1 Tagged virtual addresses in AArch64 Linux
2 =========================================
4 Author: Will Deacon <will.deacon@arm.com>
7 This document briefly describes the provision of tagged virtual
8 addresses in the AArch64 translation system and their potential uses
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:
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.
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
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.
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.