diff options
author | Mauro Carvalho Chehab <mchehab+samsung@kernel.org> | 2019-06-12 14:52:38 -0300 |
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committer | Jonathan Corbet <corbet@lwn.net> | 2019-06-14 14:20:27 -0600 |
commit | b693d0b372afb39432e1c49ad7b3454855bc6bed (patch) | |
tree | a3bcb0d94dab9032e96f036d65bd24f197218f43 /Documentation/arm64/tagged-pointers.rst | |
parent | 305a99eb98af22996e9771078b7a19978732ed41 (diff) |
docs: arm64: convert docs to ReST and rename to .rst
The documentation is in a format that is very close to ReST format.
The conversion is actually:
- add blank lines in order to identify paragraphs;
- fixing tables markups;
- adding some lists markups;
- marking literal blocks;
- adjust some title markups.
At its new index.rst, let's add a :orphan: while this is not linked to
the main index.rst file, in order to avoid build warnings.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Diffstat (limited to 'Documentation/arm64/tagged-pointers.rst')
-rw-r--r-- | Documentation/arm64/tagged-pointers.rst | 68 |
1 files changed, 68 insertions, 0 deletions
diff --git a/Documentation/arm64/tagged-pointers.rst b/Documentation/arm64/tagged-pointers.rst new file mode 100644 index 000000000000..2acdec3ebbeb --- /dev/null +++ b/Documentation/arm64/tagged-pointers.rst @@ -0,0 +1,68 @@ +========================================= +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. |