This patch adds support for slowing down qemu and saving cpu load.
The '-brake' command line parameter or monitor command 'brake'
determines how long to wait in the main_loop with respect to duration of
time that qemu used for the emulation itself.
Thus e.g. zero value runs at full speed, 1 at 1/2 speed, 10 at 1/11
speed, etc.
A drawback of this approach is that the timer interrupt can be missed if
raised too often. This happens if the loop_main divided by (1+brake) is
less than the frequency of PIC interrupts.
In such case the time of emulated system runs slowly.
This problem occurs mainly for Linux emulation where kernel parameter HZ
is of several hunderds.
For DOS emulation, where the timer interrupts are raised usualy at 18Hz,
this approach works nice, and one can play old DOS games at reasonable
speed and save batteries.
Mirek
diff -aur ../../orig/qemu-0.8.2/monitor.c ./monitor.c
--- ../../orig/qemu-0.8.2/monitor.c 2006-07-22 19:23:34.0 +0200
+++ ./monitor.c 2006-11-27 18:11:34.0 +0100
@@ -276,6 +276,14 @@
term_printf("Invalid CPU index\n");
}
+static void do_brake_set(int multiplier)
+{
+if (multiplier < 0)
+term_printf("Current brake multiplier %d\n", brake_mult);
+else
+brake_mult = multiplier;
+}
+
static void do_info_jit(void)
{
dump_exec_info(NULL, monitor_fprintf);
@@ -1198,6 +1206,8 @@
#endif
{ "stopcapture", "i", do_stop_capture,
"capture index", "stop capture" },
+{ "brake", "i", do_brake_set,
+ "multiplier", "set brake multiplier (non-negative integer), negative value prints actual setting" },
{ NULL, NULL, },
};
diff -aur ../../orig/qemu-0.8.2/vl.c ./vl.c
--- ../../orig/qemu-0.8.2/vl.c 2006-07-22 19:23:34.0 +0200
+++ ./vl.c 2006-11-27 18:25:22.0 +0100
@@ -156,6 +156,7 @@
#endif
int acpi_enabled = 1;
int fd_bootchk = 1;
+int brake_mult = 0;
/***/
/* x86 ISA bus support */
@@ -5045,8 +5046,39 @@
static CPUState *cur_cpu;
+
+int timeval_subtract(struct timeval *result, struct timeval *x, struct timeval *y) {
+ /* Perform the carry for the later subtraction by updating y. */
+ if (x->tv_usec < y->tv_usec) {
+ int nsec = (y->tv_usec - x->tv_usec) / 100 + 1;
+ y->tv_usec -= 100 * nsec;
+ y->tv_sec += nsec;
+ }
+ if (x->tv_usec - y->tv_usec > 100) {
+ int nsec = (x->tv_usec - y->tv_usec) / 100;
+ y->tv_usec += 100 * nsec;
+ y->tv_sec -= nsec;
+ }
+
+ /* Compute the time remaining to wait.
+ * tv_usec is certainly positive. */
+ result->tv_sec = x->tv_sec - y->tv_sec;
+ result->tv_usec = x->tv_usec - y->tv_usec;
+
+ /* Return 1 if result is negative. */
+ return x->tv_sec < y->tv_sec;
+}
+
+
int main_loop(void)
{
+struct timeval tti, tti2, tti3, ttix;
+//#define BRAKE_DEBUG
+#ifdef BRAKE_DEBUG
+int64_t mti, mti1, mti2, mti3;
+int prcnt=0;
+#endif
+
int ret, timeout;
#ifdef CONFIG_PROFILER
int64_t ti;
@@ -5055,6 +5087,11 @@
cur_cpu = first_cpu;
for(;;) {
+
+gettimeofday(&tti, NULL);
+#ifdef BRAKE_DEBUG
+mti = profile_getclock();
+#endif
if (vm_running) {
env = cur_cpu;
@@ -5113,6 +5150,41 @@
#ifdef CONFIG_PROFILER
dev_time += profile_getclock() - ti;
#endif
+
+gettimeofday(&tti2, NULL);
+#ifdef BRAKE_DEBUG
+mti2 = profile_getclock();
+#endif
+
+{
+struct timespec ts;
+int delus, delunc=0;
+sigset_t sis, osis;
+
+timeval_subtract(&ttix, &tti2, &tti);
+delus = ttix.tv_usec*brake_mult - delunc;
+ts.tv_sec=0;
+ts.tv_nsec=delus*1000;
+
+sigfillset(&sis);
+sigprocmask(SIG_BLOCK, &sis, &osis);
+if (delus>0) while (nanosleep(&ts, &ts));
+sigprocmask(SIG_SETMASK, &osis, NULL);
+#ifdef BRAKE_DEBUG
+mti3 = profile_getclock();
+#endif
+gettimeofday(&tti3, NULL);
+
+timeval_subtract(&ttix, &tti3, &tti2);
+delunc = ttix.tv_usec-delus;
+#ifdef BRAKE_DEBUG
+if (!((++prcnt)%128)) {
+printf("cycles %qd, \t%qd, \t%qd, \t%qd \%\n", (mti1 - mti), (mti2 - mti1), (mti3 - mti2), (100*(mti1 - mti))/(mti3-mti) );
+//printf("del[us] %d, \t%d\n", delus, delunc);
+//printf("ts[ns] = %d\n", ts.tv_nsec);
+}
+#endif
+}
}
cpu_disable_ticks();
return ret;
@@ -5224,6 +5296,7 @@
#endif
"-loadvm filestart right away with a saved state (loadvm in monitor)\n"
"-vnc displaystart a VNC server on display\n"
+ "-brake multiplierenables brake system which slows down qemu `multiplier'-times\n"
"\n"
"During emulation, the following keys are useful:\n"
"ctrl-alt-f toggle full screen\n"
@@ -5302,6 +5375,7 @@
QEMU_OPTION_smp,
QEMU_OPTION_vnc,
QEMU_OP
