[Beignet] Haswell issues

[Rebecca N. Palmer]

> I will raise this issue to Linux i915 kernel team. And hope we can get a
> clean fix in Linux kernel side finally.

I agree that's a good idea, but given how long it is likely to take, I'd 
like to at least give a clear user-friendly error in the meantime.

-Is it likely that "drm_intel_gem_bo_context_exec() failed: Invalid 
argument" (since 9e8874c) affects _all_ Haswell hardware?  If so, I'd 
consider this an improvement over the previous "some work (as long as 
you don't use __local), some silently do nothing", and put the 
i915_cmd_parser workaround in the error message.

-What happens when you do try to use __local (without 
https://01.org/zh/beignet/downloads/linux-kernel-patch-hsw-support): an 
explicit error, or wrong results?

-It was suggested some time ago that i915.enable_ppgtt=2 might help; 
does it?  (This kernel parameter can only be set at boot time, and is an 
"unsafe parameter" i.e. "may crash your system", but would be easier 
than recompiling Linux.)