detect rdtscp instruction at runtime

.gitignore

@@ -1,2 +1,3 @@
 meltdown
 *.o
+rdtscp.h

Makefile

@@ -3,7 +3,12 @@ CFLAGS += -O2 -msse2
 
 all: meltdown
 
+meltdown.o: rdtscp.h
+
 meltdown: meltdown.o
 
+rdtscp.h: detect_rdtscp.sh
+	./detect_rdtscp.sh >$@
+
 clean:
-	rm -f meltdown.o meltdown
+	rm -f meltdown.o meltdown rdtscp.h

detect_rdtscp.sh

@@ -0,0 +1,28 @@
+#!/bin/sh
+
+cat <<-'EOF'
+	static inline int
+	get_access_time(volatile char *addr)
+	{
+		unsigned long long time1, time2;
+EOF
+
+if grep -q rdtscp /proc/cpuinfo; then
+	cat <<-'EOF'
+		unsigned junk;
+		time1 = __rdtscp(&junk);
+		(void)*addr;
+		time2 = __rdtscp(&junk);
+	EOF
+else
+	cat <<-'EOF'
+		time1 = __rdtsc();
+		(void)*addr;
+		_mm_mfence();
+		time2 = __rdtsc();
+	EOF
+fi
+cat <<-'EOF'
+		return time2 - time1;
+	}
+EOF

meltdown.c

@@ -10,12 +10,10 @@
 
 #include <x86intrin.h>
 
+#include "rdtscp.h"
+
 //#define DEBUG 1
 
-/* comment out if getting illegal insctructions error */
-#ifndef HAVE_RDTSCP
-# define HAVE_RDTSCP 1
-#endif
 
 #if !(defined(__x86_64__) || defined(__i386__))
 # error "Only x86-64 and i386 are supported at the moment"
@@ -86,25 +84,6 @@ speculate(unsigned long addr)
 #endif
 }
 
-static inline int
-get_access_time(volatile char *addr)
-{
-	int time1, time2, junk;
-	volatile int j;
-
-#if HAVE_RDTSCP
-	time1 = __rdtscp(&junk);
-	j = *addr;
-	time2 = __rdtscp(&junk);
-#else
-	time1 = __rdtsc();
-	j = *addr;
-	_mm_mfence();
-	time2 = __rdtsc();
-#endif
-
-	return time2 - time1;
-}
 
 static int cache_hit_threshold;
 static int hist[VARIANTS_READ];