arm64: hyp-stub: Zero x0 on successful stub handling

We now return HVC_STUB_ERR when a stub hypercall fails, but we
leave whatever was in x0 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/arm64/kernel/hyp-stub.S

@@ -73,8 +73,10 @@ el1_sync:
 
 	/* Someone called kvm_call_hyp() against the hyp-stub... */
 	ldr	x0, =HVC_STUB_ERR
+	eret
 
-9:	eret
+9:	mov	x0, xzr
+	eret
 ENDPROC(el1_sync)
 
 .macro invalid_vector	label

arch/arm64/kvm/hyp-init.S

@@ -154,13 +154,13 @@ reset:
 	/* Install stub vectors */
 	adr_l	x5, __hyp_stub_vectors
 	msr	vbar_el2, x5
-	b	exit
+	mov	x0, xzr
+	eret
 
 1:	/* Bad stub call */
 	ldr	x0, =HVC_STUB_ERR
-
-exit:
 	eret
+
 ENDPROC(__kvm_handle_stub_hvc)
 
 	.ltorg