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authorDave Martin <Dave.Martin@arm.com>2019-06-12 17:00:32 +0100
committerWill Deacon <will.deacon@arm.com>2019-06-13 10:07:19 +0100
commit41040cf7c5f0f26c368bc5d3016fed3a9ca6dba4 (patch)
treea2f093d70015881344167376fecacc49b8a76fe5 /Documentation/arm64
parent01d57485fcdb9f9101a10a18e32d5f8b023cab86 (diff)
arm64/sve: Fix missing SVE/FPSIMD endianness conversions
The in-memory representation of SVE and FPSIMD registers is different: the FPSIMD V-registers are stored as single 128-bit host-endian values, whereas SVE registers are stored in an endianness-invariant byte order. This means that the two representations differ when running on a big-endian host. But we blindly copy data from one representation to another when converting between the two, resulting in the register contents being unintentionally byteswapped in certain situations. Currently this can be triggered by the first SVE instruction after a syscall, for example (though the potential trigger points may vary in future). So, fix the conversion functions fpsimd_to_sve(), sve_to_fpsimd() and sve_sync_from_fpsimd_zeropad() to swab where appropriate. There is no common swahl128() or swab128() that we could use here. Maybe it would be worth making this generic, but for now add a simple local hack. Since the byte order differences are exposed in ABI, also clarify the documentation. Cc: Alex Bennée <alex.bennee@linaro.org> Cc: Peter Maydell <peter.maydell@linaro.org> Cc: Alan Hayward <alan.hayward@arm.com> Cc: Julien Grall <julien.grall@arm.com> Fixes: bc0ee4760364 ("arm64/sve: Core task context handling") Fixes: 8cd969d28fd2 ("arm64/sve: Signal handling support") Fixes: 43d4da2c45b2 ("arm64/sve: ptrace and ELF coredump support") Signed-off-by: Dave Martin <Dave.Martin@arm.com> [will: Fix typos in comments and docs spotted by Julien] Signed-off-by: Will Deacon <will.deacon@arm.com>
Diffstat (limited to 'Documentation/arm64')
-rw-r--r--Documentation/arm64/sve.txt16
1 files changed, 16 insertions, 0 deletions
diff --git a/Documentation/arm64/sve.txt b/Documentation/arm64/sve.txt
index 9940e924a47e..5689fc9a976a 100644
--- a/Documentation/arm64/sve.txt
+++ b/Documentation/arm64/sve.txt
@@ -56,6 +56,18 @@ model features for SVE is included in Appendix A.
is to connect to a target process first and then attempt a
ptrace(PTRACE_GETREGSET, pid, NT_ARM_SVE, &iov).
+* Whenever SVE scalable register values (Zn, Pn, FFR) are exchanged in memory
+ between userspace and the kernel, the register value is encoded in memory in
+ an endianness-invariant layout, with bits [(8 * i + 7) : (8 * i)] encoded at
+ byte offset i from the start of the memory representation. This affects for
+ example the signal frame (struct sve_context) and ptrace interface
+ (struct user_sve_header) and associated data.
+
+ Beware that on big-endian systems this results in a different byte order than
+ for the FPSIMD V-registers, which are stored as single host-endian 128-bit
+ values, with bits [(127 - 8 * i) : (120 - 8 * i)] of the register encoded at
+ byte offset i. (struct fpsimd_context, struct user_fpsimd_state).
+
2. Vector length terminology
-----------------------------
@@ -124,6 +136,10 @@ the SVE instruction set architecture.
size and layout. Macros SVE_SIG_* are defined [1] to facilitate access to
the members.
+* Each scalable register (Zn, Pn, FFR) is stored in an endianness-invariant
+ layout, with bits [(8 * i + 7) : (8 * i)] stored at byte offset i from the
+ start of the register's representation in memory.
+
* If the SVE context is too big to fit in sigcontext.__reserved[], then extra
space is allocated on the stack, an extra_context record is written in
__reserved[] referencing this space. sve_context is then written in the