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

[Julien Grall <julien.grall@citrix.com>]

Hi Ian,

On 02/04/2015 10:13, Ian Campbell wrote:
> On Wed, 2015-04-01 at 13:02 +0100, Julien Grall wrote:
>> On 01/04/15 12:46, Ian Campbell wrote:
>>> On Mon, 2015-03-30 at 16:47 +0100, Julien Grall wrote:
>>>>> In any case ITS commands are processed in synchronously. So any VCPU that
>>>>> send ITS commands is blocked.
>>>
>>> What exactly is synchronous here? Is it just the "translate vits into
>>> requests queued with the physical its driver" phase or does it also
>>> include waiting for the physical its' response and translating that back
>>> into a v response?
>>
>>  From the spec, the processing of command is asynchronous.
>
> I was asking about the implementation of our emulation of it, not about
> the hardware itself. I understood that the underlying h/w is
> asynchronous.

Sorry I though you were asking about the h/w. I think the end of my mail 
answered the question on our implementation.

>
>>   The vCPU has
>> to poll a register in order to know if the ITS has finished to execute
>> the command.
>>
>> A vCPU may decide to execute other things while the ITS is processing
>> commands.
>>
>> The implementation suggested by Vijay, both the vCPU and CPU is blocked
>> while the ITS command are processing.
>
> 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.

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.

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

Regards,

-- 
Julien Grall