1========================================= 2Tagged virtual addresses in AArch64 Linux 3========================================= 4 5Author: Will Deacon <will.deacon@arm.com> 6 7Date : 12 June 2013 8 9This document briefly describes the provision of tagged virtual 10addresses in the AArch64 translation system and their potential uses 11in AArch64 Linux. 12 13The kernel configures the translation tables so that translations made 14via TTBR0 (i.e. userspace mappings) have the top byte (bits 63:56) of 15the virtual address ignored by the translation hardware. This frees up 16this byte for application use. 17 18 19Passing tagged addresses to the kernel 20-------------------------------------- 21 22All interpretation of userspace memory addresses by the kernel assumes 23an address tag of 0x00, unless the application enables the AArch64 24Tagged Address ABI explicitly 25(Documentation/arm64/tagged-address-abi.rst). 26 27This includes, but is not limited to, addresses found in: 28 29 - pointer arguments to system calls, including pointers in structures 30 passed to system calls, 31 32 - the stack pointer (sp), e.g. when interpreting it to deliver a 33 signal, 34 35 - the frame pointer (x29) and frame records, e.g. when interpreting 36 them to generate a backtrace or call graph. 37 38Using non-zero address tags in any of these locations when the 39userspace application did not enable the AArch64 Tagged Address ABI may 40result in an error code being returned, a (fatal) signal being raised, 41or other modes of failure. 42 43For these reasons, when the AArch64 Tagged Address ABI is disabled, 44passing non-zero address tags to the kernel via system calls is 45forbidden, and using a non-zero address tag for sp is strongly 46discouraged. 47 48Programs maintaining a frame pointer and frame records that use non-zero 49address tags may suffer impaired or inaccurate debug and profiling 50visibility. 51 52 53Preserving tags 54--------------- 55 56Non-zero tags are not preserved when delivering signals. This means that 57signal handlers in applications making use of tags cannot rely on the 58tag information for user virtual addresses being maintained for fields 59inside siginfo_t. One exception to this rule is for signals raised in 60response to watchpoint debug exceptions, where the tag information will 61be preserved. 62 63The architecture prevents the use of a tagged PC, so the upper byte will 64be set to a sign-extension of bit 55 on exception return. 65 66This behaviour is maintained when the AArch64 Tagged Address ABI is 67enabled. 68 69 70Other considerations 71-------------------- 72 73Special care should be taken when using tagged pointers, since it is 74likely that C compilers will not hazard two virtual addresses differing 75only in the upper byte. 76