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    SubjectRe: + stupid-hack-to-make-mainline-build.patch added to -mm tree
    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.
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