diff --git a/Makefile.target b/Makefile.target
index f9a9da7..61bd0cb 100644
--- a/Makefile.target
+++ b/Makefile.target
@@ -91,6 +91,7 @@ all: $(PROGS) stap
 
 #########################################################
 # cpu emulator library
+obj-y += external_sim.o
 obj-y += exec.o
 obj-y += accel/
 obj-$(CONFIG_TCG) += tcg/tcg.o tcg/tcg-op.o tcg/optimize.o
diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index f290f48..f42546a 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -40,6 +40,7 @@
 #include "hw/irq.h"
 
 #include "hw/boards.h"
+#include "external_sim.h"
 
 /* This check must be after config-host.h is included */
 #ifdef CONFIG_EVENTFD
@@ -1863,6 +1864,15 @@ int kvm_cpu_exec(CPUState *cpu)
     do {
         MemTxAttrs attrs;
 
+        if (external_sim_enabled()) {
+            /* Pause here while synchronizing with a simulation clock.
+             * We do not want to execute instructions past the synchronization
+             * deadline, but it is ok to update the states of other equipment
+             * like timers, i/o devices, etc.
+             */
+            external_sim_sync();
+        }
+
         if (cpu->vcpu_dirty) {
             kvm_arch_put_registers(cpu, KVM_PUT_RUNTIME_STATE);
             cpu->vcpu_dirty = false;
diff --git a/cpus.c b/cpus.c
index 114c29b..877e65c 100644
--- a/cpus.c
+++ b/cpus.c
@@ -2060,3 +2060,11 @@ void dump_drift_info(FILE *f, fprintf_function cpu_fprintf)
         cpu_fprintf(f, "Max guest advance   NA\n");
     }
 }
+
+void kick_all_vcpus(void)
+{
+    CPUState *cpu;
+    CPU_FOREACH(cpu) {
+        qemu_cpu_kick(cpu);
+    }
+}
diff --git a/docs/external-sim.txt b/docs/external-sim.txt
new file mode 100644
index 0000000..c5953fe
--- /dev/null
+++ b/docs/external-sim.txt
@@ -0,0 +1,59 @@
+= Synchronizing the virtual clock with an external source =
+
+QEMU has a protocol for synchronizing its virtual clock
+with the clock of a simulator in which QEMU is embedded
+as a component. This options is enabled with the -external_sim
+argument, and it should generally be accompanied by the
+following additional command line arguments:
+
+-icount 1,sleep=off -rtc clock=vm
+  or
+-enable-kvm -cpu kvm=off,-tsc,-kvmclock -rtc clock=vm
+
+The -external_sim argument is used to supply a file descriptor
+for a Unix socket, which is used for synchronization.
+The procedure for launching QEMU in is synchronization
+mode has three steps:
+
+(1) Create a socket pair with the Linux socketpair function.
+    The code segment that does this might look like
+
+       int socks[2];
+       socketpair(AF_UNIX,SOCK_STREAM,0,socks);
+
+(2) Fork QEMU with the appropriate command line arguments.
+    The -external_sim part of the argument will look something like
+
+       -external_sim sock=socks[1]
+
+(3) After forking QEMU, close sock[1] and retain the
+    sock[0] for communicating with QEMU.
+
+The synchronization protocol is very simple. To start, the
+external simulator writes an integer to its socket with
+the amount of time in microseconds that QEMU is allowed to
+advance. The code segment that does this might look like:
+
+    uint32_t ta = htonl(1000); // Advance by 1 millisecond
+    write(sock[0],&ta,sizeof(uint32_t));
+
+The external simulator can then advance its clock by this
+same amount. During this time, QEMU and the external simulator
+will be executing in parallel. When the external simulator
+completes its time advance, it waits for QEMU by reading from
+its socket. The value read will be the actual number of
+virtual microseconds by which QEMU has advanced its virtual clock.
+This will be greater than or equal to the requested advance.
+The code that does this might look like:
+
+   uint32_t ta;
+   read(fd,&ta,sizeof(uint32_t));
+   ta = ntohl(ta);
+
+These steps are repeated until either (1) the external simulator
+closes its socket thereby causing QEMU to terminate or (2) QEMU
+stops executing (e.g., if the emulated computer is shutdown) and
+causes a read or write error on the simulator's socket.
+
+You can find an example of a simulator using this protocol in
+the adevs simulation package at http://sourceforge.net/projects/adevs/
diff --git a/external_sim.c b/external_sim.c
new file mode 100644
index 0000000..e6ab3d5
--- /dev/null
+++ b/external_sim.c
@@ -0,0 +1,157 @@
+#include "qemu/osdep.h"
+#include "qemu/timer.h"
+#include "sysemu/cpus.h"
+#include "sysemu/kvm.h"
+#include "external_sim.h"
+
+/* This is a Linux only feature */
+
+#ifndef _WIN32
+
+static bool enabled = false, syncing = true;
+static unsigned elapsed; /* initialized to zero */
+static int time_advance = -1;
+static int fd = -1;
+static int64_t t;
+static QEMUTimer *sync_timer;
+static QemuMutex external_sim_mutex;
+static QemuCond external_sim_cond;
+
+bool external_sim_enabled(void)
+{
+    return enabled;
+}
+
+void external_sim_sync(void)
+{
+    /* kvm-all.c will call this function before running
+     * instructions with kvm. Because syncing will be
+     * true while external_sim is waiting for a new time advance
+     * from the simulation, no instructions will execute
+     * while the machine is supposed to be suspended in
+     * simulation time.
+     */
+    qemu_mutex_lock(&external_sim_mutex);
+    while (syncing) {
+        qemu_cond_wait(&external_sim_cond, &external_sim_mutex);
+    }
+    qemu_mutex_unlock(&external_sim_mutex);
+}
+
+static void cleanup_and_exit(void)
+{
+    /* Close the socket and quit */
+    close(fd);
+    exit(0);
+}
+
+static void start_emulator(void)
+{
+    if (kvm_enabled()) {
+        /* Setting syncing to false tells kvm-all that
+         * it can execute guest instructions.
+         */
+        qemu_mutex_lock(&external_sim_mutex);
+        syncing = false;
+        qemu_mutex_unlock(&external_sim_mutex);
+        qemu_cond_signal(&external_sim_cond);
+        /* Restart the emulator clock */
+        cpu_enable_ticks();
+    }
+}
+
+static void stop_emulator(void)
+{
+    if (kvm_enabled()) {
+        /* Tell the emulator that it is not allowed to
+         * execute guest instructions.
+         */
+        qemu_mutex_lock(&external_sim_mutex);
+        syncing = true;
+        qemu_mutex_unlock(&external_sim_mutex);
+        /* Kick KVM off of the CPU and stop the emulator clock. */
+        cpu_disable_ticks();
+        kick_all_vcpus();
+    }
+}
+
+static void write_mem_value(unsigned val)
+{
+    uint32_t msg = htonl(val);
+    if (write(fd, &msg, sizeof(uint32_t)) != sizeof(uint32_t)) {
+        /* If the socket is no good, then assume this is an
+         * indication that we should exit.
+         */
+        cleanup_and_exit();
+    }
+}
+
+static unsigned read_mem_value(void)
+{
+    uint32_t msg;
+    if (read(fd, &msg, sizeof(uint32_t)) != sizeof(uint32_t)) {
+        /* If the socket is no good, then assume this is an
+         * indication that we should exit.
+         */
+        cleanup_and_exit();
+    }
+    return ntohl(msg);
+}
+
+static void schedule_next_event(void)
+{
+    /* Read time advance from the socket */
+    time_advance = read_mem_value();
+    assert(t == 0 ||
+        abs(t - qemu_clock_get_us(QEMU_CLOCK_VIRTUAL)) <= time_advance);
+    /* Schedule the next synchronization point */
+    timer_mod(sync_timer, t + time_advance);
+    /* Note that we need to read the time advance again on the next pass */
+    time_advance = -1;
+    /* Start advancing cpu ticks and the wall clock */
+    start_emulator();
+}
+
+static void sync_func(void *data)
+{
+    /* Stop advancing cpu ticks and the wall clock */
+    stop_emulator();
+    /* Report the actual elapsed time to the external simulator. */
+    int64_t tnow = qemu_clock_get_us(QEMU_CLOCK_VIRTUAL);
+    elapsed = tnow - t;
+    write_mem_value(elapsed);
+    /* Update our time of last event */
+    t = tnow;
+    /* Schedule the next event */
+    schedule_next_event();
+}
+
+void setup_external_sim(QemuOpts *opts)
+{
+    /* The module has been enabled */
+    enabled = true;
+    if (kvm_enabled()) {
+        qemu_mutex_init(&external_sim_mutex);
+        qemu_cond_init(&external_sim_cond);
+    }
+    /* Stop the clock while the simulation is initialized */
+    stop_emulator();
+    /* Initialize the simulation clock */
+    t = 0;
+    /* Get the communication socket */
+    fd = qemu_opt_get_number(opts, "sock", 0);
+    /* Start the timer to ensure time warps advance the clock */
+    sync_timer = timer_new_us(QEMU_CLOCK_VIRTUAL, sync_func, NULL);
+    /* Get the time advance that is requested by the simulation */
+    schedule_next_event();
+}
+
+#else
+
+void setup_external_sim(QemuOpts *opts)
+{
+    fprintf(stderr, "-external_sim is not supported on Windows, exiting\n");
+    exit(0);
+}
+
+#endif
diff --git a/external_sim.h b/external_sim.h
new file mode 100644
index 0000000..38ee348
--- /dev/null
+++ b/external_sim.h
@@ -0,0 +1,36 @@
+/*
+ * This work is licensed under the terms of the GNU GPL
+ * version 2. Seethe COPYING file in the top-level directory.
+ *
+ * A module for pacing the rate of advance of the computer
+ * clock in reference to an external simulation clock. The
+ * basic approach used here is adapted from QBox from Green
+ * Socs. The mode of operation is as follows:
+ *
+ * The simulator uses pipes to exchange time advance data.
+ * The external simulator starts the exchange by forking a
+ * QEMU process and passing is descriptors for a read and
+ * write pipe. Then the external simulator writes an integer
+ * (native endian) to the pipe to indicate the number of
+ * microseconds that QEMU should advance. QEMU advances its
+ * virtual clock by this amount and writes to its write pipe
+ * the actual number of microseconds that have advanced.
+ * This process continues until a pipe on either side is
+ * closed, which will either cause QEMU to exit (if the
+ * external simulator closes a pipe) or raise SIGPIPE in the
+ * external simulator (if QEMU closes a pipe).
+ *
+ * Authors:
+ *   James Nutaro <nutaro@gmail.com>
+ *
+ */
+#ifndef EXTERNAL_SIM_H
+#define EXTERNAL_SIM_H
+
+#include "qemu-options.h"
+
+void external_sim_sync(void);
+bool external_sim_enabled(void);
+void setup_external_sim(QemuOpts *opts);
+
+#endif
diff --git a/include/sysemu/cpus.h b/include/sysemu/cpus.h
index 731756d..7fde006 100644
--- a/include/sysemu/cpus.h
+++ b/include/sysemu/cpus.h
@@ -10,6 +10,7 @@ void resume_all_vcpus(void);
 void pause_all_vcpus(void);
 void cpu_stop_current(void);
 void cpu_ticks_init(void);
+void kick_all_vcpus(void);
 
 void configure_icount(QemuOpts *opts, Error **errp);
 extern int use_icount;
diff --git a/qemu-options.hx b/qemu-options.hx
index 32d9378..6321115 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -4479,6 +4479,21 @@ contents of @code{iv.b64} to the second secret
 
 ETEXI
 
+DEF("external_sim", HAS_ARG, QEMU_OPTION_external_sim, \
+    "-external_sim sock=fd\n" \
+    "                enable synchronization of the virtual clock \n" \
+    "                with an external simulation clock\n", QEMU_ARCH_ALL)
+STEXI
+@item -external_sim sock=@var{fd0}
+@findex -external_sim
+Qemu will use the supplied socket to synchronize its virtual clock with
+an external simulation clock. Qemu will wait until a time slice size in
+microseconds is supplied on the socket. Then it will execute for at
+least that number of virtual microseconds before writing the actual
+virtual time that has elapsed in microseconds to the socket. This
+cycle will repeat until a zero time advance is requested, which
+will cause qemu to exit.
+ETEXI
 
 HXCOMM This is the last statement. Insert new options before this line!
 STEXI
diff --git a/vl.c b/vl.c
index e9012bb..4746c17 100644
--- a/vl.c
+++ b/vl.c
@@ -128,6 +128,8 @@ int main(int argc, char **argv)
 #include "qapi/qmp/qerror.h"
 #include "sysemu/iothread.h"
 
+#include "external_sim.h"
+
 #define MAX_VIRTIO_CONSOLES 1
 #define MAX_SCLP_CONSOLES 1
 
@@ -240,6 +242,20 @@ static struct {
     { .driver = "virtio-vga",           .flag = &default_vga       },
 };
 
+static QemuOptsList qemu_external_sim_opts = {
+    .name = "external_sim",
+    .implied_opt_name = "",
+    .merge_lists = true,
+    .head = QTAILQ_HEAD_INITIALIZER(qemu_external_sim_opts.head),
+    .desc = {
+        {
+            .name = "sock",
+            .type = QEMU_OPT_NUMBER,
+        },
+        { /* end of list */ }
+    },
+};
+
 static QemuOptsList qemu_rtc_opts = {
     .name = "rtc",
     .head = QTAILQ_HEAD_INITIALIZER(qemu_rtc_opts.head),
@@ -823,7 +839,7 @@ int qemu_timedate_diff(struct tm *tm)
             struct tm tmp = *tm;
             tmp.tm_isdst = -1; /* use timezone to figure it out */
             seconds = mktime(&tmp);
-	}
+    }
     else
         seconds = mktimegm(tm) + rtc_date_offset;
 
@@ -3055,6 +3071,7 @@ int main(int argc, char **argv, char **envp)
     int cyls, heads, secs, translation;
     QemuOpts *opts, *machine_opts;
     QemuOpts *hda_opts = NULL, *icount_opts = NULL, *accel_opts = NULL;
+    QemuOpts *external_sim_opts = NULL;
     QemuOptsList *olist;
     int optind;
     const char *optarg;
@@ -3101,6 +3118,7 @@ int main(int argc, char **argv, char **envp)
     module_call_init(MODULE_INIT_QOM);
     monitor_init_qmp_commands();
 
+    qemu_add_opts(&qemu_external_sim_opts);
     qemu_add_opts(&qemu_drive_opts);
     qemu_add_drive_opts(&qemu_legacy_drive_opts);
     qemu_add_drive_opts(&qemu_common_drive_opts);
@@ -4007,6 +4025,14 @@ int main(int argc, char **argv, char **envp)
                     exit(1);
                 }
                 break;
+            case QEMU_OPTION_external_sim:
+                external_sim_opts =
+                    qemu_opts_parse_noisily(
+                        qemu_find_opts("external_sim"),optarg,true);
+                if (!external_sim_opts) {
+                    exit(1);
+                }
+                break;
             case QEMU_OPTION_incoming:
                 if (!incoming) {
                     runstate_set(RUN_STATE_INMIGRATE);
@@ -4560,6 +4586,16 @@ int main(int argc, char **argv, char **envp)
     /* spice needs the timers to be initialized by this point */
     qemu_spice_init();
 
+    if (external_sim_opts) {
+        if (!(rtc_clock == QEMU_CLOCK_VIRTUAL && (
+              (icount_opts && !qemu_opt_get_bool(icount_opts, "sleep", true)) ||
+              kvm_enabled()))) {
+            error_report("-external_sim requires -rtc clock=vm and either "
+                "icount -1,sleep=off or -enable-kvm");
+            exit(1);
+        }
+        setup_external_sim(external_sim_opts);
+    }
     cpu_ticks_init();
     if (icount_opts) {
         if (!tcg_enabled()) {
