Re: question: HPET for multiple VMs

Anthony Liguori
2008-03-23T01:08:40Z

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? Better yet, do you think there is a reasonable kvmctl
harness we could write to quantify the PIT accuracy?

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.

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 :-)

Regards,

Anthony Liguori