Re: [Xen-devel] [PATCH 1/4] x86/IRQ: don't keep EOI timer running without need

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, May 08, 2019 at 06:46:25AM -0600, Jan Beulich wrote:
> The timer needs to remain active only until all pending IRQ instances
> have seen EOIs from their respective domains. Stop it when the in-flight
> count has reached zero in desc_guest_eoi(). Note that this is race free
> (with __do_IRQ_guest()), as the IRQ descriptor lock is being held at
> that point.
> 
> Also pull up stopping of the timer in __do_IRQ_guest() itself: Instead
> of stopping it immediately before re-setting, stop it as soon as we've
> made it past any early returns from the function (and hence we're sure
> it'll get set again).
> 
> Finally bail from the actual timer handler in case we find the timer
> already active again by the time we've managed to acquire the IRQ
> descriptor lock. Without this we may forcibly EOI an IRQ immediately
> after it got sent to a guest. For this, timer_is_active() gets split out
> of active_timer(), deliberately moving just one of the two ASSERT()s (to
> allow the function to be used also on a never initialized timer).

AFAICT timer_is_active is exclusively used in irq_guest_eoi_timer_fn,
which must have initialized the timer in order for
irq_guest_eoi_timer_fn to be called, and hence I'm not sure why you
need to be able to call timer_is_active with an uninitialized timer.

Is this maybe used by other patches?

Thanks, Roger.

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