Re: [Xen-devel] Re: [RFC PATCH V4 4/5] cpuidle: driver for xen

[Len Brown <lenb@kernel.org>]

> >>>>> xen_arch_setup() does this:
> >>>>>
> >>>>>         pm_idle = default_idle;
> >>>>>         boot_option_idle_override = IDLE_HALT;
> > What happens on a Xen kernel if these lines are not there?
> > Does Xen export the C-states tables to Dom0 kernel, and the Dom0
> > kernel has an acpi processor driver, and thus it would try to
> > use all the C-states?
> 
> If they're no there it tries to use the Intel cpuidle driver, which
> fails (just hangs forever in idle, I think).
> 
> > If yes, must Xen show those tables to Dom?
> > If it did not, then the lines above would not be necessary,
> > as in the absence of any C-states, the kernel should
> > use halt by default.
> 
> The dom0 kernel gets all the ACPI state so it can get all the juicy
> goodness from it.  It does extract the C state info, but passes it back
> to Xen rather than use it itself.  We don't generally try to filter the
> ACPI state before letting dom0 see it (DMAR being the exception, since
> dom0 really has no business knowing about that).
> 
> (We have this basic problem that neither Xen nor dom0 are the ideal
> owners of ACPI.  In principle Xen should own ACPI as the most privileged
> "OS", but it really only cares about things like power states, interrupt
> routing, system topology, busses, etc.  But dom0 cares about lid
> switches, magic keyboard keys, volume controls, video output switching,
> etc, etc.  At the moment it seems to work best if dom0 do all ACPI
> processing then pass Xen the parts it needs, which are generally
> fixed-at-startup config info items.)

Okay.
Since a Xen kernel can also boot on bare iron, and thus
includes cpuidle, acpi_idle, intel_idle; and when
in Dom0 mode it simply wants to run HLT in idle...

I think what we want to do is simply disable cpuidle
when booted in Dom0 mode.  That will allow it to
fall back to default_idle w/o xen_setup needing to
tinker with installing an idle driver.

I'll send a patch in the next series.
If you can try it out, that would be great.

thanks,
-Len