On Wed, Aug 20, 2025 at 07:29:05PM +0100, Mark Brown wrote:
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 would 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 succeed while reads of NT_ARM_SVE will fail. Read/write access to the FPSIMD registers in non-streaming mode is available via REGSET_FPR.
sve_set_common() already avoids allocating SVE storage when doing a FPSIMD formatted write and allocating SME storage when doing a NT_ARM_SVE write so we change the function to validate the new case and skip setting a vector length for it.
The aim 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.
Signed-off-by: Mark Brown broonie@kernel.org
Documentation/arch/arm64/sve.rst | 5 +++++ arch/arm64/kernel/ptrace.c | 40 +++++++++++++++++++++++++++++++++------- 2 files changed, 38 insertions(+), 7 deletions(-)
It would be good to have an Ack from the userspace side (GDB?) so that we know that the minimal ABI change you're proposing is sufficient.
In the meantime, I'll pick up the first two selftests as they are independent of this part.
Will