arm64: User access library functions

This patch add support for various user access functions. These
functions use the standard LDR/STR instructions and not the LDRT/STRT
variants in order to allow kernel addresses (after set_fs(KERNEL_DS)).

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Acked-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Olof Johansson <olof@lixom.net>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>

arch/arm64/lib/clear_user.S

@@ -0,0 +1,58 @@
+/*
+ * Based on arch/arm/lib/clear_user.S
+ *
+ * Copyright (C) 2012 ARM Ltd.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#include <linux/linkage.h>
+#include <asm/assembler.h>
+
+	.text
+
+/* Prototype: int __clear_user(void *addr, size_t sz)
+ * Purpose  : clear some user memory
+ * Params   : addr - user memory address to clear
+ *          : sz   - number of bytes to clear
+ * Returns  : number of bytes NOT cleared
+ *
+ * Alignment fixed up by hardware.
+ */
+ENTRY(__clear_user)
+	mov	x2, x1			// save the size for fixup return
+	subs	x1, x1, #8
+	b.mi	2f
+1:
+USER(9f, str	xzr, [x0], #8	)
+	subs	x1, x1, #8
+	b.pl	1b
+2:	adds	x1, x1, #4
+	b.mi	3f
+USER(9f, str	wzr, [x0], #4	)
+	sub	x1, x1, #4
+3:	adds	x1, x1, #2
+	b.mi	4f
+USER(9f, strh	wzr, [x0], #2	)
+	sub	x1, x1, #2
+4:	adds	x1, x1, #1
+	b.mi	5f
+	strb	wzr, [x0]
+5:	mov	x0, #0
+	ret
+ENDPROC(__clear_user)
+
+	.section .fixup,"ax"
+	.align	2
+9:	mov	x0, x2			// return the original size
+	ret
+	.previous