x86/spec-ctrl: Introduce options to control VERW flushing

x86/spec-ctrl: Introduce options to control VERW flushing

The Microarchitectural Data Sampling vulnerability is split into categories
with subtly different properties:

 MLPDS - Microarchitectural Load Port Data Sampling
 MSBDS - Microarchitectural Store Buffer Data Sampling
 MFBDS - Microarchitectural Fill Buffer Data Sampling
 MDSUM - Microarchitectural Data Sampling Uncacheable Memory

MDSUM is a special case of the other three, and isn't distinguished further.

These issues pertain to three microarchitectural buffers.  The Load Ports, the
Store Buffers and the Fill Buffers.  Each of these structures are flushed by
the new enhanced VERW functionality, but the conditions under which flushing
is necessary vary.

For this concise overview of the issues and default logic, the abbreviations
SP (Store Port), FB (Fill Buffer), LP (Load Port) and HT (Hyperthreading) are
used for brevity:

 * Vulnerable hardware is divided into two categories - parts which suffer
   from SP only, and parts with any other combination of vulnerabilities.

 * SP only has an HT interaction when the thread goes idle, due to the static
   partitioning of resources.  LP and FB have HT interactions at all points,
   due to the competitive sharing of resources.  All issues potentially leak
   data across the return-to-guest transition.

 * The microcode which implements VERW flushing also extends MSR_FLUSH_CMD, so
   we don't need to do both on the HVM return-to-guest path.  However, the
   Knights range of processors are immune to L1TF (therefore have no
   MSR_FLUSH_CMD), but are vulnerable to MDS, so do require VERW on the HVM
   path.

Note that we deliberately support mds=1 even without MD_CLEAR in case the
microcode has been updated but the feature bit not exposed.

This is part of XSA-297, CVE-2018-12126, CVE-2018-12127, CVE-2018-12130, CVE-2019-11091.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 3c04c258ab40405a74e194d9889a4cbc7abe94b4)