Rust startup code for Raspberry Pi3 through use of inline assembly - linker

Question

I'm writing a bare metal code for Raspberry Pi 3 in Rust, however, I have problem with the code that is placed #0x80000 as it is not the _start function.
The compiler is set for AArch64 architecture and I'm using LLD as linker.
# .cargo/config
[build]
target = "aarch64-unknown-none"
[target.aarch64-unknown-none]
rustflags = [
# uncomment to use rustc LLD linker
"-C", "link-arg=-Tlayout.ld",
"-C", "linker=lld-link",
"-Z", "linker-flavor=ld.lld",
]
The first function to be called after startup: (get core ID and let continue only the primary, others are stopped; setup stack for the primary and init memory)
#[link_section = ".reset_vector"]
#[no_mangle]
pub extern "C" fn _start() -> !{
unsafe {
// Halt all cores but the primary
asm!(" mrs x1, mpidr_el1
and x1, x1, #3
cmp x1, #0
bne halt"::::"volatile");
// Setup stack pointer
asm!(" mov sp, #0x80000"::::"volatile");
}
init_runtime();
main();
loop{}
}
fn init_runtime() {
extern "C" {
static mut _sbss: u64;
static mut _ebss: u64;
static mut _sdata: u64;
static mut _edata: u64;
static _sidata: u64;
}
unsafe{
// Zero the BSS section in RAM
r0::zero_bss(&mut _sbss, &mut _ebss);
// Copy variables in DATA section in FLASH to RAM
r0::init_data(&mut _sdata, &mut _edata, &_sidata);
}
}
Function to halt the cores except for primary:
#[no_mangle]
pub fn halt() {
unsafe {asm!("wfe"::::"volatile");}
}
I'm using the r0 crate to init the memory:
fn init_runtime() {
extern "C" {
static mut _sbss: u64;
static mut _ebss: u64;
static mut _sdata: u64;
static mut _edata: u64;
static _sidata: u64;
}
unsafe{
// Zero the BSS section in RAM
r0::zero_bss(&mut _sbss, &mut _ebss);
// Copy variables in DATA section in FLASH to RAM
r0::init_data(&mut _sdata, &mut _edata, &_sidata);
}
}
Finally the linker script:
ENTRY(_start)
SECTIONS {
. = 0x80000;
.text : {
KEEP(*(.reset_vector));
__reset_vector = ABSOLUTE(.);
*(.text .text.* .gnu.linkonce.t*)
}
.rodata : {
*(.rodata .rodata.* .gnu.linkonce.r*)
}
.data : {
_sdata = .;
*(.data .data.* .gnu.linkonce.d*)
_edata = ALIGN(8);
}
.bss (NOLOAD) : {
. = ALIGN(32);
_bss = .;
*(.bss .bss.*)
*(COMMON)
_ebss = ALIGN(8);
}
__bss_length = (__bss_end - __bss_start);
/DISCARD/ : { *(.comment) *(.gnu*) *(.note*) *(.eh_frame*) }
}
Here is the disassembly:
(gdb) disassemble 0x0000000000080000, 0x000000000008035c
Dump of assembler code from 0x80000 to 0x8035c:
=> 0x0000000000080000 <core::mem::uninitialized+0>: sub sp, sp, #0x10
0x0000000000080004 <core::mem::uninitialized+4>: ldr x0, [sp, #8]
0x0000000000080008 <core::mem::uninitialized+8>: str x0, [sp]
0x000000000008000c <core::mem::uninitialized+12>: ldr x0, [sp]
0x0000000000080010 <core::mem::uninitialized+16>: add sp, sp, #0x10
0x0000000000080014 <core::mem::uninitialized+20>: ret
The instruction from function _start need to be placed #0x80000 but this is not the case as there are those of core::mem::uninitialized.
How to modify the linker script so that the mrs x1, mpidr_el1 will be the first instruction to be executed?

Answer 1

After a long night struggle I figured out how to do it.
First create a new section in the linker:
ENTRY(reset)
SECTIONS {
. = 0x80000;
.reset : {
KEEP(*(.reset))
. = ALIGN(8);
}
.text : {
*(.text .text.* .gnu.linkonce.t*)
}
...
And modify the code:
#[link_section=".reset"]
#[no_mangle]
#[naked]
pub extern "C" fn reset () {
unsafe {
// Halt all cores but the primary and set stack pointer
asm!(" mrs x1, mpidr_el1
and x1, x1, #3
cmp x1, #0
bne halt
mov sp, #0x80000
b init
"::::"volatile");
}
}
And the rest:
#[naked]
#[no_mangle]
pub fn init() {
extern "C" {
static mut _sbss: u64;
static mut _ebss: u64;
static mut _sdata: u64;
static mut _edata: u64;
static _sidata: u64;
}
unsafe{
// Zero the BSS section in RAM
r0::zero_bss(&mut _sbss, &mut _ebss);
// Copy variables in DATA section in FLASH to RAM
r0::init_data(&mut _sdata, &mut _edata, &_sidata);
}
extern "Rust" {
fn main() -> !;
}
unsafe { main(); }
}
By splitting the code the initialization in reset stays at 0x80000.
Disassembly of section .reset:
0000000000080000 <reset>:
80000: d53800a1 mrs x1, mpidr_el1
80004: 92400421 and x1, x1, #0x3
80008: f100003f cmp x1, #0x0
8000c: 54001481 b.ne 8029c <halt> // b.any
80010: b26d03ff mov sp, #0x80000 // #524288
80014: 1400008d b 80248 <init>
80018: d65f03c0 ret
8001c: 00000000 .inst 0x00000000 ; undefined