Add write_cr4()

src/hulk/cpu.d

@@ -60,6 +60,11 @@ ulong read_cr4()
     return __asm!ulong("mov %cr4, %rax", "={rax}");
 }
 
+void write_cr4(ulong v)
+{
+    __asm("mov $0, %cr4", "r", v);
+}
+
 ulong rdmsr(uint msr)
 {
     return __asm!ulong(`