Improve trustzone support: take2

[jonathanpallant]

This PR merges #648 and #666 into one cohesive whole.

[jonathanpallant]

I have tested this bootloader function on an STM32U5:

Hello, this is secure-loader. Configuring peripherals...
...SAU configured
...GTZC1 configured
Booting NS application at 0x08200000...
SECURE FAULT:
- SFAR = 20000000
- Attribution Unit Violation

#[cortex_m_rt::entry]
fn main() -> ! {
    let mut bsp: bsp::SecureBoard = bsp::SecureBoard::new();

    // Enable secure fault handler
    bsp.scb
        .enable(cortex_m::peripheral::scb::Exception::SecureFault);

    // Say hello
    println!("Hello, this is secure-loader. Configuring peripherals...");

    // Configure peripherals
    bsp.configure_sau();
    println!("...SAU configured");
    bsp.set_sram3_nonsecure();
    println!("...GTZC1 configured");

    let ns_app_base = bsp::hal::ns_addr::FLASH2_START;
    println!("Booting NS application at 0x{:08x}...", ns_app_base);
    // Boot nonsecure world
    unsafe {
        cortex_m::asm::bootload_ns(ns_app_base as *const u32, bsp.scb_ns);
    }
}

[jonathanpallant]

VLSTM is apparently a NOP if you don't have an FPU, so maybe we should unconditionally do it? Avoids issues if you were using the FPU despite running a soft-float (EABI) target.

See https://developer.arm.com/documentation/dui0801/l/Floating-point-Instructions--32-bit-/VLSTM--A32-

[diondokter]

That sounds good. Keeps complexity low and it always does the correct thing. The only cost is 2 bytes of flash and 1 clock cycle. Easily worth it

[jonathanpallant]

These changes also allow a SecureFault handler that looks like this:

#[cortex_m_rt::exception]
fn SecureFault() {
    println!("SECURE FAULT:");
    // Safety: No-one else is using the SAU, so this won't race, plus the registers we want are read-only
    let sau = unsafe { &*cortex_m::peripheral::SAU::PTR };
    let sfsr = sau.sfsr.read();
    if sfsr.sfarvalid() {
        println!("- SFAR = {:08x}", sau.sfar.read().address());
    }
    if sfsr.invep() {
        println!("- Invalid Entry Point");
    }
    if sfsr.invis() {
        println!("- Invalid Integrity Signature");
    }
    if sfsr.inver() {
        println!("- Invalid Exception Return");
    }
    if sfsr.auviol() {
        println!("- Attribution Unit Violation");
    }
    if sfsr.invtran() {
        println!("- Invalid Transition");
    }
    if sfsr.lsperr() {
        println!("- Lazy state preservation error");
    }
    if sfsr.lserr() {
        println!("- Lazy state error");
    }
    loop {}
}

[therealprof]

LGTM!