x86/mm/pkeys: Add functions to fetch PKRU

This adds the raw instruction to access PKRU as well as some
accessor functions that correctly handle when the CPU does not
support the instruction.  We don't use it here, but we will use
read_pkru() in the next patch.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave@sr71.net>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: linux-mm@kvack.org
Link: http://lkml.kernel.org/r/20160212210215.15238D34@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/pgtable.h

@@ -99,6 +99,14 @@ static inline int pte_dirty(pte_t pte)
 	return pte_flags(pte) & _PAGE_DIRTY;
 }
 
+
+static inline u32 read_pkru(void)
+{
+	if (boot_cpu_has(X86_FEATURE_OSPKE))
+		return __read_pkru();
+	return 0;
+}
+
 static inline int pte_young(pte_t pte)
 {
 	return pte_flags(pte) & _PAGE_ACCESSED;

arch/x86/include/asm/special_insns.h

@@ -98,6 +98,28 @@ static inline void native_write_cr8(unsigned long val)
 }
 #endif
 
+#ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
+static inline u32 __read_pkru(void)
+{
+	u32 ecx = 0;
+	u32 edx, pkru;
+
+	/*
+	 * "rdpkru" instruction.  Places PKRU contents in to EAX,
+	 * clears EDX and requires that ecx=0.
+	 */
+	asm volatile(".byte 0x0f,0x01,0xee\n\t"
+		     : "=a" (pkru), "=d" (edx)
+		     : "c" (ecx));
+	return pkru;
+}
+#else
+static inline u32 __read_pkru(void)
+{
+	return 0;
+}
+#endif
+
 static inline void native_wbinvd(void)
 {
 	asm volatile("wbinvd": : :"memory");