ARM: hyp-stub: Zero r0 on successful stub handling

We now return HVC_STUB_ERR when a stub hypercall fails, but we
leave whatever was in r0 on success. Zeroing it on return seems
like a good idea.
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <cdall@linaro.org>

arch/arm/kernel/hyp-stub.S

@@ -215,8 +215,10 @@ __hyp_stub_do_trap:
 	beq	__hyp_stub_exit
 
 	ldr	r0, =HVC_STUB_ERR
+	__ERET
 
 __hyp_stub_exit:
+	mov	r0, #0
 	__ERET
 ENDPROC(__hyp_stub_do_trap)
 

arch/arm/kvm/init.S

@@ -155,8 +155,10 @@ reset:
 	b	exit
 
 1:	ldr	r0, =HVC_STUB_ERR
+	eret
 
 exit:
+	mov	r0, #0
 	eret
 ENDPROC(__kvm_handle_stub_hvc)