diff --git a/sys/amd64/include/cpufunc.h b/sys/amd64/include/cpufunc.h
index 7123e7b8f58a..a0ea1b2f1701 100644
--- a/sys/amd64/include/cpufunc.h
+++ b/sys/amd64/include/cpufunc.h
@@ -391,7 +391,7 @@ rdtscp(void)
 {
 	uint32_t low, high;
 
-	__asm __volatile("rdtscp" : "=a" (low), "=d" (high));
+	__asm __volatile("rdtscp" : "=a" (low), "=d" (high) : : "ecx");
 	return (low | ((uint64_t)high << 32));
 }
 
diff --git a/sys/i386/include/cpufunc.h b/sys/i386/include/cpufunc.h
index 44ba3ff4c4a3..da9a3e3a0869 100644
--- a/sys/i386/include/cpufunc.h
+++ b/sys/i386/include/cpufunc.h
@@ -373,7 +373,7 @@ rdtscp(void)
 {
 	uint64_t rv;
 
-	__asm __volatile("rdtscp" : "=A" (rv));
+	__asm __volatile("rdtscp" : "=A" (rv) : : "ecx");
 	return (rv);
 }