Do wrmsr bit-twiddling in Rust

Signed-off-by: Joe Richey <joerichey@google.com>

Author: josephlr

src/asm/asm.s

@@ -224,21 +224,19 @@ _x86_64_asm_write_cr4:
 .global _x86_64_asm_rdmsr
 .p2align 4
 _x86_64_asm_rdmsr:
-    mov   %edi,%ecx
+    mov    %edi, %ecx    # First param is the MSR number
     rdmsr
-    shl    $0x20,%rdx   # shift edx to upper 32bit
-    mov    %eax,%eax    # clear upper 32bit of rax
-    or     %rdx,%rax    # or with rdx
+    shl    $32,  %rdx    # shift edx to upper 32bit
+    mov    %eax, %eax    # clear upper 32bit of rax
+    or     %rdx, %rax    # or with rdx
     retq
 
 .global _x86_64_asm_wrmsr
 .p2align 4
 _x86_64_asm_wrmsr:
-    mov   %edi,%ecx
-    movq  %rsi,%rax
-    movq  %rsi,%rdx
-    shr   $0x20,%rdx
-    wrmsr
+    movl  %edi, %ecx    # First param is the MSR number
+    movl  %esi, %eax    # Second param is the low 32-bits
+    wrmsr               # Third param (high 32-bits) is already in %edx
     retq
 
 .global _x86_64_asm_hlt

src/asm/mod.rs

@@ -214,7 +214,7 @@ extern "C" {
         any(target_env = "gnu", target_env = "musl"),
         link_name = "_x86_64_asm_wrmsr"
     )]
-    pub(crate) fn x86_64_asm_wrmsr(msr: u32, value: u64);
+    pub(crate) fn x86_64_asm_wrmsr(msr: u32, low: u32, high: u32);
 
     #[cfg_attr(
         any(target_env = "gnu", target_env = "musl"),

src/registers/model_specific.rs

@@ -142,20 +142,19 @@ mod x86_64 {
         /// effects.
         #[inline]
         pub unsafe fn write(&mut self, value: u64) {
+            let low = value as u32;
+            let high = (value >> 32) as u32;
+
             #[cfg(feature = "inline_asm")]
-            {
-                let low = value as u32;
-                let high = (value >> 32) as u32;
-                asm!(
-                    "wrmsr",
-                    in("ecx") self.0,
-                    in("eax") low, in("edx") high,
-                    options(nostack, preserves_flags),
-                );
-            }
+            asm!(
+                "wrmsr",
+                in("ecx") self.0,
+                in("eax") low, in("edx") high,
+                options(nostack, preserves_flags),
+            );
 
             #[cfg(not(feature = "inline_asm"))]
-            crate::asm::x86_64_asm_wrmsr(self.0, value);
+            crate::asm::x86_64_asm_wrmsr(self.0, low, high);
         }
     }