[Intel-gfx] [RFCv2 10/14] drm/i915: update PDPs by condition when submit the LRC context

[Joonas Lahtinen]

Hi,

On to, 2016-02-25 at 15:02 +0000, Wang, Zhi A wrote:
> 
> -----Original Message-----
> From: Tian, Kevin 
> Sent: Wednesday, February 24, 2016 4:50 PM
> To: Wang, Zhi A; intel-gfx at lists.freedesktop.org; igvt-g at lists.01.org
> Cc: Lv, Zhiyuan; Niu, Bing; Song, Jike; daniel.vetter at ffwll.ch; Cowperthwaite, David J; chris at chris-wilson.co.uk; joonas.lahtinen at linux.intel.com
> Subject: RE: [RFCv2 10/14] drm/i915: update PDPs by condition when submit the LRC context
> 
> > 
> > From: Wang, Zhi A
> > Sent: Thursday, February 18, 2016 7:42 PM
> > 
> > Previously the PDPs inside the ring context are updated at:
> > 
> > - When populate a LRC context
> > - Before submitting a LRC context (only for 32 bit PPGTT, as the amount
> > of used PDPs may change)
> > 
> > This patch postpones the PDPs upgrade to submission time, and will update
> > it by condition if the PPGTT is 48b. Under GVT-g, one GVT context will be
> > used by different guest, the PPGTT instance related to the context might
> > be changed before the submission time. And this patch gives GVT context
> > a chance to load the new PPGTT instance into an initialized context.
> Could you elaborate why we share one GVT context across different guest?
> A natural thought is that we'll create one GVT context per every guest
> context...
> 
> [Zhi] We don't have context creation/destroy notification in guest i915 driver.
> Because in our implementation we need an unique context id to anchor the
> relationship between shadow context and guest context, while i915 uses GGTT
> address as context id. In each context pin/unpin, the context id may be changes.

Does not this lead to plenty of unnecessary storing and restoring of
the context parameters?

I would imagine this to destroy performance.

Regards, Joonas

> 
> So it's not necessary to allocate multiple GVT context here. 
> 
> Thanks
> Kevin
-- 
Joonas Lahtinen
Open Source Technology Center
Intel Corporation