aarch64: arch_buffer_validate() implementation

This leverages the AT (address translation) instruction to test for
given access permission. The result is then provided in the PAR_EL1
register.

Thanks to @jharris-intel for the suggestion.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>

arch/arm/core/aarch64/userspace.S

@@ -49,6 +49,42 @@ strlen_done:
 	str	w4, [x2]
 	ret
 
+/*
+ * int arch_buffer_validate(void *addr, size_t size, int write)
+ */
+
+GTEXT(arch_buffer_validate)
+SECTION_FUNC(TEXT, arch_buffer_validate)
+
+	add	x1, x1, x0
+	mrs	x3, DAIF
+	msr	DAIFSET, #DAIFSET_IRQ_BIT
+
+abv_loop:
+	cbnz	w2, 1f
+	at	S1E0R, x0
+	b	2f
+1:	at	S1E0W, x0
+
+2:	orr	x0, x0, #(CONFIG_MMU_PAGE_SIZE - 1)
+	add	x0, x0, #1
+
+	isb
+	mrs	x4, PAR_EL1
+	tbnz	x4, #0, abv_fail
+
+	cmp	x0, x1
+	blo	abv_loop
+
+	msr	DAIF, x3
+	mov	x0, #0
+	ret
+
+abv_fail:
+	msr	DAIF, x3
+	mov	x0, #-1
+	ret
+
 /*
  * Routine to jump into userspace
  *