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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.