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

[Dmitry Osipenko <dmitry.osipenko@collabora.com>]

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.

Summarizing:

  - only BLOB_MEM_GUEST resources require udmabuf
  - /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

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.

-- 
Best regards,
Dmitry