Will Deacon | d50240a | 2013-06-12 16:28:04 +0100 | [diff] [blame] | 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, with the following caveats: |
| 15 | |
| 16 | (1) The kernel requires that all user addresses passed to EL1 |
| 17 | are tagged with tag 0x00. This means that any syscall |
| 18 | parameters containing user virtual addresses *must* have |
| 19 | their top byte cleared before trapping to the kernel. |
| 20 | |
Will Deacon | 374ed9d | 2013-09-17 11:46:23 +0100 | [diff] [blame] | 21 | (2) Non-zero tags are not preserved when delivering signals. |
| 22 | This means that signal handlers in applications making use |
| 23 | of tags cannot rely on the tag information for user virtual |
| 24 | addresses being maintained for fields inside siginfo_t. |
| 25 | One exception to this rule is for signals raised in response |
| 26 | to watchpoint debug exceptions, where the tag information |
Will Deacon | d50240a | 2013-06-12 16:28:04 +0100 | [diff] [blame] | 27 | will be preserved. |
| 28 | |
| 29 | (3) Special care should be taken when using tagged pointers, |
| 30 | since it is likely that C compilers will not hazard two |
Will Deacon | 374ed9d | 2013-09-17 11:46:23 +0100 | [diff] [blame] | 31 | virtual addresses differing only in the upper byte. |
Will Deacon | d50240a | 2013-06-12 16:28:04 +0100 | [diff] [blame] | 32 | |
| 33 | The architecture prevents the use of a tagged PC, so the upper byte will |
| 34 | be set to a sign-extension of bit 55 on exception return. |