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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:54 EST.

1			Tagged virtual addresses in AArch64 Linux
2			=========================================
3	
4	Author: Will Deacon <will.deacon@arm.com>
5	Date  : 12 June 2013
6	
7	This document briefly describes the provision of tagged virtual
8	addresses in the AArch64 translation system and their potential uses
9	in AArch64 Linux.
10	
11	The kernel configures the translation tables so that translations made
12	via TTBR0 (i.e. userspace mappings) have the top byte (bits 63:56) of
13	the virtual address ignored by the translation hardware. This frees up
14	this byte for application use.
15	
16	
17	Passing tagged addresses to the kernel
18	--------------------------------------
19	
20	All interpretation of userspace memory addresses by the kernel assumes
21	an address tag of 0x00.
22	
23	This includes, but is not limited to, addresses found in:
24	
25	 - pointer arguments to system calls, including pointers in structures
26	   passed to system calls,
27	
28	 - the stack pointer (sp), e.g. when interpreting it to deliver a
29	   signal,
30	
31	 - the frame pointer (x29) and frame records, e.g. when interpreting
32	   them to generate a backtrace or call graph.
33	
34	Using non-zero address tags in any of these locations may result in an
35	error code being returned, a (fatal) signal being raised, or other modes
36	of failure.
37	
38	For these reasons, passing non-zero address tags to the kernel via
39	system calls is forbidden, and using a non-zero address tag for sp is
40	strongly discouraged.
41	
42	Programs maintaining a frame pointer and frame records that use non-zero
43	address tags may suffer impaired or inaccurate debug and profiling
44	visibility.
45	
46	
47	Preserving tags
48	---------------
49	
50	Non-zero tags are not preserved when delivering signals. This means that
51	signal handlers in applications making use of tags cannot rely on the
52	tag information for user virtual addresses being maintained for fields
53	inside siginfo_t. One exception to this rule is for signals raised in
54	response to watchpoint debug exceptions, where the tag information will
55	be preserved.
56	
57	The architecture prevents the use of a tagged PC, so the upper byte will
58	be set to a sign-extension of bit 55 on exception return.
59	
60	
61	Other considerations
62	--------------------
63	
64	Special care should be taken when using tagged pointers, since it is
65	likely that C compilers will not hazard two virtual addresses differing
66	only in the upper byte.
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