Re: [RFC PATCH v2 13/22] xen/arm: its: Add virtual ITS command support

[Ian Campbell <ian.campbell@citrix.com>]

On Thu, 2015-04-02 at 12:06 +0100, Julien Grall wrote:

> > Can we just enqueue with the hardware and use the guest vcpu polling
> > loop to trigger us to check for completion?
> 
> Enqueue may be long. I was thinking about suggesting to use a tasklet 
> for processing ITS command.

We don't need to enqueue everything the guest gives us at once, we could
only do a subset and pickup the rest later as things complete at the
physical ITS.

> But I don't know how much we can do in Xen with them.
> 
>  > What would happen if a guest
> > never polled, I suppose we would have to catch it some other way?
> 
> The specification (see 4.9.9 PRD03-GENC-010745 24.0) defines 2 different 
> way to be notify for completion:
> 	1) Polling: Reading GITS_CREADR in a loop
> 	2) Receiving an interrupt (see 4.9.10) by using the command MAPI.

I would expect Xen itself to use the second option at the host level,
which would then drive the completion via the vGITS_CREADR or the
guest's virtualised interrupt.

That means the pCPU is free during the ITS processing, which is surely
what we want.

> A guest would be buggy if it doesn't implement one of this solution. And 
> therefore may not run on real h/w.

I was more concerned about it wedging the hypervisor somehow with a
large number of completed but not released operations.

Ian.