Move entry point (_start) to platform module

Move the entry point (the _start function) to the platform specific
subfolder, instead of main.rs. This enables platform specific entry
points, for instance QEMU will load the hard ID in a0 and the device
tree in a1.

config/linker-script.x

@@ -6,24 +6,22 @@ SECTIONS
   . = 0x80000000;
 
   /* Output a text section, starting with the entry point */
-  .entry_point : ALIGN(0x1000) {
-    *(.entry_point)
-  }
   .text : ALIGN(0x4) {
+    _start
     *(.text)
     *(.text.*)
   }
 
   /* Output the rodata */
-  .rodata : ALIGN(0x1000) {
+  .rodata : ALIGN(0x8) {
     KEEP(*(__*))
     *(.rodata)
     *(.rodata.*)
   }
 
   /* Finally, all data                                         */
   /* NOTE: no need to page-align bss, both bss and data are RW */
-  .data : ALIGN(0x1000) {
+  .data : ALIGN(0x8) {
     KEEP(*(__*))
     *(.data)
     *(.data.*)

src/arch/metal.rs

@@ -5,6 +5,7 @@ use core::ptr;
 
 use super::{Architecture, MCause, Mode};
 use crate::virt::VirtContext;
+use crate::{_stack_end, main};
 
 /// Bare metal RISC-V runtime.
 pub struct Metal {}
@@ -197,6 +198,25 @@ fn read_mtvec() -> usize {
     return mtvec;
 }
 
+// —————————————————————————————— Entry Point ——————————————————————————————— //
+
+global_asm!(
+r#"
+.text
+.global _start
+_start:
+    ld sp, __stack_addr
+    j {main}
+
+// Store the address of the stack in memory
+// That way it can be loaded as an absolute value
+__stack_addr:
+    .dword {stack}
+"#,
+    main = sym main,
+    stack = sym _stack_end,
+);
+
 // ————————————————————————————— Context Switch ————————————————————————————— //
 
 global_asm!(

src/main.rs

@@ -7,7 +7,6 @@ mod logger;
 mod platform;
 mod virt;
 
-use core::arch::asm;
 use core::panic::PanicInfo;
 
 use arch::{pmpcfg, Arch, Architecture};
@@ -19,31 +18,16 @@ use crate::virt::VirtContext;
 
 // Defined in the linker script
 extern "C" {
-    static _stack_start: usize;
-    static _stack_end: usize;
+    pub(crate) static _stack_start: usize;
+    pub(crate) static _stack_end: usize;
 }
 
-#[no_mangle]
-#[link_section = ".entry_point"]
-pub unsafe extern "C" fn _start() -> ! {
-    /// Address of the top of the stack (stack grow towerd lower addresses)
-    static STACK: &'static usize = unsafe { &_stack_end };
-
-    // Initialize stack pointer and jump into main
-    // TODO: zero-out the BSS (QEMU might do it for us, but real hardware will not)
-    asm!(
-        "mv sp, {stack}",
-        "j {main}",
-        main = sym main,
-        stack = in(reg) STACK,
-        options(noreturn)
-    );
-}
-
-extern "C" fn main() -> ! {
+pub(crate) extern "C" fn main(hart_id: usize, device_tree_blob_addr: usize) -> ! {
     init();
     log::info!("Hello, world!");
+    log::info!("Hart ID: {}", hart_id);
     log::info!("mstatus: 0x{:x}", Arch::read_mstatus());
+    log::info!("DTS address: 0x{:x}", device_tree_blob_addr);
 
     log::info!("Preparing jump into payload");
     let payload_addr = Plat::load_payload();