Re: [PATCH] drm/i915: Add OACONTROL to the command parser register whitelist.

[Kenneth Graunke <kenneth@whitecape.org>]

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On 03/26/2014 11:26 AM, Volkin, Bradley D wrote:
> On Wed, Mar 26, 2014 at 10:37:44AM -0700, Kenneth Graunke wrote:
>> On 03/26/2014 09:38 AM, Daniel Vetter wrote:
>>> On Wed, Mar 26, 2014 at 09:03:58AM -0700, Volkin, Bradley D wrote:
>>>> On Tue, Mar 25, 2014 at 11:21:23PM -0700, Daniel Vetter wrote:
>>>>> On Tue, Mar 25, 2014 at 10:52:03PM -0700, Kenneth Graunke wrote:
>>>>>> Mesa needs to be able to write OACONTROL in order to expose the
>>>>>> Observability Architecture's performance counters via OpenGL.
>>>>>>
>>>>>> Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
>>>>>
>>>>> Thanks a lot for quickly tracking this down. Now when we've talked about
>>>>> OA a little while ago we concluded that mesa should clear OACONTROL again
>>>>> before the batch ends to make sure that userspace can't unduly observe
>>>>> other processes. So I think it'd be worth to keep track of this with a
>>>>> flag (set when OACONTROL is != 0 and reset when the batch loads 0). Also
>>>>> we need to make sure that userspace sets the right OACONTROL modes (not
>>>>> the one which streams into a global gtt buffer essentially). So some
>>>>> additional work required.
>>>>
>>>> Ok, I'll look into this. And apologies for not catching it myself.
>>>>
>>>> If we have to do additional checks on fields within the registers then I
>>>> suppose we'll need to limit those registers to MI_LOAD_REGISTER_IMM. That
>>>> might require separate whitelists for MI_LOAD_REGISTER_IMM/MEM. Not the end
>>>> of the world, but certainly some additional complexity.
>>>>
>>>> For the resetting check, are there other registers in the current list that
>>>> should have this tracking? If so, is 0 the reset value in all cases?
>>>>
>>>> Let me know if there is anything in the works that would require additional
>>>> registers or different uses of any registers.
>>>
>>> Afaik there's no other register we want to reset again. I think all other
>>> register we might want to clear are already part of hw contexts, so no
>>> chance to leak stuff (e.g. the streamout registers and a end-of-pipe
>>> counters). Ken might know of something I've missed.
>>> -Daniel
>>
>> Right, I think it's just OACONTROL.  I don't think there's a need to
>> filter particular values.
> 
> Sorry, I don't follow you here. Can you clarify what you mean r.e. filtering?

Oh, I meant I don't think we need to do additional checks on "fields
within the registers" (to quote your email) or restrict the values we
can write to the registers.  Just the ability to write the whole
register to any arbitrary value should be sufficient.

Thanks Brad!

--Ken


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