Re: [Xen-devel] [PATCH] Deferrable Timer

[Dave Winchell <dwinchell@xxxxxxxxxxxxxxx>]

Ke,

Yu, Ke wrote:

> Dave,
>
> Glad to see there is deferrable timer application. Please go ahead with
> that. And I will keep you updated if there is finding in my side.
ok.

> BTW, Could you please elaborate more on the "guest-handles-missed-tick"
> case? Since there is no need to inject missed tick to guest, which timer
> would be used as deferrable timer?
>  
Hpet.c uses set_timer for hpet comparator/timer 0. When that timer expires,
a clock interrupt may be injected to the guest. This timer
is normally set to expire at the next period boundary.
We could, instead, have it expire over a range of say, several periods.

Vpt.c works in a similar fashion for its periodic timer. Other clocksources,
e.g. pit, rtc, are layred on vpt.c with interface create_periodic_timer.
I can imagine an option passed to create_periodic_timer signifying that
a deferrable timer may be used.

Ideally, the deferrable timer would have an option where a set of allowable
timeout values, rather than a range, could be provided. If it had this 
option, we could keep
the timeouts on the integer*period time line. Otherwise I need to warp 
the comparator
as discussed below. I anticipate that there may be some problems with
warping.

I realize that specifying a range gives you more options for combining 
timeouts.
I don't mind trying to solve the warping problem.

One further option would be a deferrable timer with a range fallowed by 
a non-deferrable
timer to get back on the integer*period timeline for interrupt delivery.

thanks,
Dave


> Best Regards
> Ke 
>
> Dave Winchell wrote:
>  
>
> > Ke,
> >
> > One would think that hpet or vpt support for the
> > guest-handles-missed-ticks policy would be a good application for a
> > deferrable timer. 
> > If a deferrable timer were used, then the comparator (cmp) would have
> > to 
> > be warped to a non-integer multiple of the period. This is because
> > Linux reads the comparator register to estimate the delay since the
> > interrupt 
> > was posted.
> > I don't think warping like this will be a problem. At some point, I
> > can test this.
> >
> > I think we could use the deferrable timer for the
> > guest-does-not-handle-missed-ticks
> > policy as well.
> >
> > Any investigation that you want to do in the platform timer area would
> > be fine.
> > Or I can do it, but that will probably be after I do the vpt.c/hpet.c
> > integration
> > work.
> >
> > thanks,
> > Dave
> >
> >    
> >
> > > Best Regards
> > > Ke
> > >
> > > _______________________________________________
> > > Xen-devel mailing list
> > > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > > http://lists.xensource.com/xen-devel
> > >      
>  

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