RE: [PATCH v2 4/4] virtio-gpu: Don't require udmabuf when blob support is enabled

[Kasireddy, Vivek]

Hi Dmitry,

> 
> On 9/27/22 11:32, Gerd Hoffmann wrote:
> > On Mon, Sep 26, 2022 at 09:32:40PM +0300, Dmitry Osipenko wrote:
> >> On 9/23/22 15:32, Gerd Hoffmann wrote:
> >>> On Tue, Sep 13, 2022 at 12:50:22PM +0200, Antonio Caggiano wrote:
> >>>> From: Dmitry Osipenko <dmitry.osipenko@collabora.com>
> >>>>
> >>>> Host blobs don't need udmabuf, it's only needed by guest blobs. The host
> >>>> blobs are utilized by the Mesa virgl driver when persistent memory 
> >>>> mapping
> >>>> is needed by a GL buffer, otherwise virgl driver doesn't use blobs.
> >>>> Persistent mapping support bumps GL version from 4.3 to 4.5 in guest.
> >>>> Relax the udmabuf requirement.
> >>>
> >>> What about blob=on,virgl=off?
> >>>
> >>> In that case qemu manages the resources and continued to require
> >>> udmabuf.
> >>
> >> The udmabuf is used only by the blob resource-creation command in Qemu.
> >> I couldn't find when we could hit that udmabuf code path in Qemu because
> >> BLOB_MEM_GUEST resource type is used only by crosvm+Venus when crosvm
> >> uses a dedicated render-server for virglrenderer.
> >
> > Recent enough linux guest driver will use BLOB_MEM_GUEST resources
> > with blob=on + virgl=off
> 
> I reproduced this case today using "-device
> virtio-gpu-device,blob=true". You're right, it doesn't work without udmabuf.
> 
> [    8.369306] virtio_gpu_dequeue_ctrl_func: 90 callbacks suppressed
> [    8.369311] [drm:virtio_gpu_dequeue_ctrl_func] *ERROR* response
> 0x1200 (command 0x105)
> [    8.371848] [drm:virtio_gpu_dequeue_ctrl_func] *ERROR* response
> 0x1205 (command 0x104)
> [    8.373085] [drm:virtio_gpu_dequeue_ctrl_func] *ERROR* response
> 0x1200 (command 0x105)
> [    8.376273] [drm:virtio_gpu_dequeue_ctrl_func] *ERROR* response
> 0x1205 (command 0x104)
> [    8.416972] [drm:virtio_gpu_dequeue_ctrl_func] *ERROR* response
> 0x1200 (command 0x105)
> [    8.418841] [drm:virtio_gpu_dequeue_ctrl_func] *ERROR* response
> 0x1205 (command 0x104)
> 
> I see now why you're wanting to keep the udmabuf requirement for
> blob=on,virgl=off.
> 
> 
> >>   - /dev/udmabuf isn't accessible by normal user
> >>   - udmabuf driver isn't shipped by all of the popular Linux distros,
> >> for example Debian doesn't ship it
> >
> > That's why blob resources are off by default.
> >
> >> Because of all of the above, I don't think it makes sense to
> >> hard-require udmabuf at the start of Qemu. It's much better to fail
> >> resource creation dynamically.
> >
> > Disagree.  When virgl/venus is enabled, then yes, qemu would let
> > virglrenderer manage resources and I'm ok with whatever requirements
> > virglrenderer has.  When qemu manages resources by itself udmabuf is
> > a hard requirement for blob support though.
> 
> Let's try to relax the udmabuf requirement only for blob=on,virgl=on.
> I'll update this patch.
[Kasireddy, Vivek] In addition to what Gerd mentioned and what you have
discovered, I wanted to briefly add that we use Udmabuf/Guest Blobs for
Headless GPU passthrough use-cases (blob=on and virgl=off). Here are some
more details about our use-case:
https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9592#note_851314

Thanks,
Vivek

> 
> Thank you very much for the review!
> 
> --
> Best regards,
> Dmitry
>