From e4624435f38b34e7ce827070aa0f8b533a37c07e Mon Sep 17 00:00:00 2001 From: Jonathan Corbet Date: Mon, 12 Jun 2023 06:06:39 -0600 Subject: docs: arm64: Move arm64 documentation under Documentation/arch/ Architecture-specific documentation is being moved into Documentation/arch/ as a way of cleaning up the top-level documentation directory and making the docs hierarchy more closely match the source hierarchy. Move Documentation/arm64 into arch/ (along with the Chinese equvalent translations) and fix up documentation references. Cc: Will Deacon Cc: Alex Shi Cc: Hu Haowen Cc: Paolo Bonzini Acked-by: Catalin Marinas Reviewed-by: Yantengsi Signed-off-by: Jonathan Corbet --- Documentation/arch/arm64/tagged-pointers.rst | 88 ++++++++++++++++++++++++++++ 1 file changed, 88 insertions(+) create mode 100644 Documentation/arch/arm64/tagged-pointers.rst (limited to 'Documentation/arch/arm64/tagged-pointers.rst') diff --git a/Documentation/arch/arm64/tagged-pointers.rst b/Documentation/arch/arm64/tagged-pointers.rst new file mode 100644 index 000000000000..81b6c2a770dd --- /dev/null +++ b/Documentation/arch/arm64/tagged-pointers.rst @@ -0,0 +1,88 @@ +========================================= +Tagged virtual addresses in AArch64 Linux +========================================= + +Author: Will Deacon + +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, unless the application enables the AArch64 +Tagged Address ABI explicitly +(Documentation/arch/arm64/tagged-address-abi.rst). + +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 when the +userspace application did not enable the AArch64 Tagged Address ABI may +result in an error code being returned, a (fatal) signal being raised, +or other modes of failure. + +For these reasons, when the AArch64 Tagged Address ABI is disabled, +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 +--------------- + +When delivering signals, non-zero tags are not preserved in +siginfo.si_addr unless the flag SA_EXPOSE_TAGBITS was set in +sigaction.sa_flags when the signal handler was installed. This means +that signal handlers in applications making use of tags cannot rely +on the tag information for user virtual addresses being maintained +in these fields unless the flag was set. + +Due to architecture limitations, bits 63:60 of the fault address +are not preserved in response to synchronous tag check faults +(SEGV_MTESERR) even if SA_EXPOSE_TAGBITS was set. Applications should +treat the values of these bits as undefined in order to accommodate +future architecture revisions which may preserve the bits. + +For signals raised in response to watchpoint debug exceptions, the +tag information will be preserved regardless of the SA_EXPOSE_TAGBITS +flag setting. + +Non-zero tags are never preserved in sigcontext.fault_address +regardless of the SA_EXPOSE_TAGBITS flag setting. + +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. + +This behaviour is maintained when the AArch64 Tagged Address ABI is +enabled. + + +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. -- cgit v1.2.3