_default_start_trap and _default_setup_interrupts

Author: romancardenas

riscv-rt/CHANGELOG.md

@@ -31,6 +31,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Replace weak definition of `_pre_init_trap` with `PROVIDE(_pre_init_trap =  _default_abort)`.
 - Now, `_default_abort` is 4-byte aligned (required by `_pre_init_trap`)
 - Removed `.init.trap` section, as it is no longer required.
+- Replace weak definition of `_start_trap` with `PROVIDE(_start_trap = _default_start_trap)`.
+- Replace weak definition of `_setup_interrupts` with `PROVIDE(_setup_interrupts = _default_setup_interrupts)`.
+- Now, `_default_start_trap` is 4-byte aligned instead of target width-aligned.
 
 ## [v0.14.0] - 2025-02-18
 

riscv-rt/link.x.in

@@ -31,10 +31,18 @@ PROVIDE(abort = _default_abort);
   _pre_init_trap defaults to _default_abort. Note that _pre_init_trap must be 4-byte aligned */
 PROVIDE(_pre_init_trap = _default_abort);
 
-/* Default trap entry point. The riscv-rt crate provides a weak alias of this function,
-   which saves caller saved registers, calls _start_trap_rust, restores caller saved registers
-   and then returns. Users can override this alias by defining the symbol themselves */
-EXTERN(_start_trap);
+/* Default trap entry point. If not _start_trap symbol is provided, then _start_trap maps to
+   _default_start_trap, which saves caller saved registers, calls _start_trap_rust, restores
+   caller saved registers and then returns. Note that _start_trap must be 4-byte aligned */
+EXTERN(_default_start_trap);
+PROVIDE(_start_trap = _default_start_trap);
+
+/* Default interrupt setup entry point. If not _setup_interrupts symbol is provided, then
+   _setup_interrupts maps to _default_setup_interrupts, which in direct mode sets the value
+   of the xtvec register to _start_trap and, in vectored mode, sets its value to
+   _vector_table and enables vectored mode. */
+EXTERN(_default_setup_interrupts);
+PROVIDE(_setup_interrupts = _default_setup_interrupts);
 
 /* Default exception handler. By default, the exception handler is abort.
    Users can override this alias by defining the symbol themselves */
@@ -195,6 +203,9 @@ BUG(riscv-rt): start of .heap is not 4-byte aligned");
 ASSERT(_pre_init_trap % 4 == 0, "
 BUG(riscv-rt): _pre_init_trap is not 4-byte aligned");
 
+ASSERT(_start_trap % 4 == 0, "
+BUG(riscv-rt): _start_trap is not 4-byte aligned");
+
 ASSERT(_stext + SIZEOF(.text) < ORIGIN(REGION_TEXT) + LENGTH(REGION_TEXT), "
 ERROR(riscv-rt): The .text section must be placed inside the REGION_TEXT region.
 Set _stext to an address smaller than 'ORIGIN(REGION_TEXT) + LENGTH(REGION_TEXT)'");

riscv-rt/macros/src/lib.rs

@@ -475,12 +475,13 @@ pub fn llvm_arch_patch(_input: TokenStream) -> TokenStream {
     q.into()
 }
 
-/// Generates weak `_start_trap` function in assembly.
+/// Generates `_default_start_trap` function in assembly.
+/// If no `_start_trap` function is defined, the linker will use this function as the default.
 ///
 /// This implementation stores all registers in the trap frame and calls `_start_trap_rust`.
 /// The trap frame is allocated on the stack and deallocated after the call.
 #[proc_macro]
-pub fn weak_start_trap(_input: TokenStream) -> TokenStream {
+pub fn default_start_trap(_input: TokenStream) -> TokenStream {
     let arch = RiscvArch::try_from_env().unwrap();
 
     let width = arch.width();
@@ -504,9 +505,9 @@ pub fn weak_start_trap(_input: TokenStream) -> TokenStream {
         r#"
 core::arch::global_asm!(
 ".section .trap, \"ax\"
-.align {width}
-.weak _start_trap
-_start_trap:
+.align 4 /* Alignment required for xtvec */
+.global _default_start_trap
+_default_start_trap:
     addi sp, sp, - {trap_size} * {width}
     {store}
     add a0, sp, zero

riscv-rt/src/asm.rs

@@ -237,23 +237,24 @@ _mp_hook:
     j 1b
 2:  li a0, 1
     ret",
-    // Default implementation of `_setup_interrupts` sets the trap vector to `_start_trap`.
+    // Default implementation of `_setup_interrupts` sets the trap vector to `_start_trap` in direct mode.
+    // In vectored mode, it sets the trap vector to `_vector_table`.
     // Users can override this function by defining their own `_setup_interrupts`
-    ".weak _setup_interrupts
-_setup_interrupts:",
+    ".global _default_setup_interrupts
+_default_setup_interrupts:",
     #[cfg(not(feature = "v-trap"))]
-    "la t0, _start_trap", // _start_trap is 16-byte aligned, so it corresponds to the Direct trap mode
+    "la t0, _start_trap", // _start_trap is 4-byte aligned, so it corresponds to the Direct trap mode
     #[cfg(feature = "v-trap")]
     "la t0, _vector_table
-    ori t0, t0, 0x1", // _vector_table is 16-byte aligned, so we must set the bit 0 to activate the Vectored trap mode
+    ori t0, t0, 0x1", // _vector_table is at least 4-byte aligned, so we must set the bit 0 to activate the Vectored trap mode
     #[cfg(feature = "s-mode")]
     "csrw stvec, t0",
     #[cfg(not(feature = "s-mode"))]
     "csrw mtvec, t0",
     "ret",
 );
 
-riscv_rt_macros::weak_start_trap!();
+riscv_rt_macros::default_start_trap!();
 
 #[cfg(feature = "v-trap")]
 riscv_rt_macros::vectored_interrupt_trap!();