arm64: hyp-stub: Implement HVC_RESET_VECTORS stub hypercall

Let's define a new stub hypercall that resets the HYP configuration
to its default: hyp-stub vectors, and MMU disabled.

Of course, for the hyp-stub itself, this is a trivial no-op.
Hypervisors will have a bit more work to do.
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: James Morse <james.morse@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <cdall@linaro.org>

arch/arm64/include/asm/virt.h

@@ -44,6 +44,14 @@
  */
 #define HVC_SOFT_RESTART 2
 
+/*
+ * HVC_RESET_VECTORS - Restore the vectors to the original HYP stubs
+ */
+#define HVC_RESET_VECTORS 3
+
+/* Max number of HYP stub hypercalls */
+#define HVC_STUB_HCALL_NR 4
+
 /* Error returned when an invalid stub number is passed into x0 */
 #define HVC_STUB_ERR	0xbadca11
 
@@ -70,6 +78,7 @@ extern u32 __boot_cpu_mode[2];
 
 void __hyp_set_vectors(phys_addr_t phys_vector_base);
 phys_addr_t __hyp_get_vectors(void);
+void __hyp_reset_vectors(void);
 
 /* Reports the availability of HYP mode */
 static inline bool is_hyp_mode_available(void)

arch/arm64/kernel/hyp-stub.S

@@ -73,8 +73,11 @@ el1_sync:
 	mov	x1, x3
 	br	x4				// no return
 
+3:	cmp	x0, #HVC_RESET_VECTORS
+	beq	9f				// Nothing to reset!
+
 	/* Someone called kvm_call_hyp() against the hyp-stub... */
-3:	ldr	x0, =HVC_STUB_ERR
+	ldr	x0, =HVC_STUB_ERR
 
 9:	eret
 ENDPROC(el1_sync)
@@ -127,3 +130,9 @@ ENTRY(__hyp_set_vectors)
 	hvc	#0
 	ret
 ENDPROC(__hyp_set_vectors)
+
+ENTRY(__hyp_reset_vectors)
+	mov	x0, #HVC_RESET_VECTORS
+	hvc	#0
+	ret
+ENDPROC(__hyp_reset_vectors)