intel_idle: Add support for using intel_idle in a VM guest using just hlt

intel_idle: Add support for using intel_idle in a VM guest using just hlt

In a typical VM guest, the mwait instruction is not available, leaving
only the 'hlt' instruction (which causes a VMEXIT to the host).

So for this common case, intel_idle will detect the lack of mwait, and
fail to initialize (after which another idle method would step in which
will just use hlt always).

Other (non-common) cases exist; the table below shows the before/after
for these:

+------------+--------------------------+-------------------------+
| Hypervisor | Idle method before patch | Idle method after patch |
| exposes    |                          |                         |
+============+==========================+=========================+
| nothing    | default_idle fallback    | intel_idle VM table     |
| (common)   | (straight "hlt")         |                         |
+------------+--------------------------+-------------------------+
| mwait      | intel_idle mwait table   | intel_idle mwait table  |
+------------+--------------------------+-------------------------+
| ACPI       | ACPI C1 state ("hlt")    | intel_idle VM table     |
+------------+--------------------------+-------------------------+

This is only applicable to CPUs known by intel_idle. For the bare metal
case, unknown CPU models will use the ACPI tables (when available) to
get estimates for latency and break even point for longer idle states.
In guests, the common case is that ACPI tables are not available, but
even when they are available, they can't and don't provide the latency
information for the longer (mwait based) states. For this scenario
(unknown CPU model), the default_idle mode (no ACPI) or ACPI C1 (ACPI
avaible) will be used.

By providing capability to do this with the intel_idle driver, we can
do better than the fallback or ACPI table methods. While this current
change only gets us to the existing behavior, later patches in this
series will add new capabilities such as optimized TLB flushing.

In order to do this, a simplified version of the initialization
function for VM guests is created, and this will be called if the CPU
is recognized, but mwait is not supported, and we're in a VM guest.

One thing to note is that the max latency (and break even) of this C1
state is higher than the typical bare metal C1 state. Because hlt causes
a vmexit, and the cost of vmexit + hypervisor overhead + vmenter is
typically in the order of upto 5 microseconds... even if the hypervisor
does not actually goes into a hardware power saving state.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
[ rjw: Dropped redundant checks from should_verify_mwait() ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>