Re: [PATCH 6/9] drm/i915: driver based PASID handling

[Jesse Barnes <jbarnes@virtuousgeek.org>]

On 10/07/2015 10:17 AM, David Woodhouse wrote:
> On Wed, 2015-10-07 at 09:28 -0700, Jesse Barnes wrote:
>> On 10/07/2015 09:14 AM, Daniel Vetter wrote:
>>> On Wed, Oct 07, 2015 at 08:16:42AM -0700, Jesse Barnes wrote:
>>>> On 10/07/2015 06:00 AM, David Woodhouse wrote:
>>>>> On Fri, 2015-09-04 at 09:59 -0700, Jesse Barnes wrote:
>>>>>> +
>>>>>> +       ret = handle_mm_fault(mm, vma, address,
>>>>>> +                             desc.wr_req ? FAULT_FLAG_WRITE : 0);
>>>>>> +       if (ret & VM_FAULT_ERROR) {
>>>>>> +               gpu_mm_segv(tsk, address, SEGV_ACCERR); /* ? */
>>>>>> +               goto out_unlock;
>>>>>> +       }
>>>>>> +
>>>>>
>>>>> Hm, do you need to force the SEGV there, in what ought to be generic
>>>>> IOMMU code?
>>>>>
>>>>> Can you instead just let the fault handler return an appropriate
>>>>> failure code to the IOMMU request queue and then deal with the
>>>>> resulting error on the i915 device side?
>>>>
>>>> I'm not sure if we get enough info on the i915 side to handle it
>>>> reasonably, we'll have to test that out.
>>>
>>> We do know precisely which context blew up, but without the TDR work we
>>> can't yet just kill the offender selective without affecting the other
>>> active gpu contexts.
>>
>> How?  The notification from the IOMMU queue is asynchronous...
> 
> The page request, and the response, include 'private data' which an
> endpoint can use to carry that kind of information.
> 
> In $7.5.1.1 of the VT-d specification it tells us:
> 
> 	"Private Data: The Private Data field can be used by 
> 	 Root-Complex integrated endpoints to uniquely identify
> 	 device-specific private information associated with an 
> 	 individual page request.
> 
> 	"For Intel ® Processor Graphics device, the Private Data field 
> 	 specifies the identity of the GPU advanced-context (see 
> 	 Section 3.10) sending the page request."

Ah right so we could put our private context ID in there if the PASID doesn't end up being per-context.  That would work fine (though as Daniel said we still need fancier reset to handle things more gracefully).

>>> But besides that I really don't see a reason why we need to kill the
>>> process if the gpu faults. After all if a thread sigfaults then signal
>>> goes to that thread and not some random one (or the one thread that forked
>>> the thread that blew up). And we do have interfaces to tell userspace that
>>> something bad happened with the gpu work it submitted.
> 
> I certainly don't want the core IOMMU code killing things. I really
> want to just complete the page request with an appropriate failure
> code, and let the endpoint device deal with it from there.

I think that will work, but I want to test and make sure.  In the driver mode version I took advantage of the fact that I got an unambiguous page request failure from the IOMMU along with a unique PASID to send the signal.  Getting it on the GPU side means looking at some of our existing error state bits, which is something I've been avoiding...

Jesse

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