In Ubuntu, we have runtime control for enabling/disabling IBPB via the
commandline ("noibpb") and through the proc interface
/proc/sys/kernel/ibpb_enabled. This commit simplifies the current
(broken) implementation by merging it with all the IBPB-related upstream
changes from previous commits.

What we have now is the upstream implementation for detecting the presence
of IBPB support which is used in the alternative MSR write in
indirect_branch_prediction_barrier() to enable IBPB. On top of that, this
commit adds a global state variable 'ibpb_enabled' which is set to 1 if
the CPU supports IBPB but can be overridden via the commandline "noibpb"
switch or by writting 0 or 1 to /proc/sys/kernel/ibpb_enabled at runtime.

If ibpb_enabled is 0, indirect_branch_prediction_barrier() is turned into a
no-op, same as if the platform doesn't support IBPB.

CVE-2017-5715
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>