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Subject[PATCH 4.4 001/113] x86/vdso: Use RDPID in preference to LSL when available
Date
From: Andy Lutomirski <luto@kernel.org>

commit a582c540ac1b10f0a7d37415e04c4af42409fd08 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/x86/include/asm/cpufeatures.h | 1 +
arch/x86/include/asm/vgtod.h | 7 ++++++-
2 files changed, 7 insertions(+), 1 deletion(-)

--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -301,6 +301,7 @@
/* 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 */
--- a/arch/x86/include/asm/vgtod.h
+++ b/arch/x86/include/asm/vgtod.h
@@ -83,8 +83,13 @@ 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;
}

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