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

[Andrii Anisov <andrii.anisov@xxxxxxxxx>]

Hello Julien,

On 22.11.18 19:22, Julien Grall wrote:
> My biggest worry is you are doing optimization on a vGIC that is not 
> fully compliant with how a GIC should behave (e.g edge vs level) and 
> with very fragile locking.
Yep, old VGIC locking looks pretty terrible.

> If you are interested, Andre can provides more details.
Being honest, I'm not fully understand edge vs level problem there. It 
would be good to get better view on it.
> I can't comment without any numbers here. Bear in mind that we fixed 
> bugs in Xen 4.12 (including spectre/meltdown and missing barriers) that 
> wasn't backported to Xen 4.10. It is entirely possible that it 
> introduced slowness but it also ensure the code is behaving correctly.
For sure, I know that. It was rather a political decision.

> Anyway, if there are performance regression we should investigate them 
> and discuss how we can address/limit them. Similarly for the new vGIC, 
> if you think it is too slow, then we need to know why before we get rid 
> of the old vGIC.
Yep.

> There are no guest prioritization at the moment. However, we may want to 
> introduce it to give priority to one guest over.
But still, processing of IRQs by a hypervisor (e.g. moving them from gic 
to vgic for guests IRQs) has a higher priority over any guest execution.
--
Sincerely,
Andrii Anisov.

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