On Sun, 2008-03-23 at 10:30 +0200, Avi Kivity wrote: > Anthony Liguori wrote: > > Avi Kivity wrote: > >>> And I would like to ask right and wrong to > >>> implement the functionality in terms of need > >>> and efficiency (scalability and time accuracy). > >>> > >> > >> I think that for newer kernels we already have the desired accuracy. > >> We're not always good at exploiting that accuracy; hence the recent > >> movement of the PIT implementation from userspace to the kernel. But > >> recent discussion leads me to believe it could have been implemented > >> with the userspace PIT as well. > >> > > > > What do you think is needed to get the same accuracy in userspace as > > in kernelspace? > > Some mechanism that allows us to implement kvm_inject_pit_timer_irqs() > and kvm_pit_timer_intr_post(). Specifically, information about whether > an interrupt was actually processed, and a window for injecting missed > ticks. > > > Better yet, do you think there is a reasonable kvmctl harness we could > > write to quantify the PIT accuracy? > > kvmctl doesn't implement a pit, so no. Of course we can test any > infrastructure for counting missed interrupts. > > > > > It's easy enough to count timer interrupts and use compare that to an > > external time source to get some notion of accuracy (on varying > > frequencies of course). I know you mentioned before that guest CPU > > consumption also comes into play... I'm not quite sure why though so > > I'm not sure how to simulate that. > > It's not so easy, the code is quite tricky since the cpu processes > vectors, not interrupt lines. It's also heuristic; if the guest > programs some random device to share interrupts with the pit, the > heursitic breaks down. This never happens in practice, though. > > Problems show up when both the guest and host are loaded, as then the > cpu is timesliced instead of being available on demand. >
I have patches that works for the rtc case with acpi windows, just cleaning and sending for a review. The same method should work for the pit as well. > > > > The nice thing about the CAP infrastructure is we can always move the > > PIT back to userspace. I'll happily invest some cycles here as I'm a > > big fan of getting rid of unneeded kernel code :-) > > Yes. > ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2008. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ _______________________________________________ kvm-devel mailing list kvm-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/kvm-devel
