libfringe/src/arch/or1k.rs

// This file is part of libfringe, a low-level green threading library.
// Copyright (c) edef <edef@edef.eu>,
//               whitequark <whitequark@whitequark.org>
//               Amanieu d'Antras <amanieu@gmail.com>
// Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
// http://apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or
// http://opensource.org/licenses/MIT>, at your option. This file may not be
// copied, modified, or distributed except according to those terms.

// To understand the machine code in this file, keep in mind these facts:
// * OR1K C ABI has a "red zone": 128 bytes under the top of the stack
//   that is defined to be unmolested by signal handlers, interrupts, etc.
//   Leaf functions can use the red zone without adjusting r1 or r2.
// * OR1K C ABI passes the first argument in r3. We also use r3 to pass a value
//   while swapping context; this is an arbitrary choice
//   (we clobber all registers and could use any of them) but this allows us
//   to reuse the swap function to perform the initial call.
//
// To understand the DWARF CFI code in this file, keep in mind these facts:
// * CFI is "call frame information"; a set of instructions to a debugger or
//   an unwinder that allow it to simulate returning from functions. This implies
//   restoring every register to its pre-call state, as well as the stack pointer.
// * CFA is "call frame address"; the value of stack pointer right before the call
//   instruction in the caller. Everything strictly below CFA (and inclusive until
//   the next CFA) is the call frame of the callee. This implies that the return
//   address is the part of callee's call frame.
// * Logically, DWARF CFI is a table where rows are instruction pointer values and
//   columns describe where registers are spilled (mostly using expressions that
//   compute a memory location as CFA+n). A .cfi_offset pseudoinstruction changes
//   the state of a column for all IP numerically larger than the one it's placed
//   after. A .cfi_def_* pseudoinstruction changes the CFA value similarly.
// * Simulating return is as easy as restoring register values from the CFI table
//   and then setting stack pointer to CFA.
//
// A high-level overview of the function of the trampolines when unwinding is:
// * The 2nd init trampoline puts a controlled value (written in swap to `new_cfa`)
//   into r2. This is then used as the CFA for the 1st trampoline.
// * This controlled value points to the bottom of the stack of the parent context,
//   which holds the saved r2 and r9 from the call to swap().
// * The 1st init trampoline tells the unwinder to restore r2 and r9
//   from the stack frame at r2 (in the parent stack), thus continuing
//   unwinding at the swap call site instead of falling off the end of context stack.
use stack::Stack;

pub const STACK_ALIGNMENT: usize = 4;

#[derive(Debug, Clone, Copy)]
pub struct StackPointer(*mut usize);

pub unsafe fn init(stack: &Stack, f: unsafe extern "C" fn(usize) -> !) -> StackPointer {
  #[naked]
  unsafe extern "C" fn trampoline_1() {
    asm!(
      r#"
        # gdb has a hardcoded check that rejects backtraces where frame addresses
        # do not monotonically decrease. It is turned off if the function is called
        # "__morestack" and that is hardcoded. So, to make gdb backtraces match
        # the actual unwinder behavior, we call ourselves "__morestack" and mark
        # the symbol as local; it shouldn't interfere with anything.
      __morestack:
      .local __morestack

        # Set up the first part of our DWARF CFI linking stacks together. When
        # we reach this function from unwinding, r2 will be pointing at the bottom
        # of the parent linked stack. This link is set each time swap() is called.
        # When unwinding the frame corresponding to this function, a DWARF unwinder
        # will use r2+8 as the next call frame address, restore r2 from CFA-4 and
        # restore return address (r9) from CFA-8. This mirrors what the second half
        # of `swap_trampoline` does.
        .cfi_def_cfa r2, 8
        .cfi_offset r2, -4
        .cfi_offset r9, -8

        # This nop is here so that the initial swap doesn't return to the start
        # of the trampoline, which confuses the unwinder since it will look for
        # frame information in the previous symbol rather than this one. It is
        # never actually executed.
        l.nop

      .Lend:
      .size __morestack, .Lend-__morestack
      "#
      : : : : "volatile")
  }

  #[naked]
  unsafe extern "C" fn trampoline_2() {
    asm!(
      r#"
        # Set up the second part of our DWARF CFI.
        # When unwinding the frame corresponding to this function, a DWARF unwinder
        # will restore r2 (and thus CFA of the first trampoline) from the stack slot.
        # This stack slot is updated every time swap() is called to point to the bottom
        # of the stack of the context switch just switched from.
        .cfi_def_cfa r2, 8
        .cfi_offset r2, -4
        .cfi_offset r9, -8

        # Call the provided function.
        l.lwz   r4, 8(r1)
        l.jalr  r4
        l.nop
      "#
      : : : : "volatile")
  }

  unsafe fn push(sp: &mut StackPointer, val: usize) {
    sp.0 = sp.0.offset(-1);
    *sp.0 = val
  }

  // We set up the stack in a somewhat special way so that to the unwinder it
  // looks like trampoline_1 has called trampoline_2, which has in turn called
  // swap::trampoline.
  //
  // There are 2 call frames in this setup, each containing the return address
  // followed by the r2 value for that frame. This setup supports unwinding
  // using DWARF CFI as well as the frame pointer-based unwinding used by tools
  // such as perf or dtrace.
  let mut sp = StackPointer(stack.base() as *mut usize);

  push(&mut sp, f as usize); // Function that trampoline_2 should call

  // Call frame for trampoline_2. The CFA slot is updated by swap::trampoline
  // each time a context switch is performed.
  push(&mut sp, 0xdead0cfa); // CFA slot
  push(&mut sp, trampoline_1 as usize + 4); // Return after the nop

  // Call frame for swap::trampoline. We set up the r2 value to point to the
  // parent call frame.
  let frame = sp;
  push(&mut sp, frame.0 as usize); // Pointer to parent call frame
  push(&mut sp, trampoline_2 as usize); // Entry point

  // The call frame for swap::trampoline is actually in the red zone and not
  // below the stack pointer.
  frame
}

#[inline(always)]
pub unsafe fn swap(arg: usize, old_sp: *mut StackPointer, new_sp: StackPointer,
                   new_stack: &Stack) -> usize {
  // Address of the topmost CFA stack slot.
  let new_cfa = (new_stack.base() as *mut usize).offset(-2);

  #[naked]
  unsafe extern "C" fn trampoline() {
    asm!(
      r#"
        # Save the frame pointer and link register; the unwinder uses them to find
        # the CFA of the caller, and so they have to have the correct value immediately
        # after the call instruction that invoked the trampoline.
        l.sw    -4(r1), r2
        l.sw    -8(r1), r9
        .cfi_offset r2, -4
        .cfi_offset r9, -8

        # Link the call stacks together by writing the current stack bottom
        # address to the CFA slot in the new stack.
        l.addi  r7, r1, -8
        l.sw    0(r6), r7

        # Switch to the new stack for unwinding purposes. The old stack may no
        # longer be valid now that we have modified the link.
        .cfi_def_cfa_register r5

        # Save stack pointer of the old context.
        l.sw    0(r4), r1
        # Load stack pointer of the new context.
        l.or    r1, r0, r5
        .cfi_def_cfa_register r1

        # Restore frame pointer and link register of the new context.
        l.lwz   r2, -4(r1)
        l.lwz   r9, -8(r1)

        # Return into the new context.
        l.jr    r9
        l.nop
      "#
      : : : : "volatile")
  }

  let ret: usize;
  asm!(
    r#"
      # Call the trampoline to switch to the new context.
      l.jal   ${1}
      l.nop
    "#
    : "={r3}" (ret)
    : "s" (trampoline as usize)
      "{r3}" (arg)
      "{r4}" (old_sp)
      "{r5}" (new_sp.0)
      "{r6}" (new_cfa)
    :/*"r0", "r1",  "r2",  "r3",*/"r4",  "r5",  "r6",  "r7",
      "r8",  "r9",  "r10", "r11", "r12", "r13", "r14", "r15",
      "r16", "r17", "r18", "r19", "r20", "r21", "r22", "r23",
      "r24", "r25", "r26", "r27", "r28", "r29", "r30", "r31",
      "cc", "memory"
    : "volatile");
  ret
}