[RFC 01/10] x86/speculation: Add basic support for IBPB

[KarimAllah Ahmed]

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

Expose indirect_branch_prediction_barrier() for use in subsequent patches.

[karahmed: remove the special-casing of skylake for using IBPB (wtf?),
           switch to using ALTERNATIVES instead of static_cpu_has]
[dwmw2:    set up ax/cx/dx in the asm too so it gets NOP'd out]

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: KarimAllah Ahmed <karahmed@xxxxxxxxx>
Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
---
 arch/x86/include/asm/cpufeatures.h   |  1 +
 arch/x86/include/asm/nospec-branch.h | 16 ++++++++++++++++
 arch/x86/kernel/cpu/bugs.c           |  7 +++++++
 3 files changed, 24 insertions(+)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 624d978..8ec9588 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -207,6 +207,7 @@
 #define X86_FEATURE_RETPOLINE_AMD	( 7*32+13) /* AMD Retpoline mitigation for Spectre variant 2 */
 #define X86_FEATURE_INTEL_PPIN		( 7*32+14) /* Intel Processor Inventory Number */
 
+#define X86_FEATURE_IBPB		( 7*32+16) /* Using Indirect Branch Prediction Barrier */
 #define X86_FEATURE_AMD_PRED_CMD	( 7*32+17) /* Prediction Command MSR (AMD) */
 #define X86_FEATURE_MBA			( 7*32+18) /* Memory Bandwidth Allocation */
 #define X86_FEATURE_RSB_CTXSW		( 7*32+19) /* Fill RSB on context switches */
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 4ad4108..c333c95 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -218,5 +218,21 @@ static inline void vmexit_fill_RSB(void)
 #endif
 }
 
+static inline void indirect_branch_prediction_barrier(void)
+{
+	unsigned long ax, cx, dx;
+
+	asm volatile(ALTERNATIVE("",
+				 "movl %[msr], %%ecx\n\t"
+				 "movl %[val], %%eax\n\t"
+				 "movl $0, %%edx\n\t"
+				 "wrmsr",
+				 X86_FEATURE_IBPB)
+		     : "=a" (ax), "=c" (cx), "=d" (dx)
+		     : [msr] "i" (MSR_IA32_PRED_CMD),
+		       [val] "i" (PRED_CMD_IBPB)
+		     : "memory");
+}
+
 #endif /* __ASSEMBLY__ */
 #endif /* __NOSPEC_BRANCH_H__ */
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 390b3dc..96548ff 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -249,6 +249,13 @@ static void __init spectre_v2_select_mitigation(void)
 		setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW);
 		pr_info("Filling RSB on context switch\n");
 	}
+
+	/* Initialize Indirect Branch Prediction Barrier if supported */
+	if (boot_cpu_has(X86_FEATURE_SPEC_CTRL) ||
+	    boot_cpu_has(X86_FEATURE_AMD_PRED_CMD)) {
+		setup_force_cpu_cap(X86_FEATURE_IBPB);
+		pr_info("Enabling Indirect Branch Prediction Barrier\n");
+	}
 }
 
 #undef pr_fmt
-- 
2.7.4