Use naked_asm!, delete asm_func!

This deletes our home-grown `asm_func!` macro in favor of using
`#[unsafe(naked)]` functions within Wasmtime. This is needed for
fiber-related bits right now where we need tight control over the exact
assembly of some functions. This additionally migrates s390x fiber bits
to Rust as inline assembly is now stable for s390x.

prtest:full

Author: alexcrichton

Cargo.lock

@@ -259,7 +259,6 @@ wasmtime-fiber = { path = "crates/fiber", version = "=38.0.0", package = 'wasmti
 wasmtime-jit-debug = { path = "crates/jit-debug", version = "=38.0.0", package = 'wasmtime-internal-jit-debug' }
 wasmtime-component-util = { path = "crates/component-util", version = "=38.0.0", package = 'wasmtime-internal-component-util' }
 wasmtime-component-macro = { path = "crates/component-macro", version = "=38.0.0", package = 'wasmtime-internal-component-macro' }
-wasmtime-asm-macros = { path = "crates/asm-macros", version = "=38.0.0", package = 'wasmtime-internal-asm-macros' }
 wasmtime-versioned-export-macros = { path = "crates/versioned-export-macros", version = "=38.0.0", package = 'wasmtime-internal-versioned-export-macros' }
 wasmtime-slab = { path = "crates/slab", version = "=38.0.0", package = 'wasmtime-internal-slab' }
 wasmtime-jit-icache-coherence = { path = "crates/jit-icache-coherence", version = "=38.0.0", package = 'wasmtime-internal-jit-icache-coherence' }

Cargo.toml

@@ -15,7 +15,6 @@ workspace = true
 anyhow = { workspace = true }
 cfg-if = { workspace = true }
 wasmtime-versioned-export-macros = { workspace = true }
-wasmtime-asm-macros = { workspace = true }
 
 [target.'cfg(unix)'.dependencies]
 rustix = { workspace = true, features = ["mm"] }

crates/asm-macros/Cargo.toml

@@ -22,10 +22,6 @@ fn main() {
         println!("cargo:rerun-if-changed=src/windows.c");
         build.file("src/windows.c");
         build.define("VERSIONED_SUFFIX", Some(versioned_suffix!()));
-    } else if arch == "s390x" {
-        println!("cargo:rerun-if-changed=src/stackswitch/s390x.S");
-        build.file("src/stackswitch/s390x.S");
-        build.define("VERSIONED_SUFFIX", Some(versioned_suffix!()));
     } else {
         // assume that this is included via inline assembly in the crate itself,
         // and the crate will otherwise have a `compile_error!` for unsupported

crates/asm-macros/src/lib.rs

@@ -22,6 +22,7 @@ cfg_if::cfg_if! {
     if #[cfg(not(feature = "std"))] {
         mod nostd;
         use nostd as imp;
+        mod stackswitch;
     } else if #[cfg(miri)] {
         mod miri;
         use miri as imp;
@@ -31,16 +32,12 @@ cfg_if::cfg_if! {
     } else if #[cfg(unix)] {
         mod unix;
         use unix as imp;
+        mod stackswitch;
     } else {
         compile_error!("fibers are not supported on this platform");
     }
 }
 
-// Our own stack switcher routines are used on Unix and no_std
-// platforms, but not on Windows (it has its own fiber API).
-#[cfg(any(unix, not(feature = "std")))]
-pub(crate) mod stackswitch;
-
 /// Represents an execution stack to use for a fiber.
 pub struct FiberStack(imp::FiberStack);
 

crates/fiber/Cargo.toml

@@ -8,22 +8,27 @@ cfg_if::cfg_if! {
     if #[cfg(target_arch = "aarch64")] {
         mod aarch64;
         pub(crate) use supported::*;
+        pub(crate) use aarch64::*;
     } else if #[cfg(target_arch = "x86_64")] {
         mod x86_64;
         pub(crate) use supported::*;
+        pub(crate) use x86_64::*;
     } else if #[cfg(target_arch = "x86")] {
         mod x86;
         pub(crate) use supported::*;
+        pub(crate) use x86::*;
     } else if #[cfg(target_arch = "arm")] {
         mod arm;
         pub(crate) use supported::*;
+        pub(crate) use arm::*;
     } else if #[cfg(target_arch = "s390x")] {
-        // currently `global_asm!` isn't stable on s390x so this is an external
-        // assembler file built with the `build.rs`.
+        mod s390x;
         pub(crate) use supported::*;
+        pub(crate) use s390x::*;
     } else if #[cfg(target_arch = "riscv64")]  {
         mod riscv64;
         pub(crate) use supported::*;
+        pub(crate) use riscv64::*;
     } else {
         // No support for this platform. Don't fail compilation though and
         // instead defer the error to happen at runtime when a fiber is created.
@@ -42,18 +47,6 @@ cfg_if::cfg_if! {
 )]
 mod supported {
     pub const SUPPORTED_ARCH: bool = true;
-    unsafe extern "C" {
-        #[wasmtime_versioned_export_macros::versioned_link]
-        pub(crate) fn wasmtime_fiber_init(
-            top_of_stack: *mut u8,
-            entry: extern "C" fn(*mut u8, *mut u8),
-            entry_arg0: *mut u8,
-        );
-        #[wasmtime_versioned_export_macros::versioned_link]
-        pub(crate) fn wasmtime_fiber_switch(top_of_stack: *mut u8);
-        #[wasmtime_versioned_export_macros::versioned_link]
-        pub(crate) fn wasmtime_fiber_start();
-    }
 }
 
 /// Helper module reexported in the fallback case above when the current host

crates/fiber/build.rs

@@ -18,8 +18,7 @@
 //   `DW_CFA_AARCH64_negate_ra_state` DWARF operation (aliased with the
 //   `.cfi_window_save` assembler directive) informs an unwinder about this
 
-use super::wasmtime_fiber_start;
-use wasmtime_asm_macros::asm_func;
+use core::arch::naked_asm;
 
 cfg_if::cfg_if! {
     if #[cfg(target_vendor = "apple")] {
@@ -37,10 +36,9 @@ cfg_if::cfg_if! {
     }
 }
 
-// fn(top_of_stack(%x0): *mut u8)
-asm_func!(
-    wasmtime_versioned_export_macros::versioned_stringify_ident!(wasmtime_fiber_switch),
-    concat!(
+#[unsafe(naked)]
+pub(crate) unsafe extern "C" fn wasmtime_fiber_switch(top_of_stack: *mut u8 /* x0 */) {
+    naked_asm!(concat!(
         "
             .cfi_startproc
         ",
@@ -87,14 +85,9 @@ asm_func!(
             ret
             .cfi_endproc
         ",
-    ),
-);
+    ));
+}
 
-// fn(
-//    top_of_stack(%x0): *mut u8,
-//    entry_point(%x1): extern fn(*mut u8, *mut u8),
-//    entry_arg0(%x2): *mut u8,
-// )
 // We set up the newly initialized fiber, so that it resumes execution
 // from wasmtime_fiber_start(). As a result, we need a signed address
 // of this function, so there are 2 requirements:
@@ -112,10 +105,14 @@ asm_func!(
 // TODO: Use the PACGA instruction to authenticate the saved register
 // state, which avoids creating signed pointers to
 // wasmtime_fiber_start(), and provides wider coverage.
-#[rustfmt::skip]
-asm_func!(
-    wasmtime_versioned_export_macros::versioned_stringify_ident!(wasmtime_fiber_init),
-    concat!(
+#[unsafe(naked)]
+pub(crate) unsafe extern "C" fn wasmtime_fiber_init(
+    top_of_stack: *mut u8,                        // x0
+    entry_point: extern "C" fn(*mut u8, *mut u8), // x1
+    entry_arg0: *mut u8,                          // x2
+) {
+    naked_asm!(
+        concat!(
         "
             .cfi_startproc
             hint #34 // bti c
@@ -136,16 +133,18 @@ asm_func!(
             ret
             .cfi_endproc
         ",
-    ),
-    fiber = sym wasmtime_fiber_start,
-);
+        ),
+        fiber = sym wasmtime_fiber_start,
+    );
+}
 
 // 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`.
-asm_func!(
-    wasmtime_versioned_export_macros::versioned_stringify_ident!(wasmtime_fiber_start),
-    "
+#[unsafe(naked)]
+unsafe extern "C" fn wasmtime_fiber_start() -> ! {
+    naked_asm!(
+        "
         .cfi_startproc simple
         .cfi_def_cfa_offset 0
         .cfi_escape 0x0f,    /* DW_CFA_def_cfa_expression */ \
@@ -180,5 +179,6 @@ asm_func!(
         // codebase.
         brk 0xf1b3
         .cfi_endproc
-    ",
-);
+        ",
+    );
+}