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Date: Thu, 30 Aug 2018 17:17:31 +0200 From: Tomeu Vizoso <tomeu.vizoso@...labora.com> To: Gerd Hoffmann <kraxel@...hat.com> Cc: dri-devel@...ts.freedesktop.org, David Airlie <airlied@...ux.ie>, Laurent Pinchart <laurent.pinchart@...asonboard.com>, Jonathan Corbet <corbet@....net>, Sumit Semwal <sumit.semwal@...aro.org>, Shuah Khan <shuah@...nel.org>, "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>, open list <linux-kernel@...r.kernel.org>, "open list:DMA BUFFER SHARING FRAMEWORK" <linux-media@...r.kernel.org>, "moderated list:DMA BUFFER SHARING FRAMEWORK" <linaro-mm-sig@...ts.linaro.org>, "open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>, Zach Reizner <zachr@...gle.com>, Daniel Stone <daniels@...labora.com> Subject: Re: [PATCH v6] Add udmabuf misc device 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@...ux.ie> >>>>> Cc: Tomeu Vizoso <tomeu.vizoso@...labora.com> >>>>> Cc: Laurent Pinchart <laurent.pinchart@...asonboard.com> >>>>> Cc: Daniel Vetter <daniel@...ll.ch> >>>>> Signed-off-by: Gerd Hoffmann <kraxel@...hat.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
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