Re: [PATCH 3/7] drm/i915: Always enable execlists on BDW for vgpu

[Zhiyuan Lv <zhiyuan.lv@intel.com>]

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 :-)

> -Chris
> 
> -- 
> Chris Wilson, Intel Open Source Technology Centre
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