arm64: KVM: Report SMCCC_ARCH_WORKAROUND_1 BP hardening support

A new feature of SMCCC 1.1 is that it offers firmware-based CPU
workarounds. In particular, SMCCC_ARCH_WORKAROUND_1 provides
BP hardening for CVE-2017-5715.

If the host has some mitigation for this issue, report that
we deal with it using SMCCC_ARCH_WORKAROUND_1, as we apply the
host workaround on every guest exit.

Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm/include/asm/kvm_host.h

@@ -306,4 +306,11 @@ static inline void kvm_fpsimd_flush_cpu_state(void) {}
 
 static inline void kvm_arm_vhe_guest_enter(void) {}
 static inline void kvm_arm_vhe_guest_exit(void) {}
+
+static inline bool kvm_arm_harden_branch_predictor(void)
+{
+	/* No way to detect it yet, pretend it is not there. */
+	return false;
+}
+
 #endif /* __ARM_KVM_HOST_H__ */

arch/arm64/include/asm/kvm_host.h

@@ -415,4 +415,10 @@ static inline void kvm_arm_vhe_guest_exit(void)
 {
 	local_daif_restore(DAIF_PROCCTX_NOIRQ);
 }
+
+static inline bool kvm_arm_harden_branch_predictor(void)
+{
+	return cpus_have_const_cap(ARM64_HARDEN_BRANCH_PREDICTOR);
+}
+
 #endif /* __ARM64_KVM_HOST_H__ */

include/linux/arm-smccc.h

@@ -73,6 +73,11 @@
 			   ARM_SMCCC_SMC_32,				\
 			   0, 1)
 
+#define ARM_SMCCC_ARCH_WORKAROUND_1					\
+	ARM_SMCCC_CALL_VAL(ARM_SMCCC_FAST_CALL,				\
+			   ARM_SMCCC_SMC_32,				\
+			   0, 0x8000)
+
 #ifndef __ASSEMBLY__
 
 #include <linux/linkage.h>

virt/kvm/arm/psci.c

@@ -405,13 +405,20 @@ int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)
 {
 	u32 func_id = smccc_get_function(vcpu);
 	u32 val = PSCI_RET_NOT_SUPPORTED;
+	u32 feature;
 
 	switch (func_id) {
 	case ARM_SMCCC_VERSION_FUNC_ID:
 		val = ARM_SMCCC_VERSION_1_1;
 		break;
 	case ARM_SMCCC_ARCH_FEATURES_FUNC_ID:
-		/* Nothing supported yet */
+		feature = smccc_get_arg1(vcpu);
+		switch(feature) {
+		case ARM_SMCCC_ARCH_WORKAROUND_1:
+			if (kvm_arm_harden_branch_predictor())
+				val = 0;
+			break;
+		}
 		break;
 	default:
 		return kvm_psci_call(vcpu);