Re: [RFC PATCH 6/6] drm/i915/gvt: support QEMU getting the dmabuf

[Alex Williamson <alex.williamson@redhat.com>]

On Fri, 12 May 2017 02:12:10 +0000
"Chen, Xiaoguang" <xiaoguang.chen@intel.com> wrote:

> Hi Alex and Gerd,
> 
> >-----Original Message-----
> >From: intel-gvt-dev [mailto:intel-gvt-dev-bounces@lists.freedesktop.org] On
> >Behalf Of Alex Williamson
> >Sent: Thursday, May 11, 2017 11:45 PM
> >To: Gerd Hoffmann <kraxel@redhat.com>
> >Cc: Tian, Kevin <kevin.tian@intel.com>; intel-gfx@lists.freedesktop.org; linux-
> >kernel@vger.kernel.org; zhenyuw@linux.intel.com; Lv, Zhiyuan
> ><zhiyuan.lv@intel.com>; Chen, Xiaoguang <xiaoguang.chen@intel.com>; intel-
> >gvt-dev@lists.freedesktop.org; Wang, Zhi A <zhi.a.wang@intel.com>
> >Subject: Re: [RFC PATCH 6/6] drm/i915/gvt: support QEMU getting the dmabuf
> >
> >On Thu, 11 May 2017 15:27:53 +0200
> >Gerd Hoffmann <kraxel@redhat.com> wrote:
> >  
> >>   Hi,
> >>  
> >> > While read the framebuffer region we have to tell the vendor driver which  
> >framebuffer we want to read? There are two framebuffers now in KVMGT that is
> >primary and cursor.  
> >> > There are two methods to implement this:
> >> > 1) write the plane id first and then read the framebuffer.
> >> > 2) create 2 vfio regions one for primary and one for cursor.  
> >>
> >> (3) Place information for both planes into one vfio region.
> >>     Which allows to fetch both with a single read() syscall.
> >>
> >> The question is how you'll get the file descriptor then.  If the ioctl
> >> returns the dma-buf fd only you have a racy interface:  Things can
> >> change between read(vfio-region) and ioctl(need-dmabuf-fd).
> >>
> >> ioctl(need-dma-buf) could return both dmabuf fd and plane info to fix
> >> the race, but then it is easier to go with ioctl only interface
> >> (simliar to the orginal one from dec last year) I think.  
> >
> >If the dmabuf fd is provided by a separate mdev vendor driver specific ioctl, I
> >don't see how vfio regions should be involved.  Selecting which framebuffer
> >should be an ioctl parameter.    
> Based on your last mail. I think the implementation looks like this:
> 1) user query the framebuffer information by reading the vfio region.
> 2) if the framebuffer changed(such as framebuffer's graphics address changed, size changed etc) we will need to create a new dmabuf fd.
> 3) create a new dmabuf fd using vfio device specific ioctl.
> 
> >What sort of information needs to be conveyed
> >about each plane?    
> Only plane id is needed.
> 
> >Is it static information or something that needs to be read
> >repeatedly?   
> It is static information. For our case plane id 1 represent primary plane and 3 for cursor plane. 2 means sprite plane which will not be used in our case.
> 
> >Do we need it before we get the dmabuf fd or can it be an ioctl on
> >the dmabuf fd?  
> We need it while query the framebuffer. In kernel we need the plane id to decide which plane we should decode.
> Below is my current implementation:
> 1) user first query the framebuffer(primary or cursor) and kernel decode the framebuffer and return the framebuffer information to user and also save a copy in kernel.
> 2) user compared the framebuffer and if the framebuffer changed  creating a new dmabuf fd.

If the contents of the framebuffer change or if the parameters of the
framebuffer change?  I can't image that creating a new dmabuf fd for
every visual change within the framebuffer would be efficient, but I
don't have any concept of what a dmabuf actually does.

> 3) kernel create a new dmabuf fd based on saved framebuffer information.
> 
> So we need plane id in step 1.
> In step 3 we create a dmabuf fd only using saved framebuffer information(no other information is needed).

What changes to the framebuffer require a new dmabuf fd?  Shouldn't the
user query the parameters of the framebuffer through a dmabuf fd and
shouldn't the dmabuf fd have some signaling mechanism to the user
(eventfd perhaps) to notify the user to re-evaluate the parameters?
Otherwise are you imagining that the user polls the vfio region?  Why
can a dmabuf fd not persist across changes to the framebuffer?  Can
someone explain what a dmabuf is and how it works in terms that a
non-graphics person can understand?  Thanks,

Alex