| 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, with the following caveats: |
| |
| (1) The kernel requires that all user addresses passed to EL1 |
| are tagged with tag 0x00. This means that any syscall |
| parameters containing user virtual addresses *must* have |
| their top byte cleared before trapping to the kernel. |
| |
| (2) 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. |
| |
| (3) 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. |
| |
| 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. |
