From 754af9a1d3f81e8e17dd47be81c673abc681fbe0 Mon Sep 17 00:00:00 2001 From: Will Deacon Date: Wed, 12 Jun 2013 16:28:04 +0100 Subject: arm64: mm: permit use of tagged pointers at EL0 TCR.TBI0 can be used to cause hardware address translation to ignore the top byte of userspace virtual addresses. Whilst not especially useful in standard C programs, this can be used by JITs to `tag' pointers with various pieces of metadata. This patch enables this bit for AArch64 Linux, and adds a new file to Documentation/arm64/ which describes some potential caveats when using tagged virtual addresses. Change-Id: I4c025d026144c69a2259b6562e46176f95b4e110 Signed-off-by: Will Deacon Signed-off-by: Catalin Marinas --- Documentation/arm64/tagged-pointers.txt | 34 +++++++++++++++++++++++++++++++++ 1 file changed, 34 insertions(+) create mode 100644 Documentation/arm64/tagged-pointers.txt (limited to 'Documentation') diff --git a/Documentation/arm64/tagged-pointers.txt b/Documentation/arm64/tagged-pointers.txt new file mode 100644 index 000000000000..264e9841563a --- /dev/null +++ b/Documentation/arm64/tagged-pointers.txt @@ -0,0 +1,34 @@ + 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, with the following caveats: + + (1) The kernel requires that all user addresses passed to EL1 + are tagged with tag 0x00. This means that any syscall + 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 + 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. + +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. -- cgit v1.2.3 From 7a18e70688223a37ba4c8cf5edd313e8d1bb680d Mon Sep 17 00:00:00 2001 From: Will Deacon Date: Tue, 17 Sep 2013 11:46:23 +0100 Subject: 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 Signed-off-by: Catalin Marinas --- Documentation/arm64/tagged-pointers.txt | 14 +++++++------- 1 file changed, 7 insertions(+), 7 deletions(-) (limited to 'Documentation') 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. -- cgit v1.2.3