Re: [Qemu-devel] [PATCH] Timer/clock for Linux

[Fabrice Bellard <fabrice@bellard.org>]

Hi,

Can other people confirm that it is better to always use /dev/rtc on 
Linux ? Is there a way to get the real resolution of the host timer ?

Fabrice.

Kazu wrote:
> Hi,
> 
> I made a little patch of timer/clock for Linux host. It always trys to use
> /dev/rtc.
> getitimer doesn't report a correct interval value.
> 
> http://www.h7.dion.ne.jp/~qemu-win/download/qemu-20060407-linux-timer.patch
> 
> To get a precise timer/clock., do:
> (1) Set max-user-freq 1024 as root.
> 
> [Linux host] # echo 1024 > /proc/sys/dev/rtc/max-user-freq
> 
> It is necessary to set it everytime you reboot the host OS. Write it to
> /etc/rc.d/rc.local.
> 
> (2) Apply the patch and build a binary.
> 
> (1) is necessary and (2) is optional. I think latest kernel doesn't need the
> patch. When I use 2.6.11-1.11369_FC4 host, it is necessary to apply (2). But
> when 2.6.15-1.1831_FC4 host, it is not.
> 
> If you don't set max-user-freq, the clock in Windows 2000 guest is unstable
> while IE is running.
> 
> Timer uses RTC(Real Time Clock). If it can not be used, setitimer/getitimer
> are used.
> If the timer's interval is set to 999 us, the interval becomes 2ms. It is
> the
> cause that a guest OS clock is 1/2 slow than the real time. Recent Linux
> kernel is 4ms and clock is 1/4 slow.
> 
> 
> Tested environments are:
> Windows 2000 guest            /FC4 kernel-2.6.11-1.1831_FC4 host
> FC4-i386-rescuecd.iso guest/FC4 kernel-2.6.11-1.1831_FC4 host
> 
> Windows 2000 guest            /FC4 kernel-2.6.15-1.1831_FC4 host
> FC4-i386-rescuecd.iso guest/FC4 kernel-2.6.15-1.1831_FC4 host
> 
> I have not tested Linux 2.4 host.
> The clock of guest OS on x86_64 host is unstable. 1ms interval interrupts
> occure but the clock in Windows 2000 guest on x86_64 host doesn't work well.
> 
> Regards,
> Kazu
> 
> 
> 
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