x86/bugs: Read SPEC_CTRL MSR during boot and re-use reserved bits

x86/bugs: Read SPEC_CTRL MSR during boot and re-use reserved bits

The 336996-Speculative-Execution-Side-Channel-Mitigations.pdf refers to all
the other bits as reserved. The Intel SDM glossary defines reserved as
implementation specific - aka unknown.

As such at bootup this must be taken it into account and proper masking for
the bits in use applied.

A copy of this document is available at
https://bugzilla.kernel.org/show_bug.cgi?id=199511

[ tglx: Made x86_spec_ctrl_base __ro_after_init ]

OraBug: 28041771
CVE: CVE-2018-3639

Suggested-by: Jon Masters <jcm@redhat.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
(cherry picked from commit 1b86883ccb8d5d9506529d42dbe1a5257cb30b18)
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Mihai Carabas <mihai.carabas@oracle.com>
 Conflicts:
	arch/x86/include/asm/nospec-branch.h
[As we don't have the firmware_restrict_branch_speculation_start and
 firmware_restrict_branch_speculation_end and end up with a different
 name. See commit 473ad76ea8d76f34555d764a3d5820bc1b33cabf
 "x86/speculation: Use IBRS if available before calling into firmware"]

	arch/x86/kernel/cpu/bugs.c
[File is called bugs_64.c in UEK4]

[Also the backport needs nospec-branch.h in different files ,and we can't
 use __ro_after_init]

Signed-off-by: Brian Maly <brian.maly@oracle.com>