[i386] Add functions to read and write model-specific registers

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/include/ipxe/msr.h

+#ifndef _IPXE_MSR_H
+#define _IPXE_MSR_H
+
+/** @file
+ *
+ * Model-specific registers
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER );
+
+/**
+ * Read model-specific register
+ *
+ * @v msr		Model-specific register
+ * @ret value		Value
+ */
+static inline __attribute__ (( always_inline )) uint64_t
+rdmsr ( unsigned int msr ) {
+	uint64_t value;
+
+	__asm__ __volatile__ ( "rdmsr" : "=A" ( value ) : "c" ( msr ) );
+	return value;
+}
+
+/**
+ * Write model-specific register
+ *
+ * @v msr		Model-specific register
+ * @v value		Value
+ */
+static inline __attribute__ (( always_inline )) void
+wrmsr ( unsigned int msr, uint64_t value ) {
+
+	__asm__ __volatile__ ( "wrmsr" : : "c" ( msr ), "A" ( value ) );
+}
+
+#endif /* _IPXE_MSR_H */