UBUNTU: SAUCE: x86/speculation: Expose indirect_branch_prediction_barrier()

BugLink: https://bugs.launchpad.net/bugs/1791080

Expose indirect_branch_prediction_barrier() for use in subsequent patches.
Our approach for IBRS/IBPB is still based on the Intel v4 patchset and I
did not want to completely switch that in this upstream stable set. For
that reason I skipped most changes from "x86/speculation: Add basic IBPB
(Indirect Branch Prediction Barrier) support" from the 4.4.144 update.

Only indirect_branch_prediction_barrier() I picked up because it will
later be used for an update to "x86/speculation: Use Indirect Branch
Prediction Barrier in context switch".
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

arch/x86/include/asm/nospec-branch.h

@@ -239,6 +239,19 @@ void alternative_msr_write(unsigned int msr, u64 val, unsigned int feature)
 		: "memory");
 }
 
+static inline void indirect_branch_prediction_barrier(void)
+{
+	asm volatile(ALTERNATIVE("",
+				 "movl %[msr], %%ecx\n\t"
+				 "movl %[val], %%eax\n\t"
+				 "movl $0, %%edx\n\t"
+				 "wrmsr",
+				 X86_FEATURE_USE_IBPB)
+			: : [msr] "i" (MSR_IA32_PRED_CMD),
+			    [val] "i" (PRED_CMD_IBPB)
+			: "eax", "ecx", "edx", "memory");
+}
+
 #endif /* __ASSEMBLY__ */
 
 /*