cpufreq status information

["Jan Beulich" <jbeulich@novell.com>]

Trying to understand whether CPU frequency scaling is actually working on
a system currently requires (afaics) source patches, as there is no way to
get the current state of a CPU. Even if this is intentional, this doesn't seem
very helpful when considering to make this functionality available to
customers: I'm certain quite a few will ask how they can tell whether this
is actually working.

Now, apart from the simple job of adding a sub-hypercall to retrieve the
necessary bits, I'm wondering whether this wouldn't be just one more
element that would much better be surfaced to the guest via the vCPU
info structure (or, as that's size constrained, a new construct to make
guest-read-only information available via a shared page). Other
(potential) items to make available this same way would e.g. be guest-
accessible last-exception-from/-to MSR values (as the values read would
be meaningless if read through rdmsr).

So I'm basically considering to add a generic mechanism first, and then
make cpufreq the first user of it. The question just is - use a completely
new (guest-ro) per-vCPU page, perhaps with chained descriptors rather
than a fixed layout, or extend the vCPU info structure, but e.g. require
the guest to use VCPUOP_register_vcpu_info to gain access to all
structure fields.

Thanks, Jan