On 4 July 2018 at 18:00, Gerd Hoffmann kraxel@redhat.com wrote:
On Wed, Jul 04, 2018 at 09:26:39AM +0200, Tomeu Vizoso wrote:
On 07/04/2018 07:53 AM, Gerd Hoffmann wrote:
On Tue, Jul 03, 2018 at 10:37:57AM +0200, Daniel Vetter wrote:
On Tue, Jul 03, 2018 at 09:53:58AM +0200, Gerd Hoffmann wrote:
A driver to let userspace turn memfd regions into dma-bufs.
Use case: Allows qemu create dmabufs for the vga framebuffer or virtio-gpu ressources. Then they can be passed around to display those guest things on the host. To spice client for classic full framebuffer display, and hopefully some day to wayland server for seamless guest window display.
qemu test branch: https://git.kraxel.org/cgit/qemu/log/?h=sirius/udmabuf
Cc: David Airlie airlied@linux.ie Cc: Tomeu Vizoso tomeu.vizoso@collabora.com Cc: Laurent Pinchart laurent.pinchart@ideasonboard.com Cc: Daniel Vetter daniel@ffwll.ch Signed-off-by: Gerd Hoffmann kraxel@redhat.com
I think some ack for a 2nd use-case, like virtio-wl or whatever would be really cool. To give us some assurance that this is generically useful.
Tomeu? Laurent?
Sorry, but I think I will need some help to understand how this could help in the virtio-wl case [adding Zach Reizner to CC].
Any graphics buffers that are allocated with memfd will be shared with the compositor via wl_shm, without need for dmabufs.
Within one machine, yes. Once virtualization is added to the mix things become more complicated ...
When using virtio-gpu the guest will allocate graphics buffers from normal (guest) ram, then register these buffers (which are allowed to be scattered) with the host as resource.
qemu can use memfd to allocate guest ram. Now, with the help of udmabuf, qemu can create a *host* dma-buf for the *guest* graphics buffer.
That dma-buf can be used by qemu internally (mmap it to get a linear mapping of the resource, to avoid copying). It can be passed on to spice-client, to display the guest framebuffer.
And I think it could also be quite useful to pass guest wayland windows to the host compositor, without mapping host-allocated buffers into the guest, so we don't have do deal with the "find some address space for the mapping" issue in the first place. There are more things needed to complete this of course, but it's a building block ...
There is a use case where I think we have to deal with the "find some address space" problem. For GL4.4 ARB_buffer_storage and Vulkan memory mangement there is the concept of coherent buffers between GPU and CPU. From the virgl point of view, we'd create a host buffer in GL, and then create a mapping from it on the host that we'd need to present in the guest userspace as a linear buffer.
Just in case we think this can solve all our problems :-)
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