1 // A WORD OF CAUTION
2 //
3 // This entire file basically needs to be kept in sync with itself. It's not
4 // really possible to modify just one bit of this file without understanding
5 // all the other bits. Documentation tries to reference various bits here and
6 // there but try to make sure to read over everything before tweaking things!
7 //
8 // Also at this time this file is heavily based off the x86_64 file, so you'll
9 // probably want to read that one as well.
10 
11 use core::arch::naked_asm;
12 
13 #[unsafe(naked)]
14 pub(crate) unsafe extern "C" fn wasmtime_fiber_switch(top_of_stack: *mut u8 /* r0 */) {
15     naked_asm!(
16         "
17         // Save callee-saved registers
18         push {{r4-r11,lr}}
19 
20         // Swap stacks, recording our current stack pointer
21         ldr r4, [r0, #-0x08]
22         str sp, [r0, #-0x08]
23         mov sp, r4
24 
25         // Restore and return
26         pop {{r4-r11,lr}}
27         bx lr
28         ",
29     );
30 }
31 
32 pub(crate) unsafe fn wasmtime_fiber_init(
33     top_of_stack: *mut u8,
34     entry_point: extern "C" fn(*mut u8, *mut u8),
35     entry_arg0: *mut u8,
36 ) {
37     #[repr(C)]
38     #[derive(Default)]
39     struct InitialStack {
40         r4: *mut u8,
41         r5: *mut u8,
42         r6: *mut u8,
43         r7: *mut u8,
44         r8: *mut u8,
45         r9: *mut u8,
46         r10: *mut u8,
47         r11: *mut u8,
48         lr: *mut u8,
49 
50         // unix.rs reserved space
51         last_sp: *mut u8,
52         run_result: *mut u8,
53     }
54 
55     unsafe {
56         let initial_stack = top_of_stack.cast::<InitialStack>().sub(1);
57         initial_stack.write(InitialStack {
58             r9: entry_arg0,
59             r10: entry_point as *mut u8,
60             r11: top_of_stack,
61             lr: wasmtime_fiber_start as *mut u8,
62             last_sp: initial_stack.cast(),
63             ..InitialStack::default()
64         });
65     }
66 }
67 
68 #[unsafe(naked)]
69 unsafe extern "C" fn wasmtime_fiber_start() -> ! {
70     naked_asm!(
71         "
72         .cfi_startproc simple
73         .cfi_def_cfa_offset 0
74         // See the x86_64 file for more commentary on what these CFI directives
75         // are doing. Like over there note that the relative offsets to
76         // registers here match the frame layout in `wasmtime_fiber_switch`.
77         //
78         // TODO: this is only lightly tested. This gets backtraces in gdb but
79         // not at runtime. Perhaps the libgcc at runtime was too old? Doesn't
80         // support something here? Unclear. Will need investigation if someone
81         // ends up needing this and it still doesn't work.
82         .cfi_escape 0x0f,    /* DW_CFA_def_cfa_expression */ \
83             5,               /* the byte length of this expression */ \
84             0x7d, 0x00,      /* DW_OP_breg14(%sp) + 0 */ \
85             0x06,            /* DW_OP_deref */ \
86             0x23, 0x24	 /* DW_OP_plus_uconst 0x24 */
87 
88         .cfi_rel_offset lr, -0x04
89         .cfi_rel_offset r11, -0x08
90         .cfi_rel_offset r10, -0x0c
91         .cfi_rel_offset r9, -0x10
92         .cfi_rel_offset r8, -0x14
93         .cfi_rel_offset r7, -0x18
94         .cfi_rel_offset r6, -0x1c
95         .cfi_rel_offset r5, -0x20
96         .cfi_rel_offset r4, -0x24
97 
98         mov r1, r11
99         mov r0, r9
100         blx r10
101         .cfi_endproc
102         ",
103     );
104 }
105