[Intel-gfx] [RFC PATCH 6/6] drm/i915/gvt: support QEMU getting the dmabuf

[Chen, Xiaoguang]

Hi Alex and Gerd,

>-----Original Message-----
>From: intel-gvt-dev [mailto:intel-gvt-dev-bounces at lists.freedesktop.org] On
>Behalf Of Alex Williamson
>Sent: Thursday, May 11, 2017 11:45 PM
>To: Gerd Hoffmann <kraxel at redhat.com>
>Cc: Tian, Kevin <kevin.tian at intel.com>; intel-gfx at lists.freedesktop.org; linux-
>kernel at vger.kernel.org; zhenyuw at linux.intel.com; Lv, Zhiyuan
><zhiyuan.lv at intel.com>; Chen, Xiaoguang <xiaoguang.chen at intel.com>; intel-
>gvt-dev at lists.freedesktop.org; Wang, Zhi A <zhi.a.wang at 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 at 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.
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).

Chenxg.
>Thanks,
>
>Alex
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