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use std::{cell::RefCell, ops::ControlFlow};
use crate::{
DebugError, DebugRegisters, StackFrame, exception_handling::ExceptionInterface,
unwind_pc_without_debuginfo,
};
use probe_rs::{
CoreRegister, InstructionSet, MemoryInterface, RegisterRole, RegisterValue, UnwindRule,
};
#[derive(Default)]
pub struct XtensaExceptionHandler {
/// Single-entry cache of the most recent windowed-ABI unwind, keyed by the callee's stack
/// pointer. Avoids re-reading the spill area for each register when
/// [`Self::unwind_undefined_register`] is invoked repeatedly for the same frame.
cache: RefCell<Option<UnwindCache>>,
}
struct UnwindCache {
sp: u64,
unwound: DebugRegisters,
}
impl XtensaExceptionHandler {
/// Populate [`Self::cache`] with the windowed-ABI unwind result for `callee_frame_registers`,
/// reusing the previous result if the callee SP matches.
fn ensure_cached_unwind(
&self,
memory: &mut dyn MemoryInterface,
callee_frame_registers: &DebugRegisters,
) -> Result<(), DebugError> {
let sp = callee_frame_registers.get_register_value_by_role(&RegisterRole::StackPointer)?;
if matches!(self.cache.borrow().as_ref(), Some(c) if c.sp == sp) {
return Ok(());
}
let mut scratch = callee_frame_registers.clone();
self.unwind_registers(memory, &mut scratch)?;
*self.cache.borrow_mut() = Some(UnwindCache {
sp,
unwound: scratch,
});
Ok(())
}
fn unwind_registers(
&self,
memory: &mut dyn MemoryInterface,
unwind_registers: &mut DebugRegisters,
) -> Result<(), DebugError> {
// WindowUnderflow12:
// // On entry here: a0-a11 are call[i].reg[0..11] and initially contain garbage, a12-a15 are call[i+1].reg[0..3],
// // (in particular, a13 is call[i+1]’s stack pointer) and must be preserved
// l32e a0, a13, -16 // restore a0 from call[i+1]’s frame
// l32e a1, a13, -12 // restore a1 from call[i+1]’s frame
// l32e a2, a13, -8 // restore a2 from call[i+1]’s frame
// l32e a11, a1, -12 // a11 <- call[i-1]’s sp
// l32e a3, a13, -4 // restore a3 from call[i+1]’s frame
// l32e a4, a11, -48 // restore a4 from end of call[i]’s frame
// l32e a5, a11, -44 // restore a5 from end of call[i]’s frame
// l32e a6, a11, -40 // restore a6 from end of call[i]’s frame
// l32e a7, a11, -36 // restore a7 from end of call[i]’s frame
// l32e a8, a11, -32 // restore a8 from end of call[i]’s frame
// l32e a9, a11, -28 // restore a9 from end of call[i]’s frame
// l32e a10, a11, -24 // restore a10 from end of call[i]’s frame
// l32e a11, a11, -20 // restore a11 from end of call[i]’s frame
// rfwu
// We can try and use FP to unwind SP and RA that allows us to continue unwinding.
let ra = unwind_registers.get_register_value_by_role(&RegisterRole::ReturnAddress)?;
if ra == 0 {
return Ok(());
}
// Current register values.
let sp = unwind_registers.get_register_value_by_role(&RegisterRole::StackPointer)?;
if sp < 16 {
// Stack pointer is too low.
return Err(DebugError::Other(
"Stack pointer is too low to unwind".to_string(),
));
}
let windowsize = (ra & 0xc000_0000) >> 30;
// Read A0-A3 from current stack frame's Register-Spill Area.
let mut stack_frame = [0; 4];
memory.read_32(sp - 16, &mut stack_frame)?;
let [a0, caller_sp, a2, a3] = stack_frame;
// TODO: use an architecture-appropriate value?
if (caller_sp as u64).saturating_sub(sp) > 0x1000_0000 {
// Stack pointer is too far away from the current stack pointer.
return Err(DebugError::Other(
"Stack pointer is too far away to unwind".to_string(),
));
}
let regs_from_current_frame = [
(RegisterRole::ReturnAddress, a0),
(RegisterRole::StackPointer, caller_sp),
(RegisterRole::Core("a2"), a2),
(RegisterRole::Core("a3"), a3),
];
for (role, value) in regs_from_current_frame {
let reg = unwind_registers.get_register_mut_by_role(&role).unwrap();
reg.value = Some(RegisterValue::from(value));
}
if windowsize > 1 {
// The rest of the registers are in the previous stack frame.
let Some(caller_sp) = caller_sp.checked_sub(12) else {
return Ok(());
};
let frame_sp = memory.read_word_32(caller_sp as u64)?;
// We've already read 4 registers out of windowsize * 4.
const AREGS: [&str; 8] = ["a4", "a5", "a6", "a7", "a8", "a9", "a10", "a11"];
let mut frame = [0; AREGS.len()];
let regs_to_read = windowsize * 4 - 4;
let frame_to_read = &mut frame[..regs_to_read as usize];
// For windowsize = 3(12 registers), the offset is -48
let sp_offset = 16 + 4 * regs_to_read;
let Some(frame_sp) = (frame_sp as u64).checked_sub(sp_offset) else {
return Ok(());
};
memory.read_32(frame_sp, &mut frame_to_read[..])?;
for (reg, reg_value) in AREGS.iter().zip(frame_to_read.iter().copied()) {
let reg = unwind_registers
.get_register_mut_by_role(&RegisterRole::Core(reg))
.unwrap();
reg.value = Some(RegisterValue::from(reg_value));
}
}
Ok(())
}
}
impl ExceptionInterface for XtensaExceptionHandler {
fn unwind_without_debuginfo(
&self,
unwind_registers: &mut DebugRegisters,
frame_pc: u64,
_stack_frames: &[StackFrame],
instruction_set: Option<InstructionSet>,
memory: &mut dyn MemoryInterface,
) -> ControlFlow<Option<DebugError>> {
// Use the default method to unwind PC.
unwind_pc_without_debuginfo(unwind_registers, frame_pc, instruction_set)?;
// TODO: this should be automatically handled by the caller.
match self.unwind_registers(memory, unwind_registers) {
Ok(_) => ControlFlow::Continue(()),
Err(error) => ControlFlow::Break(Some(error)),
}
}
fn unwind_undefined_register(
&self,
debug_register: &CoreRegister,
callee_frame_registers: &DebugRegisters,
_unwind_cfa: Option<u64>,
memory: &mut dyn MemoryInterface,
register_rule: &mut String,
) -> Result<Option<RegisterValue>, DebugError> {
if debug_register.register_has_role(RegisterRole::ProgramCounter) {
unreachable!("The program counter is handled separately")
}
if self
.ensure_cached_unwind(memory, callee_frame_registers)
.is_ok()
{
let cache = self.cache.borrow();
let new = cache
.as_ref()
.and_then(|c| c.unwound.get_register(debug_register.id))
.and_then(|r| r.value);
let old = callee_frame_registers
.get_register(debug_register.id)
.and_then(|r| r.value);
if new != old {
*register_rule = "Xtensa window spill (dwarf Undefined)".to_string();
return Ok(new);
}
}
Ok(match debug_register.unwind_rule {
UnwindRule::Preserve => {
*register_rule = "Preserve (dwarf Undefined)".to_string();
callee_frame_registers
.get_register(debug_register.id)
.and_then(|reg| reg.value)
}
UnwindRule::Clear => {
*register_rule = "Clear (dwarf Undefined)".to_string();
None
}
UnwindRule::SpecialRule => {
*register_rule = "Clear (no unwind rules specified)".to_string();
None
}
})
}
}