mirror of
https://github.com/torvalds/linux.git
synced 2024-11-17 09:31:50 +00:00
374ed9d18e
Commit d50240a5f6 ("arm64: mm: permit use of tagged pointers at EL0")
added support for tagged pointers in userspace, but the corresponding
update to Documentation/ contained some imprecise statements.
This patch fixes up some minor ambiguities in the text, hopefully making
it more clear about exactly what the kernel expects from user virtual
addresses.
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
35 lines
1.5 KiB
Plaintext
35 lines
1.5 KiB
Plaintext
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.
|