[Qemu-devel] Re: [kvm-devel] [PATCH] Add support for HPET periodic timer.

[Daniel P. Berrange]

On Mon, Aug 13, 2007 at 11:04:46AM +0300, Avi Kivity wrote:
> Luca Tettamanti wrote:
> > Linux operates the HPET timer in legacy replacement mode, which means that 
> > the
> > periodic interrupt of the CMOS RTC is not delivered (qemu won't be able to 
> > use
> > /dev/rtc). Add support for HPET (/dev/hpet) as a replacement for the RTC; 
> > the
> > periodic interrupt is delivered via SIGIO and is handled in the same way as 
> > the
> > RTC timer.
> > HPET must be explicitly enabled with -use-hpet.
> >
> >   
> 
> Are there any downsides to using HPET?  If not, I suggest making it the 
> default, in order to reduce complexity for the user.

Only downside is that not all machines have HPET, and many which do have it
disabled in BIOS for 'compatability' with Windows, but if you always fallback
to trying RTC too, that's not an issue.

> Something like:
> 
> - try to use HPET (unless -no-rtc selected)
> - try to use RTC  (unless -no-rtc selected)
> - fallback to normal unix facilities

If we're going to add command line args it probably makes sense  to be a
little more generic than -no-rtc or -use-hpet. Have a list of preferred
clock sources eg

  -clock hpet,rtc,unix

If -clock is omitted, then default to trying all in the priority you
describe.

Dan.
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