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 // This file is modeled after x86_64.rs and comments are not copied over. For
9 // reference be sure to review the other file. Note that the pointer size is
10 // different so the reserved space at the top of the stack is 8 bytes, not 16
11 // bytes. Still two pointers though.
12 
13 use core::arch::naked_asm;
14 
15 #[unsafe(naked)]
16 pub(crate) unsafe extern "C" fn wasmtime_fiber_switch(top_of_stack: *mut u8) {
17     naked_asm!(
18         "
19         // Load our stack-to-use
20         mov eax, 0x4[esp]
21         mov ecx, -0x8[eax]
22 
23         // Save callee-saved registers
24         push ebp
25         push ebx
26         push esi
27         push edi
28 
29         // Save our current stack and jump to the stack-to-use
30         mov -0x8[eax], esp
31         mov esp, ecx
32 
33         // Restore callee-saved registers
34         pop edi
35         pop esi
36         pop ebx
37         pop ebp
38         ret
39         ",
40     )
41 }
42 
43 #[unsafe(naked)]
44 pub(crate) unsafe extern "C" fn wasmtime_fiber_init(
45     top_of_stack: *mut u8,
46     entry_point: extern "C" fn(*mut u8, *mut u8),
47     entry_arg0: *mut u8,
48 ) {
49     naked_asm!(
50         "
51         mov eax, 4[esp]
52 
53         // move top_of_stack to the 2nd argument
54         mov -0x0c[eax], eax
55 
56         // move entry_arg0 to the 1st argument
57         mov ecx, 12[esp]
58         mov -0x10[eax], ecx
59 
60         // Move our start function to the return address which the `ret` in
61         // `wasmtime_fiber_start` will return to.
62         lea ecx, {start}
63         mov -0x14[eax], ecx
64 
65         // And move `entry_point` to get loaded into `%ebp` through the context
66         // switch. This'll get jumped to in `wasmtime_fiber_start`.
67         mov ecx, 8[esp]
68         mov -0x18[eax], ecx
69 
70         // Our stack from top-to-bottom looks like:
71         //
72         //	  * 8 bytes of reserved space per unix.rs (two-pointers space)
73         //	  * 8 bytes of arguments (two arguments wasmtime_fiber_start forwards)
74         //	  * 4 bytes of return address
75         //	  * 16 bytes of saved registers
76         //
77         // Note that after the return address the stack is conveniently 16-byte
78         // aligned as required, so we just leave the arguments on the stack in
79         // `wasmtime_fiber_start` and immediately do the call.
80         lea ecx, -0x24[eax]
81         mov -0x08[eax], ecx
82         ret
83         ",
84         start = sym wasmtime_fiber_start,
85     );
86 }
87 
88 #[unsafe(naked)]
89 unsafe extern "C" fn wasmtime_fiber_start() -> ! {
90     naked_asm!(
91         "
92         .cfi_startproc simple
93         .cfi_def_cfa_offset 0
94         .cfi_escape 0x0f, /* DW_CFA_def_cfa_expression */ \
95             5,            /* the byte length of this expression */ \
96             0x74, 0x08,   /* DW_OP_breg4 (%esp) + 8 */ \
97             0x06,         /* DW_OP_deref */ \
98             0x23, 0x14    /* DW_OP_plus_uconst 0x14 */
99 
100         .cfi_rel_offset eip, -4
101         .cfi_rel_offset ebp, -8
102         .cfi_rel_offset ebx, -12
103         .cfi_rel_offset esi, -16
104         .cfi_rel_offset edi, -20
105 
106         // Our arguments and stack alignment are all prepped by
107         // `wasmtime_fiber_init`.
108         call ebp
109         ud2
110         .cfi_endproc
111         ",
112     );
113 }
114