linux/Documentation/arm64/tagged-pointers.txt

		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.