Re: [Qemu-devel] Timing problems

Re: [Qemu-devel] Timing problems

[Sven Zenker]

Hi all,
as a new qemu user, let me first of all say that qemu, especially in
combination with kqemu is a great piece of software. Many thanks to the
developer(s).
Since I am using a Laptop with a Pentium M CPU with SpeedStep and am
forced to use XP SP2 as a guest OS (Suse 9.3 is host OS), it wouldn't
work reliably for me,  for reasons mentioned previously in this thread
(system services timing out at startup, networking problems etc.).
Really needing a working emulator bad, I came up with the following ugly
hack, which leads to a stable and network enabled XP SP 2 running in
qemu on my SpeedStep machine, with the side effect of having an accurate
real time clock in the guest OS again. It simply replaces the virtual
timer mechanism based on CPU tick count (which is totally messed up in a
SpeedStep setting) with calls to the realtime clock. It should work even
when emulation is stopped intermittently, I hope, since the  built in
"virtual clock stop" mechanism ist left unchanged. 
I modified only the I386 code, so to make this work on other machines,
more modifications would be necessary:
++++++++++++++++++++++++++++++++
qemu-0.7.1 # diff vl.c.org vl.c 
503d502
<
505d503
<
508,510c506
<     int64_t val;
<     asm volatile ("rdtsc" : "=A" (val));
<     return val;
---
>     return get_clock();
512d507
<
594,605c589
<     int64_t usec, ticks;
<
<     usec = get_clock();
<     ticks = cpu_get_real_ticks();
< #ifdef _WIN32
<     Sleep(50);
< #else
<     usleep(50 * 1000);
< #endif
<     usec = get_clock() - usec;
<     ticks = cpu_get_real_ticks() - ticks;
<     ticks_per_sec = (ticks * 1000000LL + (usec >> 1)) / usec;
---
>       ticks_per_sec = 1000000LL;  /* our real time clock resolution */

+++++++++++++++++++++++++++++++++

and 

+++++++++++++++++++++++++++++++++

qemu-0.7.1/linux-user # diff main.c.org main.c
113,115c113
<     int64_t val;
<     asm volatile ("rdtsc" : "=A" (val));
<     return val;
---
>     return get_clock();
 return get_clock();
---
>     return val;

++++++++++++++++++++++++++++++
This is of course an ugly hack and sure to affect performance, but at
least, it makes XP work for me.
Please let me know your thoughts.
Best regards,
Sven Zenker

-- 
Dr. med. Sven Zenker
Research Associate
Center for Inflammation and Regenerative Modeling
Dpt. of Critical Care Medicine
University of Pittsburgh Medical Center

Re: Re: [Qemu-devel] Timing problems

[Mulyadi Santosa]

Hello...

> It simply
> replaces the virtual timer mechanism based on CPU tick count (which
> is totally messed up in a SpeedStep setting) with calls to the
> realtime clock. It should work even when emulation is stopped
> intermittently, I hope, since the  built in "virtual clock stop"
> mechanism ist left unchanged.

Hm..... hard choice.....correctness traded for perfomance.... But 
anyway....IMHO this hack  is needed for every speed-step enabled 
machine. Perhaps...the other workaround is via cpufreqd? I don't have 
any Pentium M based PC/laptop around, so this is just a pure guess

BTW, your patch seems reversed....if you really mean you want to fetch 
realtime clock, you should use "rdtsc", right? But the patch seems 
replaced "rdtsc" with get_clock().... 

Another thing, IMHO it is better to use unified format (diff -u). More 
readable and i think it is a standart

regards

Mulyadi

Re: Re: [Qemu-devel] Timing problems

[Jim C. Brown]

On Mon, Aug 29, 2005 at 10:01:39AM +0700, Mulyadi Santosa wrote:
> Hello...
> 
> > It simply
> > replaces the virtual timer mechanism based on CPU tick count (which
> > is totally messed up in a SpeedStep setting) with calls to the
> > realtime clock. It should work even when emulation is stopped
> > intermittently, I hope, since the  built in "virtual clock stop"
> > mechanism ist left unchanged.
> 
> Hm..... hard choice.....correctness traded for perfomance.... But 
> anyway....IMHO this hack  is needed for every speed-step enabled 
> machine. Perhaps...the other workaround is via cpufreqd? I don't have 
> any Pentium M based PC/laptop around, so this is just a pure guess
> 

The other patch for this just used a constant to increment the time iirc
(based on some value in /proc).

> BTW, your patch seems reversed....if you really mean you want to fetch 
> realtime clock, you should use "rdtsc", right? But the patch seems 
> replaced "rdtsc" with get_clock().... 
> 

The values returned by rdtsc seem to vary depending on cpu frequency when
speedstep is enabled. get_clock() is actually more accurate (tho i think less
precise), at least from the user land POV.

-- 
Infinite complexity begets infinite beauty.
Infinite precision begets infinite perfection.

Re: Re: [Qemu-devel] Timing problems

[Sven Zenker]

Hi,
> Hm..... hard choice.....correctness traded for perfomance.... But 
> anyway....IMHO this hack  is needed for every speed-step enabled 
> machine. Perhaps...the other workaround is via cpufreqd? I don't have 
> any Pentium M based PC/laptop around, so this is just a pure guess
Yeah, you can also expect problems with the original versions on
multi-processor machines, I would think. Would be nice to compare
performance of the patched and original version. Unfortunately can't do
that right now because timing of the non-patched version is of course
messed up on my machine.
> 
> BTW, your patch seems reversed....if you really mean you want to fetch 
> realtime clock, you should use "rdtsc", right? But the patch seems 
> replaced "rdtsc" with get_clock().... 
rdtsc gives you the cpu's clock count, which, if CPU frequency changes,
or your code is run on different processors (multiprocessor machine),
cannot be assumed to be related to real time anymore. Resolutionwise,
the real time clock may be inferior, of course, as Jim mentioned.
Jim: could you point me to this other patch? Thanks!
> 
> Another thing, IMHO it is better to use unified format (diff -u). More 
> readable and i think it is a standart
Here you go:
+++++++++++++++++++++++++++++++++++
qemu-0.7.1 # diff -u vl.c.org vl.c
--- vl.c.org    2005-08-26 19:03:52.000000000 -0400
+++ vl.c        2005-08-28 14:43:00.000000000 -0400
@@ -500,16 +500,11 @@
     } while (h != h1);
     return ((int64_t)h << 32) | l;
 }
-
 #elif defined(__i386__)
-
 int64_t cpu_get_real_ticks(void)
 {
-    int64_t val;
-    asm volatile ("rdtsc" : "=A" (val));
-    return val;
+    return get_clock();
 }
-
 #elif defined(__x86_64__)

 int64_t cpu_get_real_ticks(void)
@@ -591,18 +586,7 @@

 void cpu_calibrate_ticks(void)
 {
-    int64_t usec, ticks;
-
-    usec = get_clock();
-    ticks = cpu_get_real_ticks();
-#ifdef _WIN32
-    Sleep(50);
-#else
-    usleep(50 * 1000);
-#endif
-    usec = get_clock() - usec;
-    ticks = cpu_get_real_ticks() - ticks;
-    ticks_per_sec = (ticks * 1000000LL + (usec >> 1)) / usec;
+       ticks_per_sec = 1000000LL;  /* our real time clock resolution */
 }

 /* compute with 96 bit intermediate result: (a*b)/c */

+++++++++++++++++++++++++++++++++++

qemu-0.7.1/linux-user # diff -u main.c.org main.c
--- main.c.org  2005-08-28 14:40:16.000000000 -0400
+++ main.c      2005-08-28 15:06:31.000000000 -0400
@@ -110,9 +110,7 @@

 int64_t cpu_get_real_ticks(void)
 {
-    int64_t val;
-    asm volatile ("rdtsc" : "=A" (val));
-    return val;
+    return get_clock();
 }

 #elif defined(__x86_64__)

+++++++++++++++++++++++++++++++++++

Best regards,
Sven

Re: Re: [Qemu-devel] Timing problems

[Mulyadi Santosa]

Hello Sven...

> rdtsc gives you the cpu's clock count, which, if CPU frequency
> changes, or your code is run on different processors (multiprocessor
> machine), cannot be assumed to be related to real time anymore.
> Resolutionwise, the real time clock may be inferior, of course, as
> Jim mentioned. Jim: could you point me to this other patch? Thanks!

Oh, so sorry, I was mistakenly made conclusion about your patch. yes, 
you are correct, get_clock() will do the trick....while rdtsc won't 
work because the clock will run vary during speed-stepping....

The unified format of diff looks great! :)

regards

Mulyadi