Re: [Qemu-devel] Timing.. was : kqemu processor feature question

[Sven Zenker]

Had the same problem on my SpeedStep machine, am using an ugly patch
currently that replaces the CPU tick based timing with calls to the
realtime clock (see my previous mails on this list). Runs XP as guest
quite reliably under Suse 9.3 (using it every day to run Windows only
software). Performance isn't the best, though, although I'm lacking
comparisons.
-Sven

Am Mittwoch, den 19.10.2005, 10:18 +0400 schrieb Brad Campbell:
> Brad Campbell wrote:
> > John R. Hogerhuis wrote:
> > 
> >> Anyway everything seems to be fitting your theory about Athlon
> >> extensions. It would be nice to catch it in the act of trying to run an
> >> Athlon instruction on a Pentium.
> > 
> > And it was all a bogus trail.. It's *timer* related!
> > I made start_rtc_timer() fail (return -1) unconditionally, and voila it 
> > boots first time every time, and any Image I throw at it..
> > So something in the win2k loader appears to be timing sensitive. I did 
> > try the -win2k-hack for a lark, but that only had random results..
> > So there is an issue with the RTC handler on this machine by the looks..
> > RTC max freq is 1024, so it's not that.. further debugging required
> 
> 
> There is something bogus happening in cpu_calibrate_ticks() on this machine
> 
> When it boots windows ok I get this..
> 
> bklaptop:/tracks>qemu -hda w2k.img -m 512 -user-net -localtime -enable-audio
> timer: min=450 us max=50188 us avg=2424 us avg_freq=412.523 Hz
> timer: min=243 us max=53085 us avg=2435 us avg_freq=410.534 Hz
> timer: min=80 us max=24302 us avg=2416 us avg_freq=413.747 Hz
> timer: min=20 us max=60886 us avg=2837 us avg_freq=352.417 Hz
> 
> When it fails to start I get this..
> 
> bklaptop:/tracks>qemu -hda w2k.img -m 512 -user-net -localtime -enable-audio
> timer: min=194 us max=226993 us avg=21626 us avg_freq=46.239 Hz
> timer: min=646 us max=550679 us avg=21923 us avg_freq=45.613 Hz
> timer: min=939 us max=203745 us avg=21966 us avg_freq=45.525 Hz
> timer: min=171 us max=540720 us avg=28920 us avg_freq=34.577 Hz
> timer: min=4641 us max=80171 us avg=21372 us avg_freq=46.789 Hz
> 
> I might start qemu 30 times, and get 1 boot from 30 starts..
> 
> If the machine is loaded when I start qemu it will start every time. (while 
> true ; do echo -n . ; 
> done) in the shell is enough to load it up..
> It *almost* looks like the tsc is stopping while the machine is in hlt state 
> and cpu_calibrate_ticks 
> is getting bogus figures somewhere..
> 
> This machine has SpeedStep, but according to the info in 
> /sys/devices/system/cpu/cpu0/cpufreq/
> It has not changed state since boot and the governor is set to max_freq..
> 
> <Cue swimmy visual wipe effect> "Some time later......"
> 
> By changing cpu_calibrate_ticks to do something silly and keep busy while 
> waiting instead of 
> sleeping, I get dead reliable results every time.. (Yes, it's horridly ugly, 
> I know it's ugly but it 
> works here)
> 
> void cpu_calibrate_ticks(void)
> {
>      int64_t usec, ticks;
> 
>      usec = get_clock();
>      ticks = cpu_get_real_ticks();
> #ifdef _WIN32
>      Sleep(50);
> #else
>      while (get_clock()-usec < 50000) {
>          get_clock();
>      }
> #endif
>      usec = get_clock() - usec;
>      ticks = cpu_get_real_ticks() - ticks;
>      ticks_per_sec = (ticks * 1000000LL + (usec >> 1)) / usec;
>      printf("Ticks_per_sec = %lld\n",ticks_per_sec);
> }
> 
> bklaptop:/tracks>qemu -hda w2k.img -user-net -m 512 -localtime
> timer: min=90 us max=10628 us avg=994 us avg_freq=1005.900 Hz
> timer: min=33 us max=22670 us avg=1003 us avg_freq=996.085 Hz
> timer: min=305 us max=2995 us avg=980 us avg_freq=1019.897 Hz
> timer: min=220 us max=25093 us avg=1020 us avg_freq=979.883 Hz
> timer: min=27 us max=9872 us avg=701 us avg_freq=1425.036 Hz
> 
> So it appears to give a much more accurate idea of tics_per_sec. (which is 
> where all my timer 
> problems on this laptop are coming from)
> 
> For testing I run cpu_calibrate_tics 8 times in the init sequence..
> This is just changing the above routine only.. no other machine modification 
> or restart...
> 
> Before
> bklaptop:/tracks>qemu -hda w2k.img -user-net -m 512 -localtime
> Ticks_per_sec = 47720193
> Ticks_per_sec = 32508223
> Ticks_per_sec = 34422776
> Ticks_per_sec = 51535607
> Ticks_per_sec = 29776765
> Ticks_per_sec = 29481406
> Ticks_per_sec = 26230393
> Ticks_per_sec = 26133451
> Winner - RTC enabled
> timer: min=2568 us max=850100 us avg=38857 us avg_freq=25.735 Hz
> timer: min=5607 us max=153629 us avg=37663 us avg_freq=26.551 Hz
> 
> After
> bklaptop:/tracks>qemu -hda w2k.img -user-net -m 512 -localtime
> Ticks_per_sec = 999800944
> Ticks_per_sec = 999876102
> Ticks_per_sec = 999892940
> Ticks_per_sec = 999842403
> Ticks_per_sec = 999893360
> Ticks_per_sec = 999882080
> Ticks_per_sec = 999877682
> Ticks_per_sec = 999871643
> Winner - RTC enabled
> timer: min=90 us max=10628 us avg=994 us avg_freq=1005.900 Hz
> timer: min=33 us max=22670 us avg=1003 us avg_freq=996.085 Hz
> 
> It's got me.. I'm off to try and find the processor datasheet on this PIII-M 
> to check the tsc info..
> 
> Regards,
> Brad