[Intel-gfx] [iGVT-g] [PATCH 3/7] drm/i915: Always enable execlists on BDW for vgpu

[Tian, Kevin]

> From: iGVT-g [mailto:igvt-g-bounces at lists.01.org] On Behalf Of Zhiyuan Lv
> Sent: Thursday, August 20, 2015 5:40 PM
> 
> On Thu, Aug 20, 2015 at 10:22:37AM +0100, Chris Wilson wrote:
> > On Thu, Aug 20, 2015 at 04:55:08PM +0800, Zhiyuan Lv wrote:
> > > Hi Chris,
> > >
> > > On Thu, Aug 20, 2015 at 09:34:05AM +0100, Chris Wilson wrote:
> > > > On Thu, Aug 20, 2015 at 03:45:20PM +0800, Zhiyuan Lv wrote:
> > > > > Broadwell hardware supports both ring buffer mode and execlist mode.
> > > > > When i915 runs inside a VM with Intel GVT-g, we allow execlist mode
> > > > > only. The reason is that GVT-g does not support the dynamic mode
> > > > > switch between ring buffer mode and execlist mode when running
> > > > > multiple virtual machines. Consider that ring buffer mode is legacy
> > > > > mode, it makes sense to drop it inside virtual machines.
> > > >
> > > > If that is the case, you should query the host as to what mode it is
> > > > running.
> > >
> > > Thanks for the reply! You mean we query the host mode, then tell the
> > > guest driver inside VM, so that it could use the same mode as host
> > > right? That might be a little complicated, and the only benefit is to
> > > support legacy ring buffer mode ...
> >
> > The only benefit being that the guest works no matter what the host
> > does?
> 
> Supporting ring buffer mode may need more work in GVT-g. When we started to
> enable BDW support, ring buffer mode used to work but was not well tested. And
> the inter-VM switch (all in ringbuffer mode) may be tricker comparing with
> driver context switch with "MI_SET_CONTEXT", because we need to switch ring
> buffer whereas driver does not. Regarding this, the EXECLIST mode looks
> cleaner. In order to support that, we may have to: 1, change more LRI commands
> to MMIO in current driver; 2, more testing/debugging of inter-VM context
> switch flow.
> 
> Based on that, I think we should really make statement that "ring buffer mode"
> is not supported by GVT-g on BDW :-)
> 

I think the primary reason is that Windows gfx driver only uses execution
list mode. They don't have plan to add back ring buffer mode. So we
have to assume only execution list for all guests powered by GVT-g (being
that dynamic mode switch is not feasible).

Thanks
Kevin