diff options
| author | Will Deacon <will.deacon@arm.com> | 2013-06-12 16:28:04 +0100 |
|---|---|---|
| committer | Ishan Mittal <imittal@nvidia.com> | 2014-04-30 11:39:03 +0530 |
| commit | b05a09e710b9e51ddb88d230ca1cca02484c1034 (patch) | |
| tree | 85349c6893a1933393722ded66db695b6300df35 /Documentation | |
| parent | e02fc30b94bdff4732449806636e31e4f84689ab (diff) | |
arm64: mm: permit use of tagged pointers at EL0
TCR.TBI0 can be used to cause hardware address translation to ignore the
top byte of userspace virtual addresses. Whilst not especially useful in
standard C programs, this can be used by JITs to `tag' pointers with
various pieces of metadata.
This patch enables this bit for AArch64 Linux, and adds a new file to
Documentation/arm64/ which describes some potential caveats when using
tagged virtual addresses.
(cherry picked from commit d50240a5f6ceaf690a77b0fccb17be51cfa151c2)
Conflicts:
Documentation/arm64/tagged-pointers.txt
Change-Id: I4366318cc1b049e8e9f642fc502e8d0b04e2ed24
Signed-off-by: Will Deacon <will.deacon@arm.com>
Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/arm64/tagged-pointers.txt | 14 |
1 files changed, 7 insertions, 7 deletions
diff --git a/Documentation/arm64/tagged-pointers.txt b/Documentation/arm64/tagged-pointers.txt index d9995f1f51b3..264e9841563a 100644 --- a/Documentation/arm64/tagged-pointers.txt +++ b/Documentation/arm64/tagged-pointers.txt @@ -18,17 +18,17 @@ this byte for application use, with the following caveats: 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 + (2) Tags are not guaranteed to be 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 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. + addresses differing only in the upper bits. 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. |