arm64: mm: permit use of tagged pointers at EL0

TCR.TBI0 can be used to cause hardware address translation to ignore the
top byte of userspace virtual addresses. Whilst not especially useful in
standard C programs, this can be used by JITs to `tag' pointers with
various pieces of metadata.

This patch enables this bit for AArch64 Linux, and adds a new file to
Documentation/arm64/ which describes some potential caveats when using
tagged virtual addresses.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Documentation/arm64/tagged-pointers.txt

@@ -0,0 +1,34 @@
+		Tagged virtual addresses in AArch64 Linux
+		=========================================
+
+Author: Will Deacon <will.deacon@arm.com>
+Date  : 12 June 2013
+
+This document briefly describes the provision of tagged virtual
+addresses in the AArch64 translation system and their potential uses
+in AArch64 Linux.
+
+The kernel configures the translation tables so that translations made
+via TTBR0 (i.e. userspace mappings) have the top byte (bits 63:56) of
+the virtual address ignored by the translation hardware. This frees up
+this byte for application use, with the following caveats:
+
+	(1) The kernel requires that all user addresses passed to EL1
+	    are tagged with tag 0x00. This means that any syscall
+	    parameters containing user virtual addresses *must* have
+	    their top byte cleared before trapping to the kernel.
+
+	(2) Tags are not guaranteed to be preserved when delivering
+	    signals. This means that signal handlers in applications
+	    making use of tags cannot rely on the tag information for
+	    user virtual addresses being maintained for fields inside
+	    siginfo_t. One exception to this rule is for signals raised
+	    in response to debug exceptions, where the tag information
+	    will be preserved.
+
+	(3) Special care should be taken when using tagged pointers,
+	    since it is likely that C compilers will not hazard two
+	    addresses differing only in the upper bits.
+
+The architecture prevents the use of a tagged PC, so the upper byte will
+be set to a sign-extension of bit 55 on exception return.

arch/arm64/include/asm/pgtable-hwdef.h

@@ -122,5 +122,6 @@
 #define TCR_TG1_64K		(UL(1) << 30)
 #define TCR_IPS_40BIT		(UL(2) << 32)
 #define TCR_ASID16		(UL(1) << 36)
+#define TCR_TBI0		(UL(1) << 37)
 
 #endif

arch/arm64/kernel/entry.S

@@ -423,6 +423,7 @@ el0_da:
 	 * Data abort handling
 	 */
 	mrs	x0, far_el1
+	bic	x0, x0, #(0xff << 56)
 	disable_step x1
 	isb
 	enable_dbg

arch/arm64/mm/proc.S

@@ -147,7 +147,7 @@ ENTRY(__cpu_setup)
 	 * both user and kernel.
 	 */
 	ldr	x10, =TCR_TxSZ(VA_BITS) | TCR_FLAGS | TCR_IPS_40BIT | \
-		      TCR_ASID16 | (1 << 31)
+		      TCR_ASID16 | TCR_TBI0 | (1 << 31)
 #ifdef CONFIG_ARM64_64K_PAGES
 	orr	x10, x10, TCR_TG0_64K
 	orr	x10, x10, TCR_TG1_64K