Add a consistent API to a feature that most modern CPUs have; a fast

counter register in-CPU.

This is to be used as a fast "timer", where linearity is more important
than time, and multiple lines in the linearity caused by multiple CPUs
in an SMP machine is not a problem.

This adds no code whatsoever to the FreeBSD kernel until it is actually
used, and then as a single-instruction inline routine (except for the
80386 and 80486 where it is some more inline code around nanotime(9).

Reviewed by:	bde, kris, jhb

1 files changed, 21 insertions, 0 deletions

diff --git a/sys/amd64/include/cpu.h b/sys/amd64/include/cpu.h
index 7b090b6..669af64 100644
--- a/sys/amd64/include/cpu.h
+++ b/sys/amd64/include/cpu.h
@@ -47,6 +47,8 @@
 #include <machine/frame.h>
 #include <machine/segments.h>
 #include <machine/globals.h>
+#include <machine/md_var.h>
+#include <machine/specialreg.h>
 
 /*
  * definitions of cpu-dependent requirements
@@ -133,6 +135,25 @@ extern char	etext[];
 
 void	fork_trampoline __P((void));
 void	fork_return __P((struct proc *, struct trapframe));
+
+/*
+ * Return contents of in-cpu fast counter as a sort of "bogo-time"
+ * for non-critical timing.
+ */
+static __inline u_int64_t
+get_cyclecount(void)
+{
+#if defined(I386_CPU) || defined(I486_CPU)
+	struct timespec tv;
+
+	if ((cpu_feature & CPUID_TSC) == 0) {
+		nanotime(&tv);
+		return (tv.tv_sec * (u_int64_t)1000000000 + tv.tv_nsec);
+	}
+#endif
+	return (rdtsc());
+}
+
 #endif
 
 #endif /* !_MACHINE_CPU_H_ */