| ========================================= |
| 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. |
| |
| |
| Passing tagged addresses to the kernel |
| -------------------------------------- |
| |
| All interpretation of userspace memory addresses by the kernel assumes |
| an address tag of 0x00. |
| |
| This includes, but is not limited to, addresses found in: |
| |
| - pointer arguments to system calls, including pointers in structures |
| passed to system calls, |
| |
| - the stack pointer (sp), e.g. when interpreting it to deliver a |
| signal, |
| |
| - the frame pointer (x29) and frame records, e.g. when interpreting |
| them to generate a backtrace or call graph. |
| |
| Using non-zero address tags in any of these locations may result in an |
| error code being returned, a (fatal) signal being raised, or other modes |
| of failure. |
| |
| For these reasons, passing non-zero address tags to the kernel via |
| system calls is forbidden, and using a non-zero address tag for sp is |
| strongly discouraged. |
| |
| Programs maintaining a frame pointer and frame records that use non-zero |
| address tags may suffer impaired or inaccurate debug and profiling |
| visibility. |
| |
| |
| Preserving tags |
| --------------- |
| |
| Non-zero tags are not 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 watchpoint debug exceptions, where the tag information will |
| be preserved. |
| |
| 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. |
| |
| |
| Other considerations |
| -------------------- |
| |
| Special care should be taken when using tagged pointers, since it is |
| likely that C compilers will not hazard two virtual addresses differing |
| only in the upper byte. |
