projects / deliverable / linux.git / blame
| Commit | Line | Data |
|---|---|---|
| d50240a5 WD |
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 | ||
| 21 | (2) Tags are not guaranteed to be preserved when delivering | |
| 22 | signals. This means that signal handlers in applications | |
| 23 | making use of tags cannot rely on the tag information for | |
| 24 | user virtual addresses being maintained for fields inside | |
| 25 | siginfo_t. One exception to this rule is for signals raised | |
| 26 | in response to debug exceptions, where the tag information | |
| 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 | |
| 31 | addresses differing only in the upper bits. | |
| 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. |