x86/vdso: Use RDPID in preference to LSL when available

RDPID is a new instruction that reads MSR_TSC_AUX quickly.  This
should be considerably faster than reading the GDT.  Add a
cpufeature for it and use it from __vdso_getcpu() when available.

Tested-by: Megha Dey <megha.dey@intel.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/4f6c3a22012d10f1c65b9ca15800e01b42c7d39d.1479320367.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/cpufeatures.h

@@ -283,6 +283,7 @@
 #define X86_FEATURE_AVX512VBMI  (16*32+ 1) /* AVX512 Vector Bit Manipulation instructions*/
 #define X86_FEATURE_PKU		(16*32+ 3) /* Protection Keys for Userspace */
 #define X86_FEATURE_OSPKE	(16*32+ 4) /* OS Protection Keys Enable */
+#define X86_FEATURE_RDPID	(16*32+ 22) /* RDPID instruction */
 
 /* AMD-defined CPU features, CPUID level 0x80000007 (ebx), word 17 */
 #define X86_FEATURE_OVERFLOW_RECOV (17*32+0) /* MCA overflow recovery support */

arch/x86/include/asm/vgtod.h

@@ -89,8 +89,13 @@ static inline unsigned int __getcpu(void)
 	 * works on all CPUs.  This is volatile so that it orders
 	 * correctly wrt barrier() and to keep gcc from cleverly
 	 * hoisting it out of the calling function.
+	 *
+	 * If RDPID is available, use it.
 	 */
-	asm volatile ("lsl %1,%0" : "=r" (p) : "r" (__PER_CPU_SEG));
+	alternative_io ("lsl %[p],%[seg]",
+			".byte 0xf3,0x0f,0xc7,0xf8", /* RDPID %eax/rax */
+			X86_FEATURE_RDPID,
+			[p] "=a" (p), [seg] "r" (__PER_CPU_SEG));
 
 	return p;
 }