kvm: testsuite: improve printf() performance

a string I/O instruction is much faster than byte-at-a-time.

Signed-off-by: Avi Kivity <avi@qumranet.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/user/test/x86/lib/printf.c b/user/test/x86/lib/printf.c
index f13bf0cc..5caec246 100644
--- a/user/test/x86/lib/printf.c
+++ b/user/test/x86/lib/printf.c
@@ -1,6 +1,7 @@
 #include "printf.h"
 #include "smp.h"
 #include <stdarg.h>
+#include "string.h"
 
 static struct spinlock lock;
 
@@ -149,8 +150,9 @@ int snprintf(char *buf, int size, const char *fmt, ...)
 
 void print_serial(const char *buf)
 {
-    while (*buf)
-	asm volatile ("out %%al, $0xf1" : : "a"(*buf++));
+    unsigned long len = strlen(buf);
+
+    asm volatile ("rep/outsb" : "+S"(buf), "+c"(len) : "d"(0xf1));
 }
 
 int printf(const char *fmt, ...)