μAMP: Asymmetric Multi-Processing on microcontrollers

May 10, 2019 by Jorge Aparicio

An asymmetric multiprocessing (AMP) system is a multiprocessor computer system where not all of the multiple interconnected central processing units (CPUs) are treated equally. – Wikipedia

What is μAMP?

microamp (styled as μAMP) is a framework (library plus cargo subcommand) for building bare-metal applications that target AMP systems.

This blog post is a deep dive into this framework which serves as the core foundation of the multi-core version of Real Time For the Masses (RTFM), which I’ll cover in the next blog post.

Why?

Historically, microcontrollers have been designed as single-core Systems On a Chip (SoCs) but newer designs are increasingly opting for an heterogeneous multi-core architecture. For example, the NXP’s LPC43xx series pairs a Cortex-M4 processor with one (or more) Cortex-M0 co-processor(s) in a single package. The goal of these designs is usually optimizing power consumption: for example, the lower power M0 can handle all the I/O and the M4 core is only activated to perform expensive floating-point / DSP computations.

The μAMP model lets us target these kind of systems but can also be applied to homogeneous multi-core systems like the dual-core real-time processor (2 ARM Cortex-R5 cores) on the Zynq UltraScale+ EG or the LPC55S69 (2 ARM Cortex-M33 cores) microcontroller.

What it looks like?

μAMP takes after CUDA’s “single source” / hybrid model in the sense that one can write a single program (crate) that will run on multiple cores of potentially different architectures. To statically partition the application across the cores one uses the conditional compilation support that’s built into the language, that is #[cfg] and cfg!.

Here’s a contrived μAMP program that targets the dual-core real-time processor (2x ARM Cortex-R5 cores) on the Zynq UltraScale+ EG.

(Here you can find the complete source code of this and other examples shown in this post)

// examples/amp-hello.rs

#![no_main]
#![no_std]

use arm_dcc::dprintln;
use panic_dcc as _; // panic handler
use zup_rt::entry;

// program entry point for both cores
#[entry]
fn main() -> ! {
    static mut X: u32 = 0;

    // `#[entry]` transforms `X` into a `&'static mut` reference
    let x: &'static mut u32 = X;

    let who_am_i = if cfg!(core = "0") { 0 } else { 1 };
    dprintln!("Hello from core {}", who_am_i);

    dprintln!("X has address {:?}", x as *mut u32);

    loop {}
}

The cargo-microamp subcommand is used to compile this program for each core.

$ # by default cargo-microamp assumses 2 cores but
$ # this can be overridden with the `-c` flag
$ cargo microamp --example amp-hello -v
(..)
"cargo" "rustc" "--example" "amp-hello" "--release" "--" \
  "--cfg" "core=\"0\"" \
  "-C" "link-arg=-Tcore0.x" \
  "-C" "link-arg=/tmp/cargo-microamp.GSj3FpvLfYTR/microamp-data.o"
    Finished dev [unoptimized + debuginfo] target(s) in 0.10s

"cargo" "rustc" "--example" "amp-hello" "--release" "--" \
  "--cfg" "core=\"1\"" \
  "-C" "link-arg=-Tcore1.x" \
  "-C" "link-arg=/tmp/cargo-microamp.GSj3FpvLfYTR/microamp-data.o"
    Finished dev [unoptimized + debuginfo] target(s) in 0.11s

This subcommand produces two images, one for each core.

$ # image for core #0
$ size -Ax target/armv7r-none-eabi/release/examples/amp-hello-0
target/armv7r-none-eabi/debug/examples/amp-hello-0  :
section             size         addr
.text              0x4fc          0x0
.local               0x0      0x20000
.bss                 0x4   0xfffc0000
.data                0x0   0xfffc0004
.rodata             0x40   0xfffc0004
.shared              0x0   0xfffe0000

$ # image for core #1
$ size -Ax target/armv7r-none-eabi/release/examples/amp-hello-1
target/armv7r-none-eabi/debug/examples/amp-hello-1  :
section              size         addr
.text              0x4fc          0x0
.local               0x0      0x20000
.bss                 0x4   0xfffd0000
.data                0x0   0xfffd0004
.rodata             0x40   0xfffd0004
.shared              0x0   0xfffe0000

As you can see the linker sections (.bss, .data, etc) of each image are placed at different addresses. The memory layout of these images is specified by the linker scripts core0.x and core1.x. These must be provided by the user or by some crate (the zup-rt crate in this case). Later I’ll talk about those linker scripts in detail.

These images can be executed independently; this is their output:

$ # on another terminal: load and run the program
$ CORE=0 xsdb -interactive debug.tcl amp-hello-0

$ # output of core #0
$ tail -f dcc0.log
Hello from core 0
X has address 0xfffc0000
$ # on another terminal: load and run the program
$ CORE=1 xsdb -interactive debug.tcl amp-hello-1

$ # the output of core #1
$ tail -f dcc1.log
Hello from core 1
X has address 0xfffd0000

Note here that each core reports a different address for variable X. I’ll get back to this later.

So far cargo-microamp doesn’t seem to offer much advantage. One could build each of these images separately by calling cargo build twice.

The magic of the framework comes in when you use the #[shared] attribute.

#[shared] memory

The static variable X we used in the previous program does not refer to the same memory location: each image has a copy of the variable X. This can be seen in the output of the program where each core reports a different address of the variable X.

To place a variable in shared memory one has to opt-in using the #[shared] attribute provided by the microamp crate. #[shared] variables can be used to synchronize program execution and / or share / exchange data. Here’s an example:

// examples/amp-shared.rs

#![no_main]
#![no_std]

use core::sync::atomic::{AtomicU8, Ordering};

use arm_dcc::dprintln;
use microamp::shared;
use panic_dcc as _; // panic handler
use zup_rt::entry;

// non-atomic variable
#[shared] // <- means: same memory location on all the cores
static mut SHARED: u64 = 0;

// used to synchronize access to `SHARED`
#[shared]
static SEMAPHORE: AtomicU8 = AtomicU8::new(CORE0);

// possible values for SEMAPHORE

const CORE0: u8 = 0;
const CORE1: u8 = 1;
const LOCKED: u8 = 2;

#[entry]
fn main() -> ! {
    let (our_turn, next_core) = if cfg!(core = "0") {
        (CORE0, CORE1)
    } else {
        (CORE1, CORE0)
    };

    dprintln!("START");

    let mut done = false;
    while !done {
        // try to acquire the lock
        while SEMAPHORE
            .compare_exchange(our_turn, LOCKED, Ordering::AcqRel, Ordering::Relaxed)
            .is_err()
        {
            // busy wait if the lock is held by the other core
        }

        // we acquired the lock; now we have exclusive access to `SHARED`
        unsafe {
            if SHARED >= 10 {
                // stop at some arbitrary point
                done = true;
            } else {
                dprintln!("{}", SHARED);

                SHARED += 1;
            }
        }

        // release the lock & unblock the other core
        SEMAPHORE.store(next_core, Ordering::Release);
    }

    dprintln!("DONE");

    loop {}
}

In this program the two cores increase the non-atomic SHARED variable in turns. The atomic SEMAPHORE variable is used to synchronized access to the SHARED variable.

Both variables are placed in shared memory so they bind to the same memory location. We can confirm this by looking at the symbols of each image.

$ # image for core #0

$ # output format: $address $size $symbol_type $symbol_name
$ arm-none-eabi-nm -CSn amp-shared-0
(..)
fffc0000 00000008 D SHARED
fffc0008 00000001 D SEMAPHORE
$ # image for core #1

$ # output format: $address $size $symbol_type $symbol_name
$ arm-none-eabi-nm -CSn amp-shared-1
(..)
fffc0000 00000008 D SHARED
fffc0008 00000001 D SEMAPHORE

If we run core #0 we’ll see ..

$ # on another terminal:
$ CORE=0 xsdb -interactive debug.tcl amp-shared-0

$ # output of core #0
$ tail -f dcc0.log
START
0

.. that the program halts because it’s waiting for the other core. Now, we run core #1 ..

$ # on another terminal:
$ CORE=1 xsdb -interactive debug.tcl amp-shared-1

$ # output of core #1
$ tail -f dcc1.log
START
1
3
5
7
9
DONE

.. and we’ll get new output from core #0.

$ # output of core #0
$ tail -f dcc0.log
START
0
2
4
6
8
DONE

That’s all the μAMP framework gives you: sound and memory safe inter-processor communication over shared memory. Sounds simple, right? The API is simple but the implementation was rather tricky. This is Rust so if something does not need unsafe then it must be memory safe and sound under all possible scenarios; that’s what makes it tricky.

Let’s now analyze the soundness of the #[shared] abstraction.

To Send or !Send

As soon as you get shared memory you can send (move) values from one core to the other: for example, a Mutex<Option<T>> can be used as a poor man’s channel. Consider this program:

#![no_main]
#![no_std]

use core::sync::atomic::{AtomicBool, Ordering};

use arm_dcc::dprintln;
use microamp::shared;
use panic_dcc as _; // panic handler
use spin::Mutex; // spin = "0.5.0"
use zup_rt::entry;

#[shared]
static CHANNEL: Mutex<Option<&'static mut [u8; 1024]>> = Mutex::new(None);

#[shared]
static READY: AtomicBool = AtomicBool::new(false);

// runs on first core
#[cfg(core = "0")]
#[entry]
fn main() -> ! {
    static mut BUFFER: [u8; 1024] = [0; 1024];

    dprintln!("BUFFER is located at address {:?}", BUFFER.as_ptr());

    // send message
    *CHANNEL.lock() = Some(BUFFER);

    // unblock core #1
    READY.store(true, Ordering::Release);

    loop {}
}

// runs on second core
#[cfg(core = "1")]
#[entry]
fn main() -> ! {
    // wait until we receive a message
    while !READY.load(Ordering::Acquire) {
        // spin wait
    }

    let buffer: &'static mut [u8; 1024] = CHANNEL.lock().take().unwrap();

    dprintln!("Received a buffer located at address {:?}", buffer.as_ptr());

    // is this sound?
    // let first = buffer[0];

    loop {}
}

This program could print something like this:

$ # output of core #0
$ tail -f dcc0.log
BUFFER is located at address 0xfffc0000
$ # output of core #1
$ tail -f dcc1.log
Received a buffer located at address 0xfffc0000

If core #1 reads or writes to the buffer it received from core #0, would the program still be sound? The answer is it depends. It depends on the target device and the memory layout of each image.

Memory location matters

Let’s take the UltraScale+ as an example. This device has many memory blocks with different performance characteristics. Each R5 core has 3 blocks of Tightly Coupled Memory (TCM) named ATCM (64 KB), BTCM0 (32 KB) and BTCM1 (32 KB). The idea behind the TCM is that it should be exclusively accessed by a single core; this makes memory access low latency and predictable.

Apart from the TCM there’s 256 KB of On-Chip Memory (OCM) divided in 4 blocks of 64 KB. This memory region is meant to be used to share data / exchange messages between the R5 cores.

Each TCM and OCM block has a different global address. The TCM blocks of each R5 core are a bit special because they are additionally mapped (aliased) at address 0. See the table below:

R5#0 view R5#1 view Global address
R5#0 ATCM 0x0000_0000 ~ 0xFFE0_0000
R5#0 BTCM0 0x0002_0000 ~ 0xFFE2_0000
R5#0 BTCM1 0x0002_8000 ~ 0xFFE2_8000
R5#1 ATCM ~ 0x0000_0000 0xFFE9_0000
R5#1 BTCM0 ~ 0x0002_0000 0xFFEB_0000
R5#1 BTCM1 ~ 0x0002_8000 0xFFEB_8000
OCM0 ~ ~ 0xFFFC_0000
OCM1 ~ ~ 0xFFFD_0000
OCM2 ~ ~ 0xFFFE_0000
OCM3 ~ ~ 0xFFFF_0000

This means that if both cores execute the operation (0x2_0000 as *const u32).read() they will actually read different memory locations and likely get different results.

An important question here is: Can core #0 access core #1’s TCM? The answer is yes, core #0 can read and write to core #1’s TCM through its global address (e.g. 0xFFE9_0000). However, this operation is very slow and will likely degrade the performance of core #1’s accesses to its own TCM.

There are many ways to arrange the memory layout of each image; some of them can make our previous program unsound. For example, if we place the .data and .bss sections of both images (core #0’s .bss contains the BUFFER variable) at address 0x2_0000 (that is in BTCM0) it would become possible to break Rust aliasing rule with no unsafe code. This can be more easily observed if we tweak our previous program like this:

// keep the rest of the program the same

#[cfg(core = "1")]
#[entry]
fn main() -> ! {
    static mut X: [u8; 1024] = [0; 1024]; // NEW!

    // wait until we receive a "message"
    while !READY.load(Ordering::Acquire) {
        // spin wait
    }

    let buffer: &'static mut [u8; 1024] = CHANNEL.lock().take().unwrap();
    let x: &'static mut [u8; 1024] = X; // NEW!

    dprintln!("Received a buffer located at address {:?}", buffer.as_ptr());
    dprintln!("X has address {:?}", x.as_ptr()); // NEW!

    // would this be sound?
    // let head = buffer[0];

    loop {}
}

Running this program produces this output:

$ # output of core #1
$ tail -f dcc1.log
Received a buffer located at address 0x20000
X has address 0x20000

💥 Mutable aliasing! 💥

Yikes, using the aliased address of the TCM (0x000?_????) leads to Undefined Behavior in safe Rust. What went wrong? On core #0 BUFFER is an owning pointer to a 1KB buffer with value 0x2_000 which is an alias for the global address 0xFFE2_000. On core #1 X is an owning pointer to a 1KB buffer with value 0x2_000 which is an alias for the global address 0xFFEB_0000. So far so good, there’s no overlap between these two pointers because they point to different memory locations (see global addresses). The problem is that sending the BUFFER pointer to the other core effectively changes where it actually points to (from global address 0xFFE2_000 to global address 0xFFEB_0000), even though its value is unchanged.

One way to fix this issue is to use the TCM global addresses (i.e. 0xFFE?_????) instead of the aliased addresses. Using global addresses everywhere changes the output of the program to:

$ tail -f dcc1.log
Received a buffer located at address 0xffe20000
X has address 0xffeb0000

No mutable aliasing in this case.

But, is there any difference between using the aliased address (e.g. 0x2_0000) and using the global address (e.g. 0xFFE2_0000) to access the TCM? Yes, there’s a huge difference. Both addresses refer to the same physical memory but the aliased address goes through the fast TCM bus, whereas the global address goes through the slower AXI interface.

But how much is a “huge” difference? We can measure it. Consider the following program:

#[cfg(core = "0")]
#[entry]
fn main() -> ! {
    static mut X: [u8; 1024] = [0; 1024];

    let start = Instant::now();
    for x in X.iter_mut() {
        unsafe {
            ptr::write_volatile(x, ptr::read_volatile(x) + 1);
        }
    }
    let end = Instant::now();

    // `checked` version to avoid panicking branches
    if let Some(dur) = end.checked_duration_since(start) {
        print(dur);
    }

    loop {}
}

// never inline to minimize impact on `main` (e.g. register spilling)
#[inline(never)]
fn print(dur: Duration) {
    dprintln!("{}", dur.as_cycles());
}

This program performs a RMW operation on each byte of a 1 KB array and the whole operation is timed. Let’s see how long this takes depending on where the array X is located.

Location X.as_ptr() Clock cycles Ratio
BTCM0 (aliased address) 0x0002_0000 1,462 1.00
BTCM0 (global address) 0xFFE2_0000 23,171 15.8
OCM0 0xFFFC_0000 13,340 9.1

Ouch, accessing the TCM through its global address is 15-16x slower. Even accessing the OCM is faster (~2x) than accessing the TCM through its global address.

Memory safe program layout

Regardless of the performance, we must default to a memory safe program layout so we’ll pick the following layout:

  • The stack will be placed at the end of aliased BTCM1 (0x3_0000). Note that the stack grows downwards, towards smaller addresses.

Rationale: References to stack variables can not be sent to a different core because they have non-static lifetimes and to place a value in a static variable it must only contain static lifetimes ( there’s an implicit T: 'static bound on all static variables).

  • Code (.text) is placed in the aliased ATCM0 (0x0).

Rationale: We actually have no choice on this one. We’ll return to this later.

  • Constants (.rodata) and static variables (.bss and .data) are placed in OCM0 (core #0) or OCM1 (core #1).

Rationale: References to static variables (&'static mut / &'static) can safely be sent across cores so they must not be placed in the aliased TCM or we’ll run into the problem described above. We choose the OCM instead of the “global” TCM (0xFFE?_????) because the former has better performance.

  • #[shared] variables (.shared) are placed in OCM2 (both cores).

Rationale: We’ll return to this later

The bottom line here is that it’s safe (as in it doesn’t require unsafe) to send static references across the core boundary so one must think about whether that’s actually sound (doesn’t result in Undefined Behavior). In particular, one must think about these scenarios:

  • Sending a string literal (&'static str) or a static reference into something immutable (e.g. &'static i32). These point into the .rodata section.

  • Sending a static reference into something mutable (e.g. &'static mut u32 or &'static AtomicU32). These point into the .bss and .data sections.

The UltraScale+ is particularly tricky because it aliases memory in hardware. To prevent footguns one must not place static variables in aliased memory even if this significantly degrades performance.

It is possible to recover the performance using #[link_section] to place a static variable in the aliased memory (see below). However, one must be careful and never send a reference to this static variable to a different core.

#[entry]
fn main() -> ! {
    // place this in the BTCM0
    #[link_section = ".btcm0.BUFFER"]
    static mut BUFFER: [u8; 128] = [0; 128];

    // prints `0x2_0000`
    dprintln!("{:?}", BUFFER.as_ptr());
}

Using #[link_section] is actually a unsafe operation even though the above program doesn’t contain any unsafe block – a lot of stuff can go horribly wrong if you misuse #[link_section]. This is something we didn’t think about carefully enough in preparation for Rust 1.0 and I hope we’ll fix by the next edition: that is #[link_section] should require the unsafe keyword somewhere and be rejected by #[deny(unsafe_code)].

Data not code

Are we completely safe with just using a certain memory layout? The answer is: in the general case, no; and in the particular case of UltraScale+, also no.

The issue is that one can not only safely send pointers to data across the core boundary; one can also safely send pointers to code. Function pointers (e.g. fn()) obviously qualify as pointers to code; less obviously, trait objects also qualify as pointers to code (they contain a vtable pointer). One should not send either across the core boundary.

But what’s the problem with pointers to code? Consider this unsafe-less, seemingly OK program that’s rejected by the μAMP framework:

// slight variation of the `amp-channel` example

#[shared]
static CHANNEL: Mutex<Option<fn()>> = Mutex::new(None);
//~ error: the trait bound `fn(): DataNotCode` is not satisfied in Mutex<Option<fn()>>

#[shared]
static READY: AtomicBool = AtomicBool::new(false);

// runs on first core
#[cfg(core = "0")]
#[entry]
fn main() -> ! {
    fn foo() {
        dprintln!("foo");
    }

    let f: fn() = foo;

    *CHANNEL.lock() = Some(f);

    // unblock core #1
    READY.store(true, Ordering::Release);

    loop {}
}

// runs on second core
#[cfg(core = "1")]
#[entry]
fn main() -> ! {
    // wait until we receive a "message"
    while !READY.load(Ordering::Acquire) {
        // spin wait
    }

    let f: fn() = CHANNEL.lock().take().unwrap();

    // is this sound?
    f();

    loop {}
}

This is a variation of amp-channel where we send a function pointer from one core to the other. Why is this program rejected?

In the general case we could have a heterogeneous device where one core uses instruction set X (e.g. Cortex-M4F Thumb2 encoded instructions) and the other core uses instruction set Y (e.g. Cortex-R5 ARM encoded instructions); executing a function encoded using instruction set X on the core that uses instruction set Y is Undefined Behavior – if you are lucky this operation will make the core jump into some exception handler but anything could happen.

That’s why we reject this operation in the #[shared] attribute using a trait bound. Both function pointers (fn(..) -> _) and trait objects (dyn Trait) are rejected.

“But the UltraScale+ is an homogeneous multi-core device! Both cores use the same instruction set so the above program ought to be OK, right?” Unfortunately, the UltraScale+ is a complex device so the above program is still not OK.

The R5 cores in the UltraScale+ can only execute code that’s located in the TCM and fetched from the aliased address range (0x000?_????). Trying to execute code located in the OCM results in a prefetch abort at runtime. Same thing if the core tries to fetch code from the global address of the TCM (0xFFE?_????).

Thus we have no choice but to place the .text section (all functions) in the aliased TCM, that is at address 0x000?_????. Once we have done that we can no longer safely send function pointers between the cores due to the aliasing problem we saw before – each core would interpret the same address 0x000?_???? as different memory locations.

How is #[shared] implemented?

Getting #[shared] to work required a bit of linker (script) magic. Let’s see how it works.

#[shared] is a procedural macro attribute (proc_macro_attribute) that performs a small source level transformation. Let’s see the expanded code:

// user input
#[shared]
static mut SHARED: u64 = 0;
// attribute expansion
#[cfg(microamp)]
#[link_section = ".shared"]
#[no_mangle]
static mut SHARED: u64 = {
    fn assert() {
        // used to check that `u64` is not a function pointer or trait object
        microamp::export::is_data::<u64>();
    }

    0
};

#[cfg(not(microamp))]
extern {
    static mut SHARED: u64;
}

The application code will be compiled without --cfg microamp so it will use the external (extern) SHARED variable. The “definition” of this external variable (its size and initial value) must be provided at link time or linking will fail. That’s where the #[cfg(microamp)] item comes in. When one invokes the cargo-microamp subcommand it first compiles the application with --cfg microamp and produces a single object file.

$ cargo microamp --example amp-shared -v
"cargo" "rustc" "--example" "amp-shared" "--" \
  "-C" "lto" \
  "--cfg" "microamp" \
  "--emit=obj" \
  "-A" "warnings" \
  "-C" "linker=microamp-true"
(..)

This object file contains all the #[shared] variables packed in a single linker section named .shared.

$ size -Ax $(find target -name '*.o')
target/armv7r-none-eabi/debug/examples/amp_shared-6ca3c73e139e6dd1.o  :
(..)
.shared                                          0x9    0x0
(..)

cargo-microamp then strips this object file from all linker sections but the one named .shared, renames it to microamp-data.o and places it in a temporary directory.

$ cargo microamp --example amp-shared -v
(..)
"arm-none-eabi-strip" \
  "-R" "*" \
  "-R" "!.shared" \
  "--strip-unneeded" \
  "/tmp/cargo-microamp.GSj3FpvLfYTR/microamp-data.o"

These are the contents of the object file after running arm-none-eabi-strip.

$ size -Ax microamp-data.o
microamp-data.o  :
section   size   addr
.shared    0x9    0x0
Total      0x9

$ # output format: $address $size $symbol_type $symbol_name
$ arm-none-eabi-nm -CSn microamp-data.o
00000000 00000008 D SHARED
00000008 00000001 D SEMAPHORE

Note that the input object files (.o) are relocatable so the addresses reported by nm are not final; only the reported size is final.

When cargo-microamp links the image for each core it passes the path to this stripped object file to the linker.

$ cargo microamp --example amp-shared -v
(..)
"cargo" "rustc" "--example" "amp-shared" "--release" "--" \
  "--cfg" "core=\"0\"" \
  "-C" "link-arg=-Tcore0.x" \
  "-C" "link-arg=/tmp/cargo-microamp.GSj3FpvLfYTR/microamp-data.o"
(..)
"cargo" "rustc" "--example" "amp-shared" "--release" "--" \
  "--cfg" "core=\"1\"" \
  "-C" "link-arg=-Tcore1.x" \
  "-C" "link-arg=/tmp/cargo-microamp.GSj3FpvLfYTR/microamp-data.o"
(..)

Let’s take a quick look at the object file produced by rustc before it’s linked with microamp-data.o

$ # core #0

$ # output format: $address $size $symbol_type $symbol_name
$ arm-none-eabi-nm -CSn amp_shared-75511011384774a7.amp_shared.du4sqj84-cgu.0.rcgu.o
(..)
                  U SEMAPHORE
                  U SHARED
00000000 00000004 T DefaultHandler
00000000 00000070 T IRQ
00000000 000001f0 T main
(..)

The #[shared] variables which are actually extern variables in the expanded code show up as “undefined” (U) symbols and have no address or size. microamp-data.o will provide the size and initial value of these symbols but the symbols are still missing a meaningful address.

Linker script

The final piece to glue all this together is the linker script – one script per core actually – which must be provided by the user or some crate. These linker scripts must be named core0.x, core1.x, etc. and they must place the input .shared section contained in the microamp-data.o file into an output section named .shared. The output .shared section must be placed at the same memory location on all images.

core*.x linker scripts will look like this:

/* showing just the part common to both core0.x and core1.x */

MEMORY
{
  /* .. */

  OCM0 : ORIGIN = 0xFFFC0000, LENGTH = 64K
  OCM1 : ORIGIN = 0xFFFD0000, LENGTH = 64K
  OCM2 : ORIGIN = 0xFFFE0000, LENGTH = 64K
  OCM3 : ORIGIN = 0xFFFF0000, LENGTH = 64K
}

SECTIONS
{
  /* .. */

  /* output section placed in OCM2 */
  .shared : ALIGN(4)
  {
    KEEP(microamp-data.o(.shared));
    . = ALIGN(4);
  } > OCM2

  /* .. */
}

The > OCM2 bit tells the linker where to place the #[shared] variables. The variables need to have the same address on all the images so we have to pick the same memory region in all the core*.x linker scripts. We pick OCM2 here and not OCM0 / OCM1 to avoid other sections like .data, which could have a different size on each image, from displacing the .shared section.

How #[shared] variables are laid out in memory is critical. The .shared section in both images must have the exact same layout or the application will run into Undefined Behavior. To understand why this linker script / attribute does what we want one needs to understand how compiler and linker optimizations can affect the memory layout of a program.

The compiler is free to optimize away unused static variables; variables optimized away by the compiler never make it to the linker. On the other hand, a linker is free to discard entire unused linker sections. This difference in granularity is important because a linker section may contain more than one variable.

rustc by default places each static variable in its own linker section, for example static mut FOO: u32 goes into a section named .data.FOO (the actual name is longer due to mangling); this lets the linker individually discard unused static variables.

Let’s look again at the part of the expansion of #[shared] that goes into microamp-data.o.

#[cfg(microamp)]
#[link_section = ".shared"]
#[no_mangle]
static mut SHARED: u64 = {
    fn assert() {
        // used to check that `u64` is not a function pointer / trait object
        microamp::export::is_data::<u64>();
    }

    0
};

To prevent the compiler from optimizing away this variable we use #[no_mangle], which implies #[used]. Note that, in any case, #[no_mangle] is required to make extern "C" { static mut SHARED: u64 } work.

All these #[shared] variables are placed in the same output section: .shared. In the linker script we use KEEP to prevent the linker from discarding the input .shared section. The linker can’t discard any particular variable in the input .shared section because it operates on linker sections not individual variables. For the same reason the linker can’t reorder the variables within the input .shared section so the variables will have the same order in all the images – the order of the variables in each image will match their order in the microamp-data.o object file, which is up to the compiler to decide.

Validation

Obviously, I got the linker scripts wrong the first time and also the second time and maybe even the third time … after all not many people fully understand what linkers are allowed to do with one’s code – IMO, it’s a great thing that most people don’t have to think about linking.

I knew I got things wrong because I added a validation pass, a sanity check if you will, to cargo-microamp. This sanity check told me if the images were broken at compile time – figuring out that the images were invalid at runtime would probably have been Really Fun to debug but I passed on that.

The sanity check works like this: after cargo-microamp links the images it proceeds to read the .shared section of each image and checks that they contain the same set of symbols (static variables) and that each of these symbols has the same address on all images.

To see this in action I’ll intentionally add an error to the linker script of the second core:

/* core1.x */
SECTIONS
{
  /* .. */

  .shared : ALIGN(4)
  {
    /* NEW let's add a 32-bit zero here for no particular reason */
    LONG(0);

    KEEP(microamp-data.o(.shared));
    . = ALIGN(4);
  } > OCM2

  /* .. */
}

This is the error reported by the tool:

$ cargo microamp --example amp-shared
(..)
Error: the layout of the `.shared` section doesn't match
amp-shared-0:
{
    0xfffe0000: Symbol {
        size: 8,
        name: "SHARED",
    },
    0xfffe0008: Symbol {
        size: 1,
        name: "SEMAPHORE",
    },
}
amp-shared-1
{
    0xfffe0008: Symbol {
        size: 8,
        name: "SHARED",
    },
    0xfffe0010: Symbol {
        size: 1,
        name: "SEMAPHORE",
    },
}

This check was good for catching bugs in the implementation of microamp but it can also catch some user errors. For example a #[shared] variable that contains a usize field is an error if one core has target_pointer_width = "32" and the other core has target_pointer_width = "64" because the shared variable will not have the same size on both images. The fix in that case would be to use something like u32 instead of usize.

#[repr(C)]

cargo-microamp’s sanity check can catch some problems with #[shared] variables but not all of them. In particular, the validation pass can’t inspect the memory layout of each #[shared] variable. Consider this program:

struct Triplet {
    x: u32,
    y: u32,
    z: u32,
}

#[shared]
static mut SHARED: Triplet = Triplet { x: 0, y: 0, z: 0 };

#[shared]
static READY: AtomicBool = AtomicBool::new(false);

#[cfg(core = "0")]
#[entry]
fn main() -> ! {
    unsafe {
        SHARED.x = 1;
        SHARED.y = 2;
    }

    // unblock core #1
    READY.store(true, Ordering::Release);

    loop {}
}

#[cfg(core = "1")]
#[entry]
fn main() -> ! {
    // wait until core #0 initializes the fields of `SHARED`
    while !READY.load(Ordering::Acquire) {}

    unsafe {
        assert_eq!(SHARED.y, 2);
        assert_eq!(SHARED.z, 0);
    }

    loop {}
}

Can these assertions fail? Maybe.

The issue here is that the layout of Rust structs is unspecified. As of two years ago the compiler is able to reorder the fields of a struct to optimize its size (reduce padding). And even before that PR landed the compiler has been able to optimize away the unused fields of a struct.

What could go wrong in this case? In theory, the compiler can optimize the program differently for each core, for example it could optimize away the field z in the first image and x in the second image:

/* core #0 */
struct Triplet {
    x: u32,
    y: u32,
    // z: u32, // never accessed so the compiler optimizes this field away
}

extern {
    // addr_of(SHARED) == 0xFFFE_0000; size_of(SHARED) == 8
    static mut SHARED: Triplet;

    // addr_of(READY) == 0xFFFE_0008; size_of(READY) == 1
    static READY: AtomicBool;
}

#[entry]
fn main() -> ! {
    unsafe {
        // *0xFFFE_0000 <- 1
        SHARED.x = 1;

        // *0xFFFE_0004 <- 2
        SHARED.y = 2;
    }

    // *0xFFFE_0008 <- 1
    READY.store(true, Ordering::Release);

    loop {}
}
/* core #1 */
struct Triplet {
    // x: u32, // never accessed so the compiler optimizes this field away
    y: u32,
    z: u32,
}

// cargo-microamp's sanity check ensures that these have
// the same address and size on both images
extern {
    // addr_of(SHARED) == 0xFFFE_0000; size_of(SHARED) == 8
    static mut SHARED: Triplet;

    // addr_of(READY) == 0xFFFE_0008; size_of(READY) == 1
    static READY: AtomicBool;
}

#[entry]
fn main() -> ! {
    // *0xFFFE_0008 == 0?
    while !READY.load(Ordering::Acquire) {}

    unsafe {
        // *0xFFFE_0000 == 2?
        assert_eq!(SHARED.y, 2);
        // *0xFFFE_0004 == 0?
        assert_eq!(SHARED.z, 0);
    }

    loop {}
}

This kind of optimization would make the application hit the assertions.

The only way to avoid this problem, that I know of, is to only use #[repr(C)] types in #[shared] variables. The framework is already using extern "C" { .. } blocks in the expanded code so compiling the above program actually produces a warning:

$ cargo microamp --example amp-triplet
warning: `extern` block uses type `Triplet` which is not FFI-safe: this struct has unspecified layout
  --> zup-rtfm/examples/amp-triplet.rs:42:20
   |
42 | static mut SHARED: Triplet = Triplet { x: 0, y: 0, z: 0 };
   |                    ^^^^
   |
   = note: #[warn(improper_ctypes)] on by default
   = help: consider adding a #[repr(C)] or #[repr(transparent)] attribute to this struct

As the warning says we should add #[repr(C)] to struct Triplet! That would prevent the compiler from removing and reordering the fields of the struct.

Safe static variables

One last bullet point: we want to keep access to static variables safe but accessing an extern static variable is unsafe so we do a slightly different expansion in that case.

// user input
#[shared]
static SEMAPHORE: AtomicU8 = AtomicU8::new(CORE0);
// attribute expansion
#[cfg(microamp)]
#[link_section = ".shared"]
#[no_mangle]
static SEMAPHORE: AtomicU8 = {
    fn assert() {
        microamp::export::is_data::<AtomicU8>();
    }

    AtomicU8::new(CORE0)
};

// the second part of the expansion is different!
#[cfg(not(microamp))]
struct SEMAPHORE;

#[cfg(not(microamp))]
impl core::ops::Deref for SEMAPHORE {
    type Target = AtomicU8;

    fn deref(&self) -> &AtomicU8 {
        extern "C" {
            static SEMAPHORE: AtomicBool;
        }

        unsafe {
            &SEMAPHORE
        }
    }
}

When the application calls SEMAPHORE.load(Ordering::Acquire) it’s actually using the proxy struct named SEMAPHORE that derefs to an external variable (also) named SEMAPHORE. Referring to the proxy struct and deref-ing it are both safe operations so this accurately mimics a normal static variable.

Outro

For now I have pre-released both microamp and cargo-microamp with version v0.1.0-alpha.1. Before I do a proper release I want to test the whole thing on a heterogenous multi-core device. I have an LPC43xx microcontroller, which has one Cortex-M0 core and one Cortex-M4F core, lying around but haven’t had time to play with it and probably won’t have time until next month. To get that working I’ll need to add a command line flag that lets you specify a different compilation target per core, maybe something like this:

$ # core #0 is ARMv6-M
$ # core #1 is ARMv7-EM
$ cargo microamp \
    --example heterogenous \
    -t0 thumbv6m-none-eabi \
    -t1 thumbv7em-none-eabihf

In any case, that’s μAMP! It’s a very small (no pun intended) API that serves as the foundation of multi-core RTFM, which I’ll cover in the next blog post. Until next time!


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Iban Eguia, Geoff Cant, Harrison Chin, Brandon Edens, whitequark, James Munns, Fredrik Lundström, Kor Nielsen, Dietrich Ayala, Hadrien Grasland, vitiral, Lee Smith, Florian Uekermann, Ivan Dubrov and 64 more people for supporting my work on Patreon.


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Jorge Aparicio