x86/mm: Set IBPB upon context switch

CVE-2017-5715 (Spectre v2 Intel)

Set IBPB on context switch with changing of page table.
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
(backported from commit 172351a2ae2c03d501e1d5933b8f50f6cd459186)
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

arch/x86/mm/tlb.c

@@ -14,6 +14,7 @@
 #include <asm/apic.h>
 #include <asm/uv/uv.h>
 #include <asm/kaiser.h>
+#include <asm/microcode.h>
 
 /*
  *	Smarter SMP flushing macros.
@@ -139,6 +140,9 @@ void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 		/* Stop flush ipis for the previous mm */
 		cpumask_clear_cpu(cpu, mm_cpumask(prev));
 
+		if (boot_cpu_has(X86_FEATURE_SPEC_CTRL))
+			native_wrmsrl(MSR_IA32_PRED_CMD, FEATURE_SET_IBPB);
+
 		/* Load per-mm CR4 state */
 		load_mm_cr4(next);