-----Original Message-----
From: dri-devel <redacted> On Behalf Of Gerd
Hoffmann
Sent: Friday, February 12, 2021 7:02 PM
To: Kasireddy, Vivek <vivek.kasireddy@intel.com>
Cc: Kim, Dongwon <redacted>; christian.koenig@amd.com;
daniel.vetter@ffwll.ch; dri-devel@lists.freedesktop.org;
virtualization@lists.linux-foundation.org; Vetter, Daniel
[off-list ref]; linux-media@vger.kernel.org
Subject: Re: [RFC v3 2/3] virtio: Introduce Vdmabuf driver

On Fri, Feb 12, 2021 at 08:15:12AM +0000, Kasireddy, Vivek wrote:

quoted

Hi Gerd,

quoted

quoted

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You don't have to use the rendering pipeline.  You can let the
i915 gpu render into a dma-buf shared with virtio-gpu, then use
virtio-gpu only for buffer sharing with the host.

[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.

quoted

quoted

Hmm, why a big mode switch?  You should be able to do that without
modifying the virtio-gpu guest driver.  On the host side qemu needs
some work to support the most recent virtio-gpu features like the
buffer uuids (assuming you use qemu userspace), right now those are only

supported by crosvm.

quoted

[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.

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.

Hi,

We've got a MR(https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9592) for this suggested implementation. Comments are welcome. Thanks.

BR,
Tina

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.

take care,
  Gerd

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