On Thu, Nov 07, 2019 at 04:04:10PM +0000, Marc Zyngier wrote:
> As I was cleaning up some of the GICv4 code to make way for GICv4.1 it
> occured to me that we could drastically reduce the impact of the GICv4
> doorbells on systems that are not oversubscribed (each vcpu "owns" a
> physical CPU).
>
> The technique borrows its logic from the way we disable WFE trapping
> when a vcpu is the only process on the CPU run-queue. If this vcpu is
> the target of VLPIs, it is then beneficial not to trap blocking WFIs
> and to leave the vcpu waiting for interrupts in guest state.
>
> All we need to do here is to track whether VLPIs are associated to a
> vcpu (which is easily done by using a counter that we update on MAPI,
> DISCARD and MOVI).
>
> It has been *very lightly* tested on a D05, and behaved pretty well in
> my limited test cases (I get almost no doorbell at all in the non
> oversubscribed case, and the usual hailstorm as soon as there is
> oversubscription). I'd welcome some testing on more current HW.
>
Reviewed-by: Christoffer Dall <christoffer.d...@arm.com>
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