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

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Jul 03, 2018 at 10:05:28AM +0100, Chris Wilson wrote:
> 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@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.

I can easily imagine that with the few per-client fences vgpu hands out
rewrites are much more common. But yeah some real data would be good. And
more reasons to get mesa off of the gtt mmaps.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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