arm64: KVM: Propagate full Spectre v2 workaround state to KVM guests

Recent commits added the explicit notion of "workaround not required" to
the state of the Spectre v2 (aka. BP_HARDENING) workaround, where we
just had "needed" and "unknown" before.

Export this knowledge to the rest of the kernel and enhance the existing
kvm_arm_harden_branch_predictor() to report this new state as well.
Export this new state to guests when they use KVM's firmware interface
emulation.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

arch/arm/include/asm/kvm_host.h

@@ -362,18 +362,24 @@ static inline void kvm_vcpu_pmu_restore_host(struct kvm_vcpu *vcpu) {}
 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)
+#define KVM_BP_HARDEN_UNKNOWN		-1
+#define KVM_BP_HARDEN_WA_NEEDED		0
+#define KVM_BP_HARDEN_NOT_REQUIRED	1
+
+static inline int kvm_arm_harden_branch_predictor(void)
 {
 	switch(read_cpuid_part()) {
 #ifdef CONFIG_HARDEN_BRANCH_PREDICTOR
 	case ARM_CPU_PART_BRAHMA_B15:
 	case ARM_CPU_PART_CORTEX_A12:
 	case ARM_CPU_PART_CORTEX_A15:
 	case ARM_CPU_PART_CORTEX_A17:
-		return true;
+		return KVM_BP_HARDEN_WA_NEEDED;
 #endif
+	case ARM_CPU_PART_CORTEX_A7:
+		return KVM_BP_HARDEN_NOT_REQUIRED;
 	default:
-		return false;
+		return KVM_BP_HARDEN_UNKNOWN;
 	}
 }
 

arch/arm64/include/asm/cpufeature.h

@@ -614,6 +614,12 @@ static inline bool system_uses_irq_prio_masking(void)
 	       cpus_have_const_cap(ARM64_HAS_IRQ_PRIO_MASKING);
 }
 
+#define ARM64_BP_HARDEN_UNKNOWN		-1
+#define ARM64_BP_HARDEN_WA_NEEDED	0
+#define ARM64_BP_HARDEN_NOT_REQUIRED	1
+
+int get_spectre_v2_workaround_state(void);
+
 #define ARM64_SSBD_UNKNOWN		-1
 #define ARM64_SSBD_FORCE_DISABLE	0
 #define ARM64_SSBD_KERNEL		1

arch/arm64/include/asm/kvm_host.h

@@ -620,9 +620,21 @@ static inline void kvm_arm_vhe_guest_exit(void)
 	isb();
 }
 
-static inline bool kvm_arm_harden_branch_predictor(void)
+#define KVM_BP_HARDEN_UNKNOWN		-1
+#define KVM_BP_HARDEN_WA_NEEDED		0
+#define KVM_BP_HARDEN_NOT_REQUIRED	1
+
+static inline int kvm_arm_harden_branch_predictor(void)
 {
-	return cpus_have_const_cap(ARM64_HARDEN_BRANCH_PREDICTOR);
+	switch (get_spectre_v2_workaround_state()) {
+	case ARM64_BP_HARDEN_WA_NEEDED:
+		return KVM_BP_HARDEN_WA_NEEDED;
+	case ARM64_BP_HARDEN_NOT_REQUIRED:
+		return KVM_BP_HARDEN_NOT_REQUIRED;
+	case ARM64_BP_HARDEN_UNKNOWN:
+	default:
+		return KVM_BP_HARDEN_UNKNOWN;
+	}
 }
 
 #define KVM_SSBD_UNKNOWN		-1

arch/arm64/kernel/cpu_errata.c

@@ -554,6 +554,17 @@ cpu_enable_cache_maint_trap(const struct arm64_cpu_capabilities *__unused)
 static bool __hardenbp_enab = true;
 static bool __spectrev2_safe = true;
 
+int get_spectre_v2_workaround_state(void)
+{
+	if (__spectrev2_safe)
+		return ARM64_BP_HARDEN_NOT_REQUIRED;
+
+	if (!__hardenbp_enab)
+		return ARM64_BP_HARDEN_UNKNOWN;
+
+	return ARM64_BP_HARDEN_WA_NEEDED;
+}
+
 /*
  * List of CPUs that do not need any Spectre-v2 mitigation at all.
  */
@@ -854,13 +865,15 @@ ssize_t cpu_show_spectre_v1(struct device *dev, struct device_attribute *attr,
 ssize_t cpu_show_spectre_v2(struct device *dev, struct device_attribute *attr,
 		char *buf)
 {
-	if (__spectrev2_safe)
+	switch (get_spectre_v2_workaround_state()) {
+	case ARM64_BP_HARDEN_NOT_REQUIRED:
 		return sprintf(buf, "Not affected\n");
-
-	if (__hardenbp_enab)
+        case ARM64_BP_HARDEN_WA_NEEDED:
 		return sprintf(buf, "Mitigation: Branch predictor hardening\n");
-
-	return sprintf(buf, "Vulnerable\n");
+        case ARM64_BP_HARDEN_UNKNOWN:
+	default:
+		return sprintf(buf, "Vulnerable\n");
+	}
 }
 
 ssize_t cpu_show_spec_store_bypass(struct device *dev,

virt/kvm/arm/psci.c

@@ -401,8 +401,16 @@ int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)
 		feature = smccc_get_arg1(vcpu);
 		switch(feature) {
 		case ARM_SMCCC_ARCH_WORKAROUND_1:
-			if (kvm_arm_harden_branch_predictor())
+			switch (kvm_arm_harden_branch_predictor()) {
+			case KVM_BP_HARDEN_UNKNOWN:
+				break;
+			case KVM_BP_HARDEN_WA_NEEDED:
 				val = SMCCC_RET_SUCCESS;
+				break;
+			case KVM_BP_HARDEN_NOT_REQUIRED:
+				val = SMCCC_RET_NOT_REQUIRED;
+				break;
+			}
 			break;
 		case ARM_SMCCC_ARCH_WORKAROUND_2:
 			switch (kvm_arm_have_ssbd()) {