Fiber support for s390x (#2870)

Add fiber support for the s390x architecture.

crates/fiber/src/arch/s390x.S

@@ -0,0 +1,112 @@
+// A WORD OF CAUTION
+//
+// This entire file basically needs to be kept in sync with itself. It's not
+// really possible to modify just one bit of this file without understanding
+// all the other bits. Documentation tries to reference various bits here and
+// there but try to make sure to read over everything before tweaking things!
+//
+// Also at this time this file is heavily based off the x86_64 file, so you'll
+// probably want to read that one as well.
+
+#include "header.h"
+
+// fn(top_of_stack(%x0): *mut u8)
+HIDDEN(wasmtime_fiber_switch)
+GLOBL(wasmtime_fiber_switch)
+.p2align 2
+TYPE(wasmtime_fiber_switch)
+FUNCTION(wasmtime_fiber_switch):
+    // Save all callee-saved registers on the stack since we're assuming
+    // they're clobbered as a result of the stack switch.
+    stmg %r6, %r15, 48(%r15)
+    aghi %r15, -64
+    std %f8, 0(%r15)
+    std %f9, 8(%r15)
+    std %f10, 16(%r15)
+    std %f11, 24(%r15)
+    std %f12, 32(%r15)
+    std %f13, 40(%r15)
+    std %f14, 48(%r15)
+    std %f15, 56(%r15)
+
+    // Load our previously saved stack pointer to resume to, and save off our
+    // current stack pointer on where to come back to eventually.
+    lg %r1, -16(%r2)
+    stg %r15, -16(%r2)
+
+    // Switch to the new stack and restore all our callee-saved registers after
+    // the switch and return to our new stack.
+    ld %f8, 0(%r1)
+    ld %f9, 8(%r1)
+    ld %f10, 16(%r1)
+    ld %f11, 24(%r1)
+    ld %f12, 32(%r1)
+    ld %f13, 40(%r1)
+    ld %f14, 48(%r1)
+    ld %f15, 56(%r1)
+    lmg %r6, %r15, 112(%r1)
+    br %r14
+SIZE(wasmtime_fiber_switch)
+
+// fn(
+//    top_of_stack(%x0): *mut u8,
+//    entry_point(%x1): extern fn(*mut u8, *mut u8),
+//    entry_arg0(%x2): *mut u8,
+// )
+HIDDEN(wasmtime_fiber_init)
+GLOBL(wasmtime_fiber_init)
+.p2align 2
+TYPE(wasmtime_fiber_init)
+FUNCTION(wasmtime_fiber_init):
+    larl %r1, FUNCTION(wasmtime_fiber_start)
+    stg %r1, -48(%r2)  // wasmtime_fiber_start - restored into %r14
+    stg %r2, -112(%r2) // top_of_stack - restored into %r6
+    stg %r3, -104(%r2) // entry_point - restored into %r7
+    stg %r4, -96(%r2)  // entry_arg0 - restored into %r8
+    aghi %r2, -160     // 160 bytes register save area
+    stg %r2, 120(%r2)  // bottom of register save area - restored into %r15
+
+    // `wasmtime_fiber_switch` has a 64 byte stack.
+    aghi %r2, -64
+    stg %r2, 208(%r2)
+    br %r14
+SIZE(wasmtime_fiber_init)
+
+.p2align 2
+TYPE(wasmtime_fiber_start)
+FUNCTION(wasmtime_fiber_start):
+.cfi_startproc simple
+
+    // See the x86_64 file for more commentary on what these CFI directives are
+    // doing. Like over there note that the relative offsets to registers here
+    // match the frame layout in `wasmtime_fiber_switch`.
+    .cfi_escape 0x0f,    /* DW_CFA_def_cfa_expression */ \
+        7,               /* the byte length of this expression */ \
+        0x7f, 0x90, 0x1, /* DW_OP_breg15 0x90 */ \
+        0x06,            /* DW_OP_deref */ \
+        0x23, 0xe0, 0x1  /* DW_OP_plus_uconst 0xe0 */
+
+    .cfi_rel_offset 6, -112
+    .cfi_rel_offset 7, -104
+    .cfi_rel_offset 8, -96
+    .cfi_rel_offset 9, -88
+    .cfi_rel_offset 10, -80
+    .cfi_rel_offset 11, -72
+    .cfi_rel_offset 12, -64
+    .cfi_rel_offset 13, -56
+    .cfi_rel_offset 14, -48
+    .cfi_rel_offset 15, -40
+
+    // Load our two arguments prepared by `wasmtime_fiber_init`.
+    lgr %r2, %r8  // entry_arg0
+    lgr %r3, %r6  // top_of_stack
+
+    // ... and then we call the function! Note that this is a function call so
+    // our frame stays on the stack to backtrace through.
+    basr %r14, %r7  // entry_point
+    // .. technically we shouldn't get here, so just trap.
+    .word 0x0000
+    .cfi_endproc
+SIZE(wasmtime_fiber_start)
+
+FOOTER