Re: [RFC PATCH v3 0/4] Deliver vGPU display vblank event to userspace

[Zhenyu Wang]

On 2019.06.28 07:43:46 +0200, Gerd Hoffmann wrote:
> On Fri, Jun 28, 2019 at 11:21:49AM +0800, Zhenyu Wang wrote:
> > On 2019.06.27 12:31:33 +0200, Gerd Hoffmann wrote:
> > > > >   Hi,
> > > > > 
> > > > > > Instead of delivering page flip events, we choose to post display
> > > > > > vblank event. Handling page flip events for both primary plane and
> > > > > > cursor plane may make user space quite busy, although we have the
> > > > > > mask/unmask mechansim for mitigation. Besides, there are some cases
> > > > > > that guest app only uses one framebuffer for both drawing and display.
> > > > > > In such case, guest OS won't do the plane page flip when the
> > > > > > framebuffer is updated, thus the user land won't be notified about the
> > > > > updated framebuffer.
> > > > > 
> > > > > What happens when the guest is idle and doesn't draw anything to the
> > > > > framebuffer?
> > > > The vblank event will be delivered to userspace as well, unless guest OS disable the pipe.
> > > > Does it make sense to vfio/display?
> > > 
> > > Getting notified only in case there are actual display updates would be
> > > a nice optimization, assuming the hardware is able to do that.  If the
> > > guest pageflips this is obviously trivial.  Not sure this is possible in
> > > case the guest renders directly to the frontbuffer.
> > > 
> > > What exactly happens when the guest OS disables the pipe?  Is a vblank
> > > event delivered at least once?  That would be very useful because it
> > > will be possible for userspace to stop polling altogether without
> > > missing the "guest disabled pipe" event.
> > > 
> > 
> > It looks like purpose to use vblank here is to replace user space
> > polling totally by kernel event? Which just act as display update
> > event to replace user space timer to make it query and update
> > planes?
> 
> I think it makes sense to design it that way, so userspace will either
> use the events (when supported by the driver) or a timer (fallback if
> not) but not both.

Agree. It's more of a userspace choice.

> 
> > Although in theory vblank is not appropriate for this which
> > doesn't align with plane update or possible front buffer rendering at
> > all, but looks it's just a compromise e.g not sending event for every
> > cursor position change, etc.
> > 
> > I think we need to define semantics for this event properly, e.g user
> > space purely depends on this event for display update, the opportunity
> > for issuing this event is controlled by driver when it's necessary for
> > update, etc. Definitely not named as vblank event or only issue at vblank,
> > that need to happen for other plane change too.
> 
> I think it should be "display update notification", i.e. userspace
> should check for plane changes and update the display.
> 
> Most events will probably come from vblank (typically plane update are
> actually committed at vblank time to avoid tearing, right?).  That is an
> implementation detail though.
> 

Yeah, vblank should be a good time, although driver might also do
optimization e.g checking actual plane change between vblank to see if
there's any real change, etc. Also that will depend on driver
implementation.

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