Re: [Xen-devel] [PATCH 1/4] xen/arm: gic: Ensure we have an ISB between ack and do_IRQ()

[Julien Grall <julien.grall@xxxxxxx>]

On 20/11/2018 18:10, Andrii Anisov wrote:
> Hello Julien,
>
>
> On 19.11.18 18:42, Julien Grall wrote:
> > There are no issue about processing IRQs before the syncs. It is the same as 
> > if an IRQ was raised from ila different pCPUs.
> > So why do you need that?
>  From my understanding of gic-vgic code (old vgic), for the IRQs targeting the 
> `current` vcpu, it leads to a faster processing under interrupts storm 
> conditions. If it was all LRs set on previous switch to a guest, a the IRQ will 
> have a chance to go directly to LR instead of setting on lr_pending queue. Also 
> inflight_irqs queue have a chance to be shorter to insert.
Do you have actual numbers? Also to be on the same page, what is your definition 
of interrupts storm?
Bear in mind that the old vGIC will be phased out soon. If you are worried about 
performance, then I would recommend to try the new vGIC and see whether it improves.
> Moreover, maybe you can explain me, what's the point of interrupts enabling 
> before for `do_IRQ()` call? Those interrupts would be grabbed and processed 
> anyway, during run through the loop in `gic_interrupt()`. So I see the only 
> outcome of interrupts enabling - spending more time for context switches.
Well, if you re-enable the interrupts you give a chance for higher priority 
interrupts to come up. This will not happen if you have interrupts disabled.
But you seem to base your assumption on interrupts storm (yet to be defined). If 
you have an interrupt storm, then you are already doomed as your guest/Xen will 
not have time to do any other work.
In any case, you need to provide number to support your optimization.

Cheers,

--
Julien Grall

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel