Re: question: HPET for multiple VMs

[Dor Laor <dor.laor@gmail.com>]

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.
> 


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