Re: [RFC PATCH 6/6] drm/i915/gvt: support QEMU getting the dmabuf

From: Gerd Hoffmann <kraxel@redhat.com>
To: Alex Williamson <alex.williamson@redhat.com>
Cc: "Chen, Xiaoguang" <xiaoguang.chen@intel.com>,
	"Tian, Kevin" <kevin.tian@intel.com>,
	"intel-gfx@lists.freedesktop.org"
	<intel-gfx@lists.freedesktop.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"zhenyuw@linux.intel.com" <zhenyuw@linux.intel.com>,
	"Lv, Zhiyuan" <zhiyuan.lv@intel.com>,
	"intel-gvt-dev@lists.freedesktop.org" 
	<intel-gvt-dev@lists.freedesktop.org>,
	"Wang, Zhi A" <zhi.a.wang@intel.com>
Subject: Re: [RFC PATCH 6/6] drm/i915/gvt: support QEMU getting the dmabuf
Date: Fri, 05 May 2017 08:55:31 +0200	[thread overview]
Message-ID: <1493967331.371.53.camel@redhat.com> (raw)
In-Reply-To: <20170504100833.199bc8ba@t450s.home>

  Hi,

> > >>Hmm, that looks like a rather strange way to return a file descriptor.
> > >>
> > >>What is the reason to not use ioctls on the vfio file handle, like
> > >>older version of these patches did?  
> > >If I understood correctly that Alex prefer not to change the ioctls on the vfio file
> > >handle like the old version.
> > >So I used this way the smallest change to general vfio framework only adding a
> > >subregion definition.
> 
> I think I was hoping we could avoid a separate file descriptor
> altogether and use a vfio region instead.

What exactly did you have in mind?  Put the framebuffer information
(struct intel_vgpu_dmabuf) into the vfio region, then access it using
read/write/mmap?

> However, it was explained
> previously why this really needs to be a separate fd and I agree that
> using a region to expose an fd is really awkward.

Now with this patchset we have *two* kinds of separate file handles.
First the anon-fd created by reading from the region.  This is then used
to run the intel ioctls on, which in turn create the other kind of file
handle (dma-buf-fd).

The dma-buf-fd really needs to be a separate fd, because it gets passed
around as handle and because this is the way dma-bufs work (guess this
is the discussion you are referring to).

I can't see a compelling reason for the anon-fd though.  I suspect this
was done due to a misunderstanding ...

cheers,
  Gerd