Re: The HPET issue on Linux

[Gleb Natapov <gleb@redhat.com>]

On Wed, Jan 06, 2010 at 01:51:54PM -0600, Anthony Liguori wrote:
> On 01/06/2010 01:44 PM, Gleb Natapov wrote:
> >On Wed, Jan 06, 2010 at 01:23:28PM -0600, Anthony Liguori wrote:
> >>On 01/06/2010 01:20 PM, Beth Kon wrote:
> >>>Beth Kon wrote:
> >>>>I will try to look into this. Since HPET is edge-triggered,
> >>>>looks like this problem is of a different nature than PIT.  Is
> >>>>this a solid failure or intermittent?
> >>>Anthony just explained that on x86, even edge-triggered interrupts
> >>>are queued in the apic and an eoi will occur, so this is not
> >>>different than the PIT.
> >>Not quite queued in the sense that multiple events will be delivered
> >>in order, but I think the point is that you can still detect whether
> >>delivery succeeded by counting APIC EOIs.
> >>
> >>The trouble is that historically we've struggled with doing this in
> >>userspace.  Maybe it's time to revisit.
> >>
> >We reinject PIT interrupts from kernel and RTC interrupts from
> >userspace.
> 
> Because we can determine that we've missed an RTC interrupt in
> userspace.  We cannot determine this with the PIT without adding a
> hook into the userspace apic that lets us know whether an injection
> failed or not.
> 
We have exactly that hook in apic already and that's how RTC determines
that interrupt was coalesced.

--
			Gleb.