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.

Passing tagged addresses to the kernel

All interpretation of userspace memory addresses by the kernel assumes an address tag of 0x00.

This includes, but is not limited to, addresses found in:

• pointer arguments to system calls, including pointers in structures passed to system calls,
• the stack pointer (sp), e.g. when interpreting it to deliver a signal,
• the frame pointer (x29) and frame records, e.g. when interpreting them to generate a backtrace or call graph.

Using non-zero address tags in any of these locations may result in an error code being returned, a (fatal) signal being raised, or other modes of failure.

For these reasons, passing non-zero address tags to the kernel via system calls is forbidden, and using a non-zero address tag for sp is strongly discouraged.

Programs maintaining a frame pointer and frame records that use non-zero address tags may suffer impaired or inaccurate debug and profiling visibility.

Preserving tags

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.

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.

Other considerations

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.