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Diffstat (limited to 'Documentation/arm64/tagged-pointers.txt')
| -rw-r--r-- | Documentation/arm64/tagged-pointers.txt | 66 |
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diff --git a/Documentation/arm64/tagged-pointers.txt b/Documentation/arm64/tagged-pointers.txt deleted file mode 100644 index a25a99e82bb1..000000000000 --- a/Documentation/arm64/tagged-pointers.txt +++ /dev/null @@ -1,66 +0,0 @@ - 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. |