diff --git a/arch/x86/tools/mcreboot-smp-x86.sh.in b/arch/x86/tools/mcreboot-smp-x86.sh.in
index 786254e2..24861333 100644
--- a/arch/x86/tools/mcreboot-smp-x86.sh.in
+++ b/arch/x86/tools/mcreboot-smp-x86.sh.in
@@ -18,6 +18,42 @@ SBINDIR="@SBINDIR@"
 KMODDIR="@KMODDIR@"
 KERNDIR="@KERNDIR@"
 
+INTERVAL=1
+LOGMODE=0
+while getopts :i:k: OPT
+do
+	case ${OPT} in
+	i)	INTERVAL=${OPTARG}
+		expr "${INTERVAL}" + 1 > /dev/null 2>&1
+		if [ $? -ge 2 ]
+		then
+			echo "invalid -i value"
+			exit 1
+		fi
+		if [ ${INTERVAL} -le 0 ]
+		then
+			echo "invalid -i value"
+			exit 1
+		fi
+		;;
+	k)	LOGMODE=${OPTARG}
+		expr "${LOGMODE}" + 1 > /dev/null 2>&1
+		if [ $? -ge 2 ]
+		then
+			echo "invalid -k value"
+			exit 1
+		fi
+		if [ ${LOGMODE} -lt 0 -o ${LOGMODE} -gt 2 ]
+		then
+			echo "invalid -k value"
+			exit 1
+		fi
+		;;
+	*)  echo "invalid option -${OPT}"
+		exit 1
+	esac
+done
+
 mem="512M@0"
 cpus=""
 ihk_ikc_irq_core=0
@@ -105,7 +141,7 @@ if ! ${SBINDIR}/ihkconfig 0 create; then echo "error: create"; exit; fi
 if ! ${SBINDIR}/ihkosctl 0 assign cpu ${cpus}; then echo "error: assign CPUs"; exit; fi
 if ! ${SBINDIR}/ihkosctl 0 assign mem ${mem}; then echo "error: assign memory"; exit; fi
 if ! ${SBINDIR}/ihkosctl 0 load ${KERNDIR}/mckernel.img; then echo "error: loading kernel image"; exit; fi
-if ! ${SBINDIR}/ihkosctl 0 kargs hidos; then echo "error: setting kernel arguments"; exit; fi
+if ! ${SBINDIR}/ihkosctl 0 kargs "hidos ksyslogd=${LOGMODE}"; then echo "error: setting kernel arguments"; exit; fi
 if ! ${SBINDIR}/ihkosctl 0 boot; then echo "error: booting"; exit; fi
 if ! insmod ${KMODDIR}/mcctrl.ko; then echo "error: inserting mcctrl.ko"; exit; fi
 if ! chown `logname` /dev/mcd* /dev/mcos*; then echo "error: chowning device files"; exit; fi
@@ -124,4 +160,7 @@ if [ "$enable_mcoverlay" != "" ]; then
 	if ! mount -t mcoverlay mcoverlay -o lowerdir=/proc/mcos0:/proc,upperdir=/tmp/mcos/mcos0_proc_upper,workdir=/tmp/mcos/mcos0_proc_work,nocopyupw,nofscheck /tmp/mcos/mcos0_proc; then echo "error: mount /tmp/mcos/mcos0_proc"; exit; fi
 	mount --make-rprivate /proc
 fi
-
+if [ ${LOGMODE} -ne 0 ]
+then
+	SBINDIR=${SBINDIR} ${SBINDIR}/mcklogd -i ${INTERVAL}
+fi
diff --git a/kernel/debug.c b/kernel/debug.c
index a58aff0d..6c9a8214 100644
--- a/kernel/debug.c
+++ b/kernel/debug.c
@@ -22,13 +22,44 @@ extern int vsnprintf(char *buf, size_t size, const char *fmt, va_list args);
 extern int sprintf(char * buf, const char *fmt, ...);
 static ihk_spinlock_t kmsg_lock;
 
+static unsigned long kprintf_lock_head(void);
+static void kprintf_unlock_head(unsigned long irqflags);
+
+static void kprintf_wait(int len, unsigned long *flags_head, int *slide) {
+	int head, tail, buf_len, mode, adj;
+
+	mode = kmsg_buf.mode;
+	while (1) {
+		adj = 0;
+		tail = kmsg_buf.tail;
+		buf_len = kmsg_buf.len;
+		head = kmsg_buf.head;
+		if (head < tail) head += buf_len;
+		if (tail + len > buf_len) adj = buf_len - tail;
+		if (head > tail && head <= tail + len + adj) {
+			if (mode != 1) {
+				*slide = 1;
+				break;
+			} else {
+				kprintf_unlock_head(*flags_head);
+				*flags_head = kprintf_lock_head();
+			}
+		} else {
+			break;
+		}
+	}
+}
+
 /* TODO: lock */
 void kputs(char *buf)
 {
 	int len = strlen(buf);
-	unsigned long flags;
+	int slide = 0;
+	unsigned long flags_tail, flags_head;
 
-	flags = __ihk_mc_spinlock_lock(&kmsg_lock);
+	flags_tail = kprintf_lock();
+	flags_head = kprintf_lock_head();
+	kprintf_wait(len, &flags_head, &slide);
 
 	if (len + kmsg_buf.tail > kmsg_buf.len) {
 		kmsg_buf.tail = 0;
@@ -39,8 +70,12 @@ void kputs(char *buf)
 	
 	memcpy(kmsg_buf.str + kmsg_buf.tail, buf, len);
 	kmsg_buf.tail += len;
-
-	__ihk_mc_spinlock_unlock(&kmsg_lock, flags);
+	if (slide == 1) {
+		kmsg_buf.head = kmsg_buf.tail + 1;
+		if (kmsg_buf.head >= kmsg_buf.len) kmsg_buf.head = 0;
+	}
+	kprintf_unlock_head(flags_head);
+	kprintf_unlock(flags_tail);
 }
 
 #define KPRINTF_LOCAL_BUF_LEN 1024
@@ -55,11 +90,23 @@ void kprintf_unlock(unsigned long irqflags)
 	__ihk_mc_spinlock_unlock(&kmsg_lock, irqflags);
 }
 
+static unsigned long kprintf_lock_head(void)
+{
+	return __ihk_mc_spinlock_lock(&kmsg_buf.lock);
+}
+
+static void kprintf_unlock_head(unsigned long irqflags)
+{
+	__ihk_mc_spinlock_unlock(&kmsg_buf.lock, irqflags);
+}
+
 /* Caller must hold kmsg_lock! */
 int __kprintf(const char *format, ...)
 {
 	int len = 0;
+	int slide = 0;
 	va_list va;
+	unsigned long flags_head;
 	char buf[KPRINTF_LOCAL_BUF_LEN];
 
 	/* Copy into the local buf */
@@ -67,6 +114,9 @@ int __kprintf(const char *format, ...)
 	len += vsnprintf(buf + len, KPRINTF_LOCAL_BUF_LEN - len - 2, format, va);
 	va_end(va);
 
+	flags_head = kprintf_lock_head();
+	kprintf_wait(len, &flags_head, &slide);
+
 	/* Append to kmsg buffer */
 	if (kmsg_buf.tail + len > kmsg_buf.len) {
 		kmsg_buf.tail = 0;
@@ -74,25 +124,33 @@ int __kprintf(const char *format, ...)
 
 	memcpy(kmsg_buf.str + kmsg_buf.tail, buf, len);
 	kmsg_buf.tail += len;
+	if (slide == 1) {
+		kmsg_buf.head = kmsg_buf.tail + 1;
+		if (kmsg_buf.head >= kmsg_buf.len) kmsg_buf.head = 0;
+	}
 
+	kprintf_unlock_head(flags_head);
 	return len;
 }
 
 int kprintf(const char *format, ...)
 {
 	int len = 0;
+	int slide = 0;
 	va_list va;
-	unsigned long flags;
+	unsigned long flags_tail, flags_head;
 	char buf[KPRINTF_LOCAL_BUF_LEN];
 
-	flags = __ihk_mc_spinlock_lock(&kmsg_lock);
-
 	/* Copy into the local buf */
 	len = sprintf(buf, "[%3d]: ", ihk_mc_get_processor_id());
 	va_start(va, format);
 	len += vsnprintf(buf + len, KPRINTF_LOCAL_BUF_LEN - len - 2, format, va);
 	va_end(va);
 
+	flags_tail = kprintf_lock();
+	flags_head = kprintf_lock_head();
+	kprintf_wait(len, &flags_head, &slide);
+
 	/* Append to kmsg buffer */
 	if (kmsg_buf.tail + len > kmsg_buf.len) {
 		kmsg_buf.tail = 0;
@@ -100,16 +158,24 @@ int kprintf(const char *format, ...)
 
 	memcpy(kmsg_buf.str + kmsg_buf.tail, buf, len);
 	kmsg_buf.tail += len;
+	if (slide == 1) {
+		kmsg_buf.head = kmsg_buf.tail + 1;
+		if (kmsg_buf.head >= kmsg_buf.len) kmsg_buf.head = 0;
+	}
 
-	__ihk_mc_spinlock_unlock(&kmsg_lock, flags);
+	kprintf_unlock_head(flags_head);
+	kprintf_unlock(flags_tail);
 
 	return len;
 }
 
-void kmsg_init(void)
+void kmsg_init(int mode)
 {
 	ihk_mc_spinlock_init(&kmsg_lock);
 	kmsg_buf.tail = 0;
 	kmsg_buf.len = sizeof(kmsg_buf.str);
+	kmsg_buf.head = 0;
+	kmsg_buf.mode = mode;
+	ihk_mc_spinlock_init(&kmsg_buf.lock);
 	memset(kmsg_buf.str, 0, kmsg_buf.len);
 }
diff --git a/kernel/include/init.h b/kernel/include/init.h
index 31717f16..c36fd3c9 100644
--- a/kernel/include/init.h
+++ b/kernel/include/init.h
@@ -14,7 +14,7 @@
 #define INIT_H
 
 extern void arch_init(void);
-extern void kmsg_init(void);
+extern void kmsg_init(int);
 extern void mem_init(void);
 extern void ikc_master_init(void);
 extern void ap_init(void);
diff --git a/kernel/include/kmsg.h b/kernel/include/kmsg.h
index db6da37f..75e82a0e 100644
--- a/kernel/include/kmsg.h
+++ b/kernel/include/kmsg.h
@@ -16,6 +16,6 @@
 void kputs(char *buf);
 int kprintf(const char *format, ...);
 
-void kmsg_init(void);
+void kmsg_init(int);
 
 #endif
diff --git a/kernel/init.c b/kernel/init.c
index 1e651c0b..3313459c 100644
--- a/kernel/init.c
+++ b/kernel/init.c
@@ -298,7 +298,15 @@ extern void ibmic_cmd_init(void);
 
 int main(void)
 {
-	kmsg_init();
+	char *ptr;
+	int mode = 0;
+
+	ptr = find_command_line("ksyslogd=");
+	if (ptr) {
+	    mode = ptr[9] - 0x30;
+	    if (mode < 0 || mode > 2) mode = 0;
+	}
+	kmsg_init(mode);
 
 	kputs("MCK started.\n");
 
diff --git a/lib/include/ihk/debug.h b/lib/include/ihk/debug.h
index f7c09b57..6b64ccc7 100644
--- a/lib/include/ihk/debug.h
+++ b/lib/include/ihk/debug.h
@@ -13,12 +13,16 @@
 #ifndef IHK_DEBUG_H
 #define IHK_DEBUG_H
 
+#include <arch-lock.h>
 #include <ihk/memconst.h>
 
 struct ihk_kmsg_buf {
 	int tail;
 	int len;
-	char str[IHK_KMSG_SIZE - sizeof(int) * 2];
+	int head;
+	int mode;
+	ihk_spinlock_t lock;
+	char str[IHK_KMSG_SIZE - sizeof(int) * 4 - sizeof(ihk_spinlock_t)];
 };
 
 extern int kprintf(const char *format, ...);