On 2012-08-14 16:37, Gleb Natapov wrote: > On Tue, Aug 14, 2012 at 04:20:06PM +0200, Jan Kiszka wrote: >> On 2012-08-14 16:03, Gleb Natapov wrote: >>> On Tue, Aug 14, 2012 at 04:00:54PM +0200, Jan Kiszka wrote: >>>> On 2012-08-05 16:03, Gleb Natapov wrote: >>>>> On Sun, Aug 05, 2012 at 05:00:37PM +0300, Avi Kivity wrote: >>>>>> On 08/05/2012 04:48 PM, Gleb Natapov wrote: >>>>>> >> >>>>>>>>>> During guest boot up, some of these jump keys will change, no? Does >>>>>>>>>> this mean a stop_machine() or equivalent? I'm worried about real-time >>>>>>>>>> response or one guest being affected by another. >>>>>>>>>> >>>>>>>>> Yes, SW enable bit changes during boot. The jump label triggerable by a >>>>>>>>> guest are rate limited though. So stop machine will not happen more then >>>>>>>>> once per second even with malicious guests. >>>>>>>> >>>>>>>> I'm not talking about a malicious guest, just a guest that is booting up >>>>>>>> normally but kills real-time response for another guest (or just induces >>>>>>>> a large hiccup in a non-real-time guest, but we don't guarantee anything >>>>>>>> for those). >>>>>>>> >>>>>>>> We don't support real-time guests now, but Jan has plans. >>>>>>>> >>>>>>> For such setup jump labels have to be compiled out from the kernel >>>>>>> completely. Anything that calls stop_machine does not play well with >>>>>>> real time. >>>>>>> >>>>>>> Guest can cause stop machine on boot today already by detecting PMU and >>>>>>> configuring NMI watchdog. >>>>>> >>>>>> The host can prevent this by leaving disabling the guest pmu. But >>>>>> disabling jump labels for real-time kernels may be acceptable too. We >>>>>> can probably to it at run time by forcing the slow path at all times. >>>>> Yes, it is possible to add module option that will force slow path if >>>>> needed. >>>> >>>> Should I write a patch or will you? Having host-side stop_machine due to >>>> such common guest operations is indeed a no-go for RT. >>>> >>> Do we support RT now? The operation are definitely not common. >> >> We also have min_timer_period_us to control the APIC timer - for the >> same use case. >> >> And regarding how common they are: Do standard OSes trigger any >> jump-label optimized switch during at least their boot-up? I thought so. >> In that case, if you co-locate RT and standard OSes on a shared host, >> you would have a conflict. >> > Yes, during boot up it happens. But it is rate limited to happen not > more than once per second. But I genuinely curious does RT guest have > any RT guaranties from QEMU/kvm combination today (with of without > jump-labels)? Yes, when avoiding userspace exits. If you have a customized RTOS guest or are lucky with some existing one, that works pretty well for periodic processing in the 1 ms range. Jan -- Siemens AG, Corporate Technology, CT RTC ITP SDP-DE 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
