On 03/06/2007 04:24 PM, Jeremy Fitzhardinge wrote: > Thomas Gleixner wrote: >>> 3) clockevent set_next_event interface is suboptimal for paravirt (and >>> probably realtime-ish uses). The problem is that the expiry is passed >>> as a relative time. On paravirt, an arbitrary amount of (stolen) time >>> may have passed since the delta was computed and when the timer device >>> is programmed, causing that next interrupt to be too far out in the >>> future. It seems a better interface for set_next_event would be to pass >>> the current time and the absolute expiry. Actually, I sent email to >>> Thomas and Ingo about this (and some other clockevents/hrtimer feedback) >>> in July 2006, but never heard back. Thoughts? >>> >> There is no problem for realtime uses, as the reprogramming path is >> running with local interrupts disabled. I can see the point for paravirt >> and I'm not opposed to change / expand the interface for that. It might >> be done by an extra clockevents feature flag, which requests absolute >> time instead of relative time. >> > > I'm not sure how much different it makes overall. It's true that > absolute time would be a more useful interface, but because the guest > vcpu can be preempted at any time, we could miss the timeout > regardless. In Xen if you set a timeout for the past you get an > immediate interrupt; I presume the clockevent code can deal with that? >
That's the problem though, you won't know to set it for the past since the expiry is relative. When the vcpu starts running again, it will set the timer to expire X ns from now, not Xns from when the timer was requested. _______________________________________________ Virtualization mailing list [email protected] https://lists.osdl.org/mailman/listinfo/virtualization
