KVM: arm64: Only insert reserved ranges when SMCCC filter is used

The reserved ranges are only useful for preventing userspace from
adding a rule that intersects with functions we must handle in KVM. If
userspace never writes to the SMCCC filter than this is all just wasted
work/memory.

Insert reserved ranges on the first call to KVM_ARM_VM_SMCCC_FILTER.
Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20231004234947.207507-3-oliver.upton@linux.devSigned-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/kvm/hypercalls.c

@@ -133,12 +133,10 @@ static bool kvm_smccc_test_fw_bmap(struct kvm_vcpu *vcpu, u32 func_id)
 						   ARM_SMCCC_SMC_64,		\
 						   0, ARM_SMCCC_FUNC_MASK)
 
-static void init_smccc_filter(struct kvm *kvm)
+static int kvm_smccc_filter_insert_reserved(struct kvm *kvm)
 {
 	int r;
 
-	mt_init(&kvm->arch.smccc_filter);
-
 	/*
 	 * Prevent userspace from handling any SMCCC calls in the architecture
 	 * range, avoiding the risk of misrepresenting Spectre mitigation status
@@ -148,14 +146,20 @@ static void init_smccc_filter(struct kvm *kvm)
 			       SMC32_ARCH_RANGE_BEGIN, SMC32_ARCH_RANGE_END,
 			       xa_mk_value(KVM_SMCCC_FILTER_HANDLE),
 			       GFP_KERNEL_ACCOUNT);
-	WARN_ON_ONCE(r);
+	if (r)
+		goto out_destroy;
 
 	r = mtree_insert_range(&kvm->arch.smccc_filter,
 			       SMC64_ARCH_RANGE_BEGIN, SMC64_ARCH_RANGE_END,
 			       xa_mk_value(KVM_SMCCC_FILTER_HANDLE),
 			       GFP_KERNEL_ACCOUNT);
-	WARN_ON_ONCE(r);
+	if (r)
+		goto out_destroy;
 
+	return 0;
+out_destroy:
+	mtree_destroy(&kvm->arch.smccc_filter);
+	return r;
 }
 
 static bool kvm_smccc_filter_configured(struct kvm *kvm)
@@ -189,6 +193,12 @@ static int kvm_smccc_set_filter(struct kvm *kvm, struct kvm_smccc_filter __user
 		goto out_unlock;
 	}
 
+	if (!kvm_smccc_filter_configured(kvm)) {
+		r = kvm_smccc_filter_insert_reserved(kvm);
+		if (WARN_ON_ONCE(r))
+			goto out_unlock;
+	}
+
 	r = mtree_insert_range(&kvm->arch.smccc_filter, start, end,
 			       xa_mk_value(filter.action), GFP_KERNEL_ACCOUNT);
 	if (r)
@@ -392,7 +402,7 @@ void kvm_arm_init_hypercalls(struct kvm *kvm)
 	smccc_feat->std_hyp_bmap = KVM_ARM_SMCCC_STD_HYP_FEATURES;
 	smccc_feat->vendor_hyp_bmap = KVM_ARM_SMCCC_VENDOR_HYP_FEATURES;
 
-	init_smccc_filter(kvm);
+	mt_init(&kvm->arch.smccc_filter);
 }
 
 void kvm_arm_teardown_hypercalls(struct kvm *kvm)