This is a tracking issue for the PR #81346.
The feature gate for the issue is #![feature(abi_c_cmse_nonsecure_call)].

Description

The TrustZone-M feature is available for targets with the Armv8-M architecture profile (thumbv8m in their target name).
LLVM, the Rust compiler and the linker are providing support for the TrustZone-M feature.

One of the things provided, with this unstable feature, is the C-cmse-nonsecure-call function ABI. This ABI is used on function pointers to non-secure code to mark a non-secure function call (see section 5.5 for details).

With this ABI, the compiler will do the following to perform the call:

• save registers needed after the call to Secure memory
• clear all registers that might contain confidential information
• clear the Least Significant Bit of the function address
• branches using the BLXNS instruction

To avoid using the non-secure stack, the compiler will constrain the number and type of parameters/return value.

The extern "C-cmse-nonsecure-call" ABI is otherwise equivalent to the extern "C" ABI.

Example

#![no_std] #![feature(abi_c_cmse_nonsecure_call)] #[no_mangle] pub fn call_nonsecure_function(addr: usize) -> u32 { let non_secure_function = unsafe { core::mem::transmute::<usize, extern "C-cmse-nonsecure-call" fn() -> u32>(addr) }; non_secure_function() }

$ rustc --emit asm --crate-type lib --target thumbv8m.main-none-eabi function.rs call_nonsecure_function: .fnstart .save {r7, lr} push {r7, lr} .setfp r7, sp mov r7, sp .pad #16 sub sp, #16 str r0, [sp, #12] ldr r0, [sp, #12] str r0, [sp, #8] b .LBB0_1 .LBB0_1: ldr r0, [sp, #8] push.w {r4, r5, r6, r7, r8, r9, r10, r11} bic r0, r0, #1 mov r1, r0 mov r2, r0 mov r3, r0 mov r4, r0 mov r5, r0 mov r6, r0 mov r7, r0 mov r8, r0 mov r9, r0 mov r10, r0 mov r11, r0 mov r12, r0 msr apsr_nzcvq, r0 blxns r0 pop.w {r4, r5, r6, r7, r8, r9, r10, r11} str r0, [sp, #4] b .LBB0_2 .LBB0_2: ldr r0, [sp, #4] add sp, #16 pop {r7, pc} 

Steps

• Initial implementation: Add a new ABI to support cmse_nonsecure_call #81346
• ABI compliance should be checked by rustc instead of LLVM: CMSE features should check ABI conformance in rustc, not LLVM #81347