Mark `call_ostd_main` as unsafe

ostd/src/arch/loongarch/boot/mod.rs

@@ -136,5 +136,7 @@ unsafe extern "C" fn loongarch_boot(
         memory_regions: parse_memory_regions(),
     });
 
-    call_ostd_main();
+    // SAFETY: The safety is guaranteed by the safety preconditions and the fact that we call it
+    // once after setting up necessary resources.
+    unsafe { call_ostd_main() };
 }

ostd/src/arch/riscv/boot/mod.rs

@@ -139,5 +139,7 @@ unsafe extern "C" fn riscv_boot(hart_id: usize, device_tree_paddr: usize) -> ! {
         memory_regions: parse_memory_regions(),
     });
 
-    call_ostd_main();
+    // SAFETY: The safety is guaranteed by the safety preconditions and the fact that we call it
+    // once after setting up necessary resources.
+    unsafe { call_ostd_main() };
 }

ostd/src/arch/x86/boot/linux_boot/mod.rs

@@ -198,6 +198,7 @@ fn parse_memory_regions(boot_params: &BootParams) -> MemoryRegionArray {
 ///
 /// - This function must be called only once at a proper timing in the BSP's boot assembly code.
 /// - The caller must follow C calling conventions and put the right arguments in registers.
+/// - If this function is called, entry points of other boot protocols must never be called.
 // SAFETY: The name does not collide with other symbols.
 #[unsafe(no_mangle)]
 unsafe extern "sysv64" fn __linux_boot(params_ptr: *const BootParams) -> ! {
@@ -218,5 +219,7 @@ unsafe extern "sysv64" fn __linux_boot(params_ptr: *const BootParams) -> ! {
         memory_regions: parse_memory_regions(params),
     });
 
-    call_ostd_main();
+    // SAFETY: The safety is guaranteed by the safety preconditions and the fact that we call it
+    // once after setting up necessary resources.
+    unsafe { call_ostd_main() };
 }

ostd/src/arch/x86/boot/multiboot/mod.rs

@@ -368,6 +368,7 @@ impl Iterator for MemoryEntryIter {
 ///
 /// - This function must be called only once at a proper timing in the BSP's boot assembly code.
 /// - The caller must follow C calling conventions and put the right arguments in registers.
+/// - If this function is called, entry points of other boot protocols must never be called.
 // SAFETY: The name does not collide with other symbols.
 #[unsafe(no_mangle)]
 unsafe extern "sysv64" fn __multiboot_entry(boot_magic: u32, boot_params: u64) -> ! {
@@ -386,5 +387,7 @@ unsafe extern "sysv64" fn __multiboot_entry(boot_magic: u32, boot_params: u64) -
         memory_regions: parse_memory_regions(mb1_info),
     });
 
-    call_ostd_main();
+    // SAFETY: The safety is guaranteed by the safety preconditions and the fact that we call it
+    // once after setting up necessary resources.
+    unsafe { call_ostd_main() };
 }

ostd/src/arch/x86/boot/multiboot2/mod.rs

@@ -143,6 +143,7 @@ fn parse_memory_regions(mb2_info: &BootInformation) -> MemoryRegionArray {
 ///
 /// - This function must be called only once at a proper timing in the BSP's boot assembly code.
 /// - The caller must follow C calling conventions and put the right arguments in registers.
+/// - If this function is called, entry points of other boot protocols must never be called.
 // SAFETY: The name does not collide with other symbols.
 #[unsafe(no_mangle)]
 unsafe extern "sysv64" fn __multiboot2_entry(boot_magic: u32, boot_params: u64) -> ! {
@@ -161,5 +162,7 @@ unsafe extern "sysv64" fn __multiboot2_entry(boot_magic: u32, boot_params: u64)
         memory_regions: parse_memory_regions(&mb2_info),
     });
 
-    call_ostd_main();
+    // SAFETY: The safety is guaranteed by the safety preconditions and the fact that we call it
+    // once after setting up necessary resources.
+    unsafe { call_ostd_main() };
 }

ostd/src/boot/mod.rs

@@ -113,13 +113,18 @@ pub(crate) fn init_after_heap() {
     });
 }
 
-/// Calls the OSTD-user defined entrypoint of the actual kernel.
+/// Initializes OSTD and then jumps to the `#[ostd::main]` entry point.
 ///
 /// Any kernel that uses the `ostd` crate should define a function marked with
-/// `ostd::main` as the entrypoint.
+/// `#[ostd::main]` as the kernel's entry function.
 ///
-/// This function should be only called from the bootloader-specific module.
-pub(crate) fn call_ostd_main() -> ! {
+/// # Safety
+///
+/// This function must be called only once at a proper timing on the BSP by the
+/// [`arch::boot`] module.
+///
+/// [`arch::boot`]: crate::arch::boot
+pub(crate) unsafe fn call_ostd_main() -> ! {
     // The entry point of kernel code, which should be defined by the package that
     // uses OSTD.
     unsafe extern "Rust" {
@@ -131,7 +136,5 @@ pub(crate) fn call_ostd_main() -> ! {
 
     // SAFETY: This external function is defined by the package that uses OSTD,
     // which should be generated by the `ostd::main` macro. So it is safe.
-    unsafe {
-        __ostd_main();
-    }
+    unsafe { __ostd_main() };
 }