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

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Date:   Thu, 27 Jun 2019 12:31:33 +0200
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     "Zhang, Tina" <tina.zhang@...el.com>
Cc:     "intel-gvt-dev@...ts.freedesktop.org" 
        <intel-gvt-dev@...ts.freedesktop.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "zhenyuw@...ux.intel.com" <zhenyuw@...ux.intel.com>,
        "Lv, Zhiyuan" <zhiyuan.lv@...el.com>,
        "Wang, Zhi A" <zhi.a.wang@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        "Yuan, Hang" <hang.yuan@...el.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>
Subject: Re: [RFC PATCH v3 0/4] Deliver vGPU display vblank event to userspace

> >   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.

cheers,
  Gerd