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

[Gerd Hoffmann <kraxel@redhat.com>]

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

>   - /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.

take care,
  Gerd