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- 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.
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