Re: [PATCH v6] Add udmabuf misc device

[Tomeu Vizoso <tomeu.vizoso@collabora.com>]

On 07/04/2018 10:00 AM, Gerd Hoffmann 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.

Guess each physical address in the iovec in 
VIRTIO_GPU_CMD_RESOURCE_ATTACH_BACKING can be passed as the offset in the 
udmabuf_create_item struct?

> 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.

Sounds good to me if the answer to the above is "yes".

> There are more things needed to
> complete this of course, but it's a building block ...

Are you thinking of anything else besides passing the winsrv protocol 
across the guest/host boundary? Just wondering if I'm missing something.

Thanks,

Tomeu