x86/speculation/mds: Add mitigation control for MDS

Now that the mitigations are in place, add a command line parameter to
control the mitigation, a mitigation selector function and a SMT update
mechanism.

This is the minimal straight forward initial implementation which just
provides an always on/off mode. The command line parameter is:

  mds=[full|off]

This is consistent with the existing mitigations for other speculative
hardware vulnerabilities.

The idle invocation is dynamically updated according to the SMT state of
the system similar to the dynamic update of the STIBP mitigation. The idle
mitigation is limited to CPUs which are only affected by MSBDS and not any
other variant, because the other variants cannot be mitigated on SMT
enabled systems.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Jon Masters <jcm@redhat.com>
Tested-by: Jon Masters <jcm@redhat.com>

Documentation/admin-guide/kernel-parameters.txt

@@ -2356,6 +2356,28 @@
 			Format: <first>,<last>
 			Specifies range of consoles to be captured by the MDA.
 
+	mds=		[X86,INTEL]
+			Control mitigation for the Micro-architectural Data
+			Sampling (MDS) vulnerability.
+
+			Certain CPUs are vulnerable to an exploit against CPU
+			internal buffers which can forward information to a
+			disclosure gadget under certain conditions.
+
+			In vulnerable processors, the speculatively
+			forwarded data can be used in a cache side channel
+			attack, to access data to which the attacker does
+			not have direct access.
+
+			This parameter controls the MDS mitigation. The
+			options are:
+
+			full    - Enable MDS mitigation on vulnerable CPUs
+			off     - Unconditionally disable MDS mitigation
+
+			Not specifying this option is equivalent to
+			mds=full.
+
 	mem=nn[KMG]	[KNL,BOOT] Force usage of a specific amount of memory
 			Amount of memory to be used when the kernel is not able
 			to see the whole system memory or for test.