Floating-Point Support

TF-M adds several configuration flags to control Floating point (FP) 1 support in TF-M Secure Processing Environment (SPE) and Non Secure Processing Environment (NSPE).

Support FP in SPE or NSPE.
Support FP Application Binary Interface (ABI) 2 types: software, hardware. SPE and NSPE shall use the same FP ABI type.
Support lazy stacking enable/disable in SPE only, NSPE is not allowed to enable/disable this feature.
Support GNU Arm Embedded Toolchain 3. GNU Arm Embedded Toolchain 10.3- 2021.10 and later version shall be used to mitigate VLLDM instruction security vulnerability 4.
Support Inter-Process Communication (IPC) 5 model in TF-M, and doesn’t support LIBRARY or SFN model.
Support Armv8.0-M mainline.
Support isolation level 1,2,3.
Does not support use FPU in First-Level Interrupt Handling (FLIH) 6 at current stage.

Please refer to Arm musca S1 7 platform as a reference implementation when you enable FP support on your platforms.

FP ABI type for SPE and NSPE

FP design in Armv8.0-M 8 architecture requires consistent FP ABI types between SPE and NSPE. Furthermore, both sides shall set up CPACR individually when FPU is used. Otherwise, No Coprocessor (NOCP) usage fault will be asserted during FP context switch between security states.

Secure and non-secure libraries are compiled with COMPILER_CP_FLAG and linked with LINKER_CP_OPTION for different FP ABI types. All those libraries shall be built with COMPLIER_CP_FLAG.

If FP ABI types mismatch error is generated during build, pleae check whether the library is compiled with COMPILER_CP_FLAG. Example:

target_compile_options(lib
    PRIVATE
        ${COMPILER_CP_FLAG}
)

CMake configurations for FP support

The following CMake configurations configure COMPILER_CP_FLAG in TF-M SPE.

CONFIG_TFM_FP are used to configure FP ABI type for secure and non-secure side both.

CONFIG_TFM_FP

FP ABI type 2 3

soft (default)

Software

hard

Hardware

FP software ABI type is default in TF-M.
CONFIG_TFM_LAZY_STACKING is used to enable/disable lazy stacking feature. This feature is only valid for FP hardware ABI type. NSPE is not allowed to enable/disable this feature. Let SPE decide the secure/non-secure shared setting of lazy stacking to avoid the possible side-path brought by flexibility.

CONFIG_TFM_LAZY_STACKING

Description

0FF

Disable lazy stacking

ON (default)

Enable lazy stacking
CONFIG_TFM_FP_ARCH specifies which FP architecture is available on the target, valid for FP hardware ABI type.

FP architecture is processor dependent. For GNUARM compiler, example value are: auto, fpv5-d16, fpv5-sp-d16, etc.

Default value of CONFIG_TFM_FP_ARCH for GNUARM compiler is fpv5-sp-d16.

This parameter shall be specified by platform. Please check compiler reference manual and processor hardware manual for more details to set correct FPU configuration for platform.

Reference

1: High-Performance Hardware Support for Floating-Point Operations
2(1,2): Float Point ABI
3(1,2): GNU Arm Embedded Toolchain
4: VLLDM instruction Security Vulnerability
5: Arm® Platform Security Architecture Firmware Framework 1.0
6: Secure Interrupt Integration Guide
7: Musca-S1 Test Chip Board
8: Armv8-M Architecture Reference Manual

CONFIG_TFM_FP	FP ABI type 2 3
soft (default)	Software
hard	Hardware

CONFIG_TFM_LAZY_STACKING	Description
0FF	Disable lazy stacking
ON (default)	Enable lazy stacking