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| author | Will Deacon <will.deacon@arm.com> | 2013-09-17 11:46:23 +0100 |
|---|---|---|
| committer | Catalin Marinas <catalin.marinas@arm.com> | 2013-09-20 09:56:06 +0100 |
| commit | 374ed9d18e658704235b84a60589888ca52ab0fe (patch) | |
| tree | fe87e717b8ccf75eb57fd693786f6fab21413cf9 /Documentation/arm64 | |
| parent | 59f67e16e6b79697241c3fd030e3da300377893e (diff) | |
| download | lwn-374ed9d18e658704235b84a60589888ca52ab0fe.tar.gz lwn-374ed9d18e658704235b84a60589888ca52ab0fe.zip | |
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.
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Diffstat (limited to 'Documentation/arm64')
| -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. |