Questions about Cx and Px state uploading from dom0

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

Hello,

I was looking at implementing ACPI Cx and Px state uploading from
FreeBSD dom0, as my main test box is considerably slower without Xen
knowing about the Px states.  That has raised a couple of questions.

1. How to figure out what features to report available by OSPM when
calling the _PDC (or _OSC) ACPI method.  I'm confused by the usage of
this from Linux: it seems to be used to detect mwait support in
xen_check_mwait but not when calling _PDC (ie: in
acpi_processor_set_pdc).  I'm also not sure what the hypercall expects
the caller to provide.  Should buf[2] be set to all the possible
features supported by the OS and Xen will trim those as required?

2. When uploading Px states, what's the meaning of the shared_type
field in xen_processor_performance?  I've looked at the usage of the
field by Xen, and first of all it seems to be a layering violation
because the values set in the field (CPUFREQ_SHARED_TYPE_*) are not
exposed as part of the public interface.  This all works for Linux
because the same values are used by Xen and the Linux kernel.
Secondly, this is not part of the data fetched from ACPI AFAICT, so
I'm unsure how the value should be calculated.  I also wonder whether
this couldn't be done by Xen itself from the uploaded Px data (but
without knowing exactly how the value should be calculated it's hard
to tell).

Thanks, Roger.