[RFC v3 2/3] virtio: Introduce Vdmabuf driver

[Kasireddy, Vivek]

Hi Gerd,

> 
> On Fri, Feb 12, 2021 at 08:15:12AM +0000, Kasireddy, Vivek wrote:
> > Hi Gerd,
> > [Kasireddy, Vivek] Just to confirm my understanding of what you are
> > suggesting, are you saying that we need to either have Weston allocate
> > scanout buffers (GBM surface/BO) using virtio-gpu and render into them
> > using i915; or have virtio-gpu allocate pages and export a dma-buf and
> > have Weston create a GBM BO by calling gbm_bo_import(fd) and render into the BO
> using i915?
> 
> Not sure what the difference between the former and the latter is.
[Kasireddy, Vivek] Oh, what I meant is whether you were suggesting that we 
create a GBM device and create a GBM surface and BOs using this device or
just create a raw/dumb GEM object and create a GBM BO by importing it. As
we just discovered, the former means we have to initialize virgl which complicates
things so we went with the latter.

> 
> > [Kasireddy, Vivek] We are only interested in Qemu UI at the moment but
> > if we were to use virtio-gpu, we are going to need to add one more vq
> > and support for managing buffers, events, etc.
> 
> Should be easy and it should not need any virtio-gpu driver changes.
[Kasireddy, Vivek] Vdmabuf v4, that implements your suggestion -- to have
Vdmabuf allocate pages --  is posted here:
https://lists.freedesktop.org/archives/dri-devel/2021-February/297841.html
and tested it with Weston Headless and Qemu:
https://gitlab.freedesktop.org/Vivek/weston/-/blob/vdmabuf/libweston/backend-headless/headless.c#L522
https://lists.nongnu.org/archive/html/qemu-devel/2021-02/msg02976.html

Having said that, after discussing with Daniel Vetter, we are now switching our
focus to virtio-gpu to compare and contrast both solutions. 

> 
> You can use virtio-gpu like a dumb scanout device.  Create a dumb bo, create a
> framebuffer for the bo, map the framebuffer to the crtc.
> 
> Then export the bo, import into i915, use it as render target.  When rendering is done flush
> (DRM_IOCTL_MODE_DIRTYFB).  Alternatively allocate multiple bo's + framebuffers
> and pageflip.
[Kasireddy, Vivek] Since we are testing with Weston, we are looking at pageflips (4 color
buffers). And, this part so far seems to work where virtio-gpu is used for kms (max_outputs=1)
and Iris/i915 is used for rendering. We are currently glueing virtio-gpu and i915 in Weston but
eventually the plan is to glue them (virgl/virtio-gpu and Iris) in Mesa if possible using KMSRO
(KMS render only) to avoid having to change Weston or X or other user-space components.

> 
> Pretty standard workflow for cases where rendering and scanout are handled by different
> devices.  As far I know not uncommon in the arm world.
> 
> Right now this will involve a memcpy() for any display update because qemu is a bit
> behind on supporting recent virtio-gpu features.
[Kasireddy, Vivek] IIUC, I think you are referring to creating the Pixman image in set_scanout.
What additional features need to be implemented or what is your recommendation in terms of
what needs to be done to turn the memcpy() into a dma-buf? Also, how should we ensure that
access to the guest fb/dmabuf is synchronized to ensure that the Guest and the Host do not access
the backing storage of the dmabuf at the same time?

Thanks,
Vivek

> 
> take care,
>   Gerd