[PATCH] x86/vdso: Use RDPID in preference to LSL when available
From: Andy Lutomirski
Date: Tue Apr 19 2016 - 18:47:50 EST
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.
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
I don't have a Cannonlake CPU (or whatever CPU I'd need for this).
Could someone who has such a beast give this a try?
Boris, could you double-check me? You're a lot more familiar with
CPUID stuff and the instruction tables than I am, and I can't fall
back to a real disassembler because the instruction is too new.
Also, it's time for someone to do UMIP. I'll see if I can convince
someone in KVM land to emulate it to make it easier to test.
arch/x86/include/asm/cpufeatures.h | 1 +
arch/x86/include/asm/special_insns.h | 8 ++++++++
arch/x86/include/asm/vgtod.h | 7 ++++++-
3 files changed, 15 insertions(+), 1 deletion(-)
diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 7bfb6b70c745..beaf2fb601ee 100644
@@ -279,6 +279,7 @@
/* 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 */
* BUG word(s)
diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
index d96d04377765..de64c404dc74 100644
@@ -301,6 +301,14 @@ static inline void pcommit_sfence(void)
#define nop() asm volatile ("nop")
+/* Read MSR_TSC_AUX using RDPID. */
+static inline unsigned int rdpid(void)
+ unsigned int pid;
+ asm (".byte 0xf3,0x0f,0xc7,0xf8" : "=a" (pid)); /* RDPID %eax/rax */
+ return pid;
#endif /* __KERNEL__ */
diff --git a/arch/x86/include/asm/vgtod.h b/arch/x86/include/asm/vgtod.h
index e728699db774..3a01996db58f 100644
@@ -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 */
+ [p] "=a" (p), [seg] "r" (__PER_CPU_SEG));