x86/umip: Select X86_INTEL_UMIP by default

UMIP does cause any performance penalty to the vast majority of x86 code
that does not use the legacy instructions affected by UMIP.

Also describe UMIP more accurately and explain the behavior that can be
expected by the (few) applications that use the affected instructions.

Suggested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi V. Shankar <ravi.v.shankar@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Cc: ricardo.neri@intel.com
Link: http://lkml.kernel.org/r/1510640985-18412-2-git-send-email-ricardo.neri-calderon@linux.intel.com
[ Spelling fixes, rewrote the changelog. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/Kconfig

@@ -1805,14 +1805,20 @@ config X86_SMAP
 	  If unsure, say Y.
 
 config X86_INTEL_UMIP
-	def_bool n
+	def_bool y
 	depends on CPU_SUP_INTEL
 	prompt "Intel User Mode Instruction Prevention" if EXPERT
 	---help---
 	  The User Mode Instruction Prevention (UMIP) is a security
 	  feature in newer Intel processors. If enabled, a general
-	  protection fault is issued if the instructions SGDT, SLDT,
-	  SIDT, SMSW and STR are executed in user mode.
+	  protection fault is issued if the SGDT, SLDT, SIDT, SMSW
+	  or STR instructions are executed in user mode. These instructions
+	  unnecessarily expose information about the hardware state.
+
+	  The vast majority of applications do not use these instructions.
+	  For the very few that do, software emulation is provided in
+	  specific cases in protected and virtual-8086 modes. Emulated
+	  results are dummy.
 
 config X86_INTEL_MPX
 	prompt "Intel MPX (Memory Protection Extensions)"