Re: [PATCH] drm/i915/oa: Disable OA on Haswell

[Lionel Landwerlin <lionel.g.landwerlin@intel.com>]

On 11/05/18 16:51, Chris Wilson wrote:
> Quoting Lionel Landwerlin (2018-05-11 16:43:02)
>> On 11/05/18 15:18, Chris Wilson wrote:
>>> Quoting Lionel Landwerlin (2018-05-11 15:14:13)
>>>> My understanding of the virtual memory addressing from the GPU is limited...
>>>> But how can the GPU poke at the kernel's allocated data?
>>>> I thought we mapped into the GPU's address space only what is allocated
>>>> through gem.
>>> Correct. The HW should only be accessing the pages through the GTT and
>>> the GTT should only contain known pages (or a pointer to the scratch
>>> page). There is maybe a hole where we are freeing the memory before
>>> the HW has finished using it (still writing through stale TLB and
>>> whatnot even though the system has reallocated the pages), but other
>>> than that quite, quite scary. Hence this awooga.
>>> -Chris
>>>
>> I managed to reproduce a kasan backtrace on the same test.
>> So it's not just the CI machine.
>>
>> But I can't even startup a gdm on that machine with drm-tip. So maybe
>> there is some much more broken...
> Don't leave us in suspense...

Your first patch (check that OA is actually disabled) seems to get rid 
of the issue on my machine.
Thanks a lot a for finding that!

Trying to find when HSW when wrong now. Same kernel works just fine on 
my SKL.

>
>> i915/perf unpins the object correctly before freeing (at which point it
>> could be reused).
> Sure, but does perf know that the OA unit has stopped writing at that
> point... That's not so clear (from my pov).

Clearly it wasn't :(

>
>> Should we ensure i915_vma_destroy() i915/perf maybe?
>>
>> It almost seems like this is an issue that could arise in other part of
>> the driver too.
> The problem of the HW continuing to access the pages after unbinding is
> inherent to the system (and what actually happens if we change PTE in
> flight is usually undefined), hence the great care we go to track HW
> activity and try not to release pages while it is still using them.
> -Chris
>