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 pub(crate) unsafe fn wasmtime_fiber_init(
44     top_of_stack: *mut u8,
45     entry_point: extern "C" fn(*mut u8, *mut u8),
46     entry_arg0: *mut u8,
47 ) {
48     // Our stack from top-to-bottom looks like:
49     //
50     //	  * 8 bytes of reserved space per unix.rs (two-pointers space)
51     //	  * 8 bytes of arguments (two arguments wasmtime_fiber_start forwards)
52     //	  * 4 bytes of return address
53     //	  * 16 bytes of saved registers
54     //
55     // Note that after the return address the stack is conveniently 16-byte
56     // aligned as required, so we just leave the arguments on the stack in
57     // `wasmtime_fiber_start` and immediately do the call.
58     #[repr(C)]
59     #[derive(Default)]
60     struct InitialStack {
61         // state that will get resumed into from a `wasmtime_fiber_switch`
62         // starting up this fiber.
63         edi: *mut u8,
64         esi: *mut u8,
65         ebx: *mut u8,
66         ebp: *mut u8,
67         return_address: *mut u8,
68 
69         // two arguments to `entry_point`
70         arg1: *mut u8,
71         arg2: *mut u8,
72 
73         // unix.rs reserved space
74         last_sp: *mut u8,
75         run_result: *mut u8,
76     }
77 
78     unsafe {
79         let initial_stack = top_of_stack.cast::<InitialStack>().sub(1);
80         initial_stack.write(InitialStack {
81             ebp: entry_point as *mut u8,
82             return_address: wasmtime_fiber_start as *mut u8,
83             arg1: entry_arg0,
84             arg2: top_of_stack,
85             last_sp: initial_stack.cast(),
86             ..InitialStack::default()
87         });
88     }
89 }
90 
91 #[unsafe(naked)]
92 unsafe extern "C" fn wasmtime_fiber_start() -> ! {
93     naked_asm!(
94         "
95         .cfi_startproc simple
96         .cfi_def_cfa_offset 0
97         .cfi_escape 0x0f, /* DW_CFA_def_cfa_expression */ \
98             5,            /* the byte length of this expression */ \
99             0x74, 0x08,   /* DW_OP_breg4 (%esp) + 8 */ \
100             0x06,         /* DW_OP_deref */ \
101             0x23, 0x14    /* DW_OP_plus_uconst 0x14 */
102 
103         .cfi_rel_offset eip, -4
104         .cfi_rel_offset ebp, -8
105         .cfi_rel_offset ebx, -12
106         .cfi_rel_offset esi, -16
107         .cfi_rel_offset edi, -20
108 
109         // Our arguments and stack alignment are all prepped by
110         // `wasmtime_fiber_init`.
111         call ebp
112         ud2
113         .cfi_endproc
114         ",
115     );
116 }
117