Documentation/arm64/tagged-pointers.txt

   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) 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
  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             virtual addresses differing only in the upper byte.
  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.