Don't use LLVM-reserved registers

Author: phil-opp

bios/boot_sector/src/fail.rs

@@ -32,7 +32,7 @@ impl<T, E> UnwrapOrFail for Result<T, E> {
 pub extern "C" fn print_char(c: u8) {
     let ax = u16::from(c) | 0x0e00;
     unsafe {
-        asm!("int 0x10", in("ax") ax, in("bx") 0);
+        asm!("push bx", "mov bx, 0", "int 0x10", "pop bx", in("ax") ax);
     }
 }
 

bios/stage-2/src/memory_map.rs

@@ -24,14 +24,18 @@ pub fn query_memory_map() -> Result<&'static mut [E820MemoryRegion], ()> {
     let mut offset = 0;
     let buf = [0u8; 24];
     loop {
-        let mut ret = 0;
-        let mut buf_written_len = 0;
+        let ret: u32;
+        let buf_written_len;
         unsafe {
             asm!(
+                "push ebx",
+                "mov ebx, edx",
+                "mov edx, 0x534D4150",
                 "int 0x15",
-                inout ("eax") 0xe820 => ret,
-                in("edx") SMAP,
-                inout("ebx") offset,
+                "mov edx, ebx",
+                "pop ebx",
+                inout("eax") 0xe820 => ret,
+                inout("edx") offset,
                 inout("ecx") buf.len() => buf_written_len,
                 in("di") &buf
             )

bios/stage-2/src/protected_mode.rs

@@ -112,12 +112,11 @@ pub fn enter_protected_mode_and_jump_to_stage_3(entry_point: *const u8, info: &m
             // align the stack
             "and esp, 0xffffff00",
             // push arguments
-            "push {info}",
+            "push {info:e}",
             // push entry point address
-            "push {entry_point}",
+            "push {entry_point:e}",
             info = in(reg) info as *const _ as u32,
             entry_point = in(reg) entry_point as u32,
-            out("ebx") _
         );
         asm!("ljmp $0x8, $2f", "2:", options(att_syntax));
         asm!(
@@ -144,13 +143,13 @@ pub fn enter_protected_mode_and_jump_to_stage_3(entry_point: *const u8, info: &m
 fn set_protected_mode_bit() -> u32 {
     let mut cr0: u32;
     unsafe {
-        asm!("mov {}, cr0", out(reg) cr0, options(nomem, nostack, preserves_flags));
+        asm!("mov {:e}, cr0", out(reg) cr0, options(nomem, nostack, preserves_flags));
     }
     let cr0_protected = cr0 | 1;
     write_cr0(cr0_protected);
     cr0
 }
 
 fn write_cr0(val: u32) {
-    unsafe { asm!("mov cr0, {}", in(reg) val, options(nostack, preserves_flags)) };
+    unsafe { asm!("mov cr0, {:e}", in(reg) val, options(nostack, preserves_flags)) };
 }

bios/stage-2/src/screen.rs

@@ -3,7 +3,7 @@ use core::{arch::asm, fmt::Write as _};
 pub fn print_char(c: u8) {
     let ax = u16::from(c) | 0x0e00;
     unsafe {
-        asm!("int 0x10", in("ax") ax, in("bx") 0);
+        asm!("push bx", "mov bx, 0", "int 0x10", "pop bx", in("ax") ax);
     }
 }
 

bios/stage-2/src/vesa.rs

@@ -35,9 +35,9 @@ impl<'a> VesaInfo<'a> {
             .split_at_mut(core::mem::size_of::<VbeInfoBlock>());
         slice.fill(0);
         let block_ptr = slice.as_mut_ptr();
-        let mut ret: u16 = 0;
+        let ret;
         unsafe {
-            asm!("mov es, bx", "int 0x10", inout("ax") 0x4f00u16 => ret, in("bx")0, in("di") block_ptr)
+            asm!("mov es, {:x}", "int 0x10", in(reg)0, inout("ax") 0x4f00u16 => ret, in("di") block_ptr)
         };
         match ret {
             0x4f => {
@@ -165,16 +165,16 @@ impl VesaModeInfo {
         slice.fill(0);
         let block_ptr = slice.as_mut_ptr();
 
-        let mut ret: u16 = 0;
+        let mut ret: u16;
         let mut target_addr = block_ptr as u32;
-        let segment = (target_addr >> 4);
+        let segment = target_addr >> 4;
         target_addr -= segment << 4;
         unsafe {
             asm!(
-                "mov es, bx", "int 0x10",
+                "mov es, {:x}", "int 0x10",
+                in(reg) segment as u16,
                 inout("ax") 0x4f01u16 => ret,
                 in("cx") mode,
-                in("bx") segment as u16,
                 in("di") target_addr as u16
             )
         };
@@ -210,8 +210,17 @@ impl VesaModeInfo {
     }
 
     pub fn enable(&self) -> Result<(), u16> {
-        let mut ret: u16 = 0;
-        unsafe { asm!("int 0x10", inout("ax") 0x4f02u16 => ret, in("bx") self.mode) };
+        let mut ret: u16;
+        unsafe {
+            asm!(
+                "push bx",
+                "mov bx, {:x}",
+                "int 0x10",
+                "pop bx",
+                in(reg) self.mode,
+                inout("ax") 0x4f02u16 => ret,
+            )
+        };
         match ret {
             0x4f => Ok(()),
             other => Err(other),

bios/stage-3/src/main.rs

@@ -36,13 +36,12 @@ pub fn enter_long_mode_and_jump_to_stage_4(info: &mut BiosInfo) {
             "and esp, 0xffffff00",
             // push arguments (extended to 64 bit)
             "push 0",
-            "push {info}",
+            "push {info:e}",
             // push entry point address (extended to 64 bit)
             "push 0",
-            "push {entry_point}",
+            "push {entry_point:e}",
             info = in(reg) info as *const _ as u32,
             entry_point = in(reg) info.stage_4.start as u32,
-            out("ebx") _
         );
         asm!("ljmp $0x8, $2f", "2:", options(att_syntax));
         asm!(