Il 13/11/2012 21:40, Marcelo Tosatti ha scritto: > On Tue, Nov 13, 2012 at 08:52:54AM +0100, Christian Ehrhardt wrote: >> > >> > Hi, >> > >> > thanks for your reply. >> > >> > On Mon, Nov 12, 2012 at 07:32:37PM -0200, Marcelo Tosatti wrote: >>>> > > > there is a bug in the emulation of the lapic time counter. In particular >>>> > > > what we are seeing is that the time counter of a periodic lapic timer >>>> > > > in the guest reads as zero 99% of the time. The patch below fixes that. >>>> > > > >>>> > > > The emulation of the lapic timer is done with the help of a hires >>>> > > > timer that expires with the same frequency as the lapic counter. >>>> > > > New expiration times for a periodic timer are calculated incrementally >>>> > > > based on the last scheduled expiration time. This ensures long term >>>> > > > accuracy of the emulated timer close to that of the underlying clock. >>>> > > > >>>> > > > The actual value of the lapic time counter is calculated from the >>>> > > > real time difference between current time and scheduled expiration time >>>> > > > of the hires timer. If this difference is negative, the hires timer >>>> > > > expired. For oneshot mode this is correctly translated into a zero value >>>> > > > for the time counter. However, in periodic mode we must use the negative >>>> > > > difference unmodified. >>>> > > > >>>> > > > regards Christian >>>> > > > >>>> > > > Fix lapic time counter read for periodic mode. >>> > > >>> > > In periodic mode the hrtimer is rearmed once expired, see >>> > > apic_timer_fn. So _get_remaining should return proper value >>> > > even if the guest is not able to process timer interrupts. >>> > > >>> > > Can you describe your specific scenario in more detail? >> > >> > In my specific case, the host is admittedly somewhat special as it >> > already is a rehosted version of linux, i.e. not running directly on >> > native hardware. It is still unclear if the host has sufficiently accurate >> > timer interrupts. This is most likely part of the problems we are seeing. >> > >> > However, AFAICS apic_timer_fn is only called once per jiffy (at least in >> > some configurations). In particular, it is not called by >> > hrtimer_get_remaining. Thus depending on the frequency of the LAPIC timer >> > in the guest there might _several_ iterations that are missed. This can >> > probably be mitigated by a hires timer interrupts. However, I think >> > the problem is still there even in that case. >> > >> > Additionally, the behaviour that I want to establish matches that of the >> > PIT timer (in a not completely obvious way, though). >> > >> > Having said that the proposed patch in my first mail is incomplete, as >> > the mod_64 does not work correctly for negative values. A fixed version >> > is below. >> > >> > regards Christian >> > >> > Signed-off-by: Christian Ehrhardt <lk@xxxxxxx> > Alright. Please add a comment from the LAPIC documentation describing > this behaviour (and a nice changelog). Thanks. > Christian, did you ever resubmit the patch? Paolo -- 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
