[PATCH 1/2] x86/vdso: Use RDPID in preference to LSL when available

[Andy Lutomirski]

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@xxxxxxxxx>
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---
 arch/x86/include/asm/cpufeatures.h | 1 +
 arch/x86/include/asm/vgtod.h       | 7 ++++++-
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index a39629206864..3068edf43abe 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -281,6 +281,7 @@
 /* Intel-defined CPU features, CPUID level 0x00000007:0 (ecx), word 16 */
 #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 */
diff --git a/arch/x86/include/asm/vgtod.h b/arch/x86/include/asm/vgtod.h
index e728699db774..3a01996db58f 100644
--- a/arch/x86/include/asm/vgtod.h
+++ b/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;
 }
-- 
2.7.4