diff options
| author | Will Deacon <will.deacon@arm.com> | 2013-09-17 11:46:23 +0100 |
|---|---|---|
| committer | Nitin Garg <nitin.garg@freescale.com> | 2014-04-21 22:35:35 -0500 |
| commit | f3348d880b759f5dad1d1882415966f5302d2c91 (patch) | |
| tree | 7645bec1044743946e5e00fff198b7bc72ff8365 /Documentation | |
| parent | 99e5fcd90f5679b352379060cd5290364a811e41 (diff) | |
arm64: documentation: tighten up tagged pointer documentation
Commit d50240a5f6ce ("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.
Change-Id: I7df342e01d5253ccacb3847449940892768d7e07
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@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 264e9841563a..d9995f1f51b3 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) 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 + (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 - addresses differing only in the upper bits. + 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. |