Re: Xen/arm: Virtual ITS command queue handling

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

On 15/05/15 12:30, Ian Campbell wrote:
>> Handling of Single vITS and multipl pITS can be made simple.
>>
>> All ITS commands except SYNC & INVALL has device id which will
>> help us to know to which pITS it should be sent.
>>
>> SYNC & INVALL can be dropped by Xen on Guest request
>>  and let Xen append where ever SYNC & INVALL is required.
>> (Ex; Linux driver adds SYNC for required commands).
>> With this assumption, all ITS commands are mapped to pITS
>> and no need of synchronization across pITS
> 
> You've ignored the second bullet its three sub-bullets, I think.

Aside ignoring the second bullet it's not possible to drop like that a
SYNC/INVALL command sent be the guest. How can you decide when a SYNC is
required or not? Why dropping "optional" SYNC would be fine? The spec
only says "This command specifies that all actions for the specified
re-distributor must be completed"...

Linux is not a good example for respecting the spec. Developers may
decide to put SYNC differently in new necessary place and we won't be
able to handle it correctly in Xen (see the vGICv3 re-dist example...).

If we go on one vITS per multiple pITS we would have to send the command
SYNC/INVALL to every pITS.

Regards,

-- 
Julien Grall