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

[Zhenyu Wang]

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? 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.

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Open Source Technology Center, Intel ltd.

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