Re: Xen/arm: Virtual ITS command queue handling

[Julien Grall <julien.grall.oss@gmail.com>]

On 15/05/15 15:04, Vijay Kilari wrote:
> On Fri, May 15, 2015 at 7:14 PM, Julien Grall <julien.grall@citrix.com> wrote:
>> On 15/05/15 14:24, Ian Campbell wrote:
>>> On Fri, 2015-05-15 at 18:44 +0530, Vijay Kilari wrote:
>>>> On Fri, May 15, 2015 at 6:23 PM, Ian Campbell <ian.campbell@citrix.com> wrote:
>>>>> On Fri, 2015-05-15 at 18:17 +0530, Vijay Kilari wrote:
>>>>>> On Fri, May 15, 2015 at 5:33 PM, Julien Grall <julien.grall@citrix.com> wrote:
>>>>>>> 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.
>>>>>>>
>>>>>>    Why can't we group the batch of commands based on pITS it has
>>>>>> to be sent?.
>>>>>
>>>>> Are you suggesting that each batch we send should be synchronous? (i.e.
>>>>> end with SYNC+INT) That doesn't seem at all desirable.
>>>>
>>>> Not only at the end of batch, SYNC can be appended based on every
>>>> command within the batch.
>>>
>>> Could be, but something to avoid I think?
>>
>> That would slow down the ITS processing (SYNC is waiting that the
>> previous command has executed).
>>
>> Also, what about INTALL? Sending it everytime would be horrible for the
>> performance because it flush the ITS cache.
> 
> INVALL is not required everytime. It can be sent only as mentioned in spec Note.
> ex; MOVI
> 
> Note: this command is expected to be used by software when it changed
> the re-configuration
> of an LPI in memory to ensure any cached copies of the old
> configuration are discarded.

INVALL is used when a large number of LPIs has been reconfigured. If you
send one by MOVI is not efficient at all and will slowdown all the
interrupts for few milliseconds. We need to use them with caution.

Usually a guest will send one for multiple MOVI command.

Regards,

-- 
Julien Grall