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

[Vijay Kilari <vijay.kilari@xxxxxxxxx>]

On Wed, Apr 29, 2015 at 9:56 PM, Vijay Kilari <vijay.kilari@xxxxxxxxx> wrote:
> Hi Julien,
>
> On Wed, Apr 29, 2015 at 7:05 PM, Julien Grall <julien.grall@xxxxxxxxxx> wrote:
>> On 29/04/15 12:56, Julien Grall wrote:
>>> As the 2 suggested approach don't seem to fit our usage, we need to find
>>> another approach.
>>
>> I think I have another approach which doesn't require interrupt neither
>> polling in EL2.
>
> I could resolve all the issues around approach 1
> only concern is generating dummy/fake device id.
>
>>
>> 1) Trap on CWRITER
>>    a) Read command for the vITS CQ
>>    b) Transform command
>>    c) Inject command
>>         - If ITS CQ full => 2)
>>         - If vITS CQ empty => 2)
>>         - Else => 1)
>> 2) Return to guest
>> 3) Trap on CREADR
>>    a) Check completion of the current batch of command
>>       - If complete => 3.b)
>>       - Else => 4)
>>    b) Update CREADR
>>    c) Check if more command and inject it (see process in 1))
>> 4) Return to guest
>>
>> Some other restrictions to add:
>>    - If there is already a batch of command in process for the domain,
>> defer the injection of new command
>>    - The number of command sent in a batch should be limited.
>>
>> I think thoses restrictions are okay because the ITS is per-domain not
>> per-VCPU.
>>
>> Although, there is a possible problem if the guest is not reading
>> CREADR. Maybe having a timer would be fine to update CREADR and inject
>> new commands.
>
>  How to know that guest is not reading CREADR and using INT mode to
> check for completion?
> One way is when INT ITS command is emulated, then we can assume that guest
> is using INT mode and use timer to update CREADR and inject new
> commands.
>
> However with INT mode, if the guest driver is checking for CREADR
> value on receiving completion interrupt requested through INT ITS command
> then CREADR will show up old value.

  On trap of CREADR we have to update it. So this can be handled

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