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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.
- 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.
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