[Intel-gfx] [PATCH v2 2/2] drm/i915: write fence reg only once on VGPU

[Chris Wilson]

Quoting Daniel Vetter (2018-07-03 09:51:03)
> On Tue, Jul 03, 2018 at 10:56:17AM +0800, Zhao Yakui wrote:
> > On VGPU scenario the read/write operation of fence_reg will be trapped
> > by the GVT-g. And then gvt-g follows the HW spec to write the fence_reg.
> > So it is unnecessary to read/write fence reg several times. This will help
> > to reduce the unnecessary trap of fence_reg mmio operation.
> > 
> > V1->V2: Fix one typo error of parameter when calling intel_vgpu_active
> > 
> > Signed-off-by: Zhao Yakui <yakui.zhao at intel.com>
> 
> Ok this makes more sense. Except you need to put the 64bit entirely into
> the vpgu block, with a comment explaining why this is safe (since the vpgu
> will take care of updating fences correctly).

Except, who cares? Are fence registers being rewritten that frequently
that special casing vgpu is worth the hassle. Part of that is that you
need to leave a hint behind in the code that (a) explains why it is safe
after having the "here be dragons" and (b) why we care.

On a more pragmatic level if fencing doesn't plateau out to steady
state, that is a worrying amount of contention -- the actual fence write
itself would be the least of my worries.
-Chris