stos/amd64: Add MSR register helpers

Signed-off-by: Chloe M. <chloe@mensia.org>

paw/stos/head/arch/amd64/msr.h

@@ -0,0 +1,56 @@
+/*
+ * Copyright (c) 2026, Chloe M.
+ * Provided under the BSD-3 clause.
+ *
+ * Description: Model specific register helpers
+ * Author:      Chloe M.
+ */
+
+#ifndef _MACHINE_MSR_H_
+#define _MACHINE_MSR_H_ 1
+
+#ifndef __ASSEMBLER__
+#include <stdef.h>
+#endif  /* !__ASSEMBLER__ */
+
+#define IA32_SPEC_CTL       0x00000048
+#define IA32_KERNEL_GS_BASE 0xC0000102
+#define IA32_GS_BASE        0xC0000101
+#define IA32_FS_BASE        0xC0000100
+#define IA32_APIC_BASE_MSR  0x0000001B
+
+#ifndef __ASSEMBLER__
+ALWAYS_INLINE static inline UQUAD
+MdRdmsr(ULONG Address)
+{
+    ULONG Low, High;
+
+    ASMV(
+        "rdmsr"
+        : "=a" (Low), "=d" (High)
+        : "c" (Address)
+        : "memory"
+    );
+
+    return ((UQUAD)High << 32) | Low;
+}
+
+ALWAYS_INLINE static inline VOID
+MdWrmsr(ULONG Address, UQUAD Value)
+{
+    ULONG Low, High;
+
+    Low = Value & 0xFFFFFFFF;
+    High = (Value >> 32) & 0xFFFFFFFF;
+
+    ASMV(
+        "wrmsr"
+        :
+        : "a" (Low), "d" (High),
+          "c" (Address)
+        : "memory"
+    );
+}
+
+#endif  /* !__ASSEMBLER__ */
+#endif  /* !_MACHINE_MSR_H_ */