KVM: arm64: Add generic check for system-supported vCPU features

To date KVM has relied on kvm_reset_vcpu() failing when the vCPU feature
flags are unsupported by the system. This is a bit messy since
kvm_reset_vcpu() is called at runtime outside of the KVM_ARM_VCPU_INIT
ioctl when it is expected to succeed. Further complicating the matter is
that kvm_reset_vcpu() must tolerate be idemptotent to the config_lock,
as it isn't consistently called with the lock held.

Prepare to move feature compatibility checks out of kvm_reset_vcpu() with
a 'generic' check that compares the user-provided flags with a computed
maximum feature set for the system.
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lore.kernel.org/r/20230920195036.1169791-2-oliver.upton@linux.devSigned-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/kvm/arm.c

@@ -1190,6 +1190,16 @@ int kvm_vm_ioctl_irq_line(struct kvm *kvm, struct kvm_irq_level *irq_level,
 	return -EINVAL;
 }
 
+static unsigned long system_supported_vcpu_features(void)
+{
+	unsigned long features = KVM_VCPU_VALID_FEATURES;
+
+	if (!cpus_have_final_cap(ARM64_HAS_32BIT_EL1))
+		clear_bit(KVM_ARM_VCPU_EL1_32BIT, &features);
+
+	return features;
+}
+
 static int kvm_vcpu_init_check_features(struct kvm_vcpu *vcpu,
 					const struct kvm_vcpu_init *init)
 {
@@ -1204,12 +1214,12 @@ static int kvm_vcpu_init_check_features(struct kvm_vcpu *vcpu,
 			return -ENOENT;
 	}
 
+	if (features & ~system_supported_vcpu_features())
+		return -EINVAL;
+
 	if (!test_bit(KVM_ARM_VCPU_EL1_32BIT, &features))
 		return 0;
 
-	if (!cpus_have_const_cap(ARM64_HAS_32BIT_EL1))
-		return -EINVAL;
-
 	/* MTE is incompatible with AArch32 */
 	if (kvm_has_mte(vcpu->kvm))
 		return -EINVAL;