? -- 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
