Currently it is not possible to disable streaming mode via ptrace on SME only systems, the interface for doing this is to write via NT_ARM_SVE but such writes will be rejected on a system without SVE support. Enable this functionality by allowing userspace to write SVE_PT_REGS_FPSIMD format data via NT_ARM_SVE with the vector length set to 0 on SME only systems. Such writes currently error since we require that a vector length is specified which should minimise the risk that existing software is relying on current behaviour.
Reads are not supported since I am not aware of any use case for this and there is some risk that an existing userspace application may be confused if it reads NT_ARM_SVE on a system without SVE. Existing kernels will return FPSIMD formatted register state from NT_ARM_SVE if full SVE state is not stored, for example if the task has not used SVE. Returning a vector length of 0 would create a risk that software could try to do things like allocate space for register state with zero sizes, while returning a vector length of 128 bits would look like SVE is supported. It seems safer to just not make the changes to add read support.
It remains possible for userspace to detect a SME only system via the ptrace interface only since reads of NT_ARM_SSVE and NT_ARM_ZA will suceed while reads of NT_ARM_SVE will fail. Read/write access to the FPSIMD registers in non-streaming mode is available via REGSET_FPR.
The aim is is to make a minimally invasive change, no operation that would previously have succeeded will be affected, and we use a previously defined interface in new circumstances rather than define completely new ABI.
The series starts with some enhancements to sve-ptrace to cover some further corners of existing behaviours in order to reduce the risk of inadvertent changes, implements the proposed new ABI, then extends both sve-ptrace and fp-ptrace to exercise it.
Signed-off-by: Mark Brown broonie@kernel.org --- Mark Brown (5): kselftest/arm64: Verify that we reject out of bounds VLs in sve-ptrace kselftest/arm64: Check that unsupported regsets fail in sve-ptrace arm64/sme: Support disabling streaming mode via ptrace on SME only systems kselftst/arm64: Test NT_ARM_SVE FPSIMD format writes on non-SVE systems kselftest/arm64: Cover disabling streaming mode without SVE in fp-ptrace
Documentation/arch/arm64/sve.rst | 5 + arch/arm64/kernel/ptrace.c | 40 ++++++-- tools/testing/selftests/arm64/fp/fp-ptrace.c | 5 +- tools/testing/selftests/arm64/fp/sve-ptrace.c | 139 +++++++++++++++++++++++++- 4 files changed, 177 insertions(+), 12 deletions(-) --- base-commit: 768361ab16ce943ef3577cea204dc81aa4a47517 change-id: 20250717-arm64-sme-ptrace-sme-only-1fb850600ea0 prerequisite-change-id: 20250808-arm64-fp-trace-macro-02ede083da51
Best regards, -- Mark Brown broonie@kernel.org