[Intel-gfx] [PATCH v14 5/7] vfio: ABI for mdev display dma-buf operation

[Zhang, Tina]

> -----Original Message-----
> From: Gerd Hoffmann [mailto:kraxel at redhat.com]
> Sent: Friday, September 29, 2017 6:21 PM
> To: Zhang, Tina <tina.zhang at intel.com>; zhenyuw at linux.intel.com; Wang, Zhi
> A <zhi.a.wang at intel.com>; Tian, Kevin <kevin.tian at intel.com>; Alex
> Williamson <alex.williamson at redhat.com>
> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>; intel-gfx at lists.freedesktop.org;
> intel-gvt-dev at lists.freedesktop.org; Lv, Zhiyuan <zhiyuan.lv at intel.com>
> Subject: Re: [PATCH v14 5/7] vfio: ABI for mdev display dma-buf operation
> 
>   Hi,
> 
> > For example, if the old reused dmabuf_obj is released just after query
> > ioctl return it,  the next get_fd ioctl would return error as the
> > dmabuf_obj has already been closed.
> 
> My branch already grabs an extra reference when creating a new dmabuf_obj,
> which will be dropped on GET_DMABUF ioctl, exactly to avoid the dmabuf_obj
> disappear between QUERY_PLANE and GET_DMABUF ioctls.
Yeah, that could solve the problem. But I'm not sure if it could be acceptable.
Zhenyu, can you share your comments?
Thanks.

BR,
Tina

> 
> Can easily be extended to handle the reuse case too.
> 
> https://www.kraxel.org/cgit/linux/commit/?h=gvt-dmabuf-v14&id=9959109ae
> 52cf15e119715a6b7de080fb849e3d2
> 
> While being at it also cleanup properly on close (so we don't leak structs in case
> userspace never calls GET_DMABUF for a plane).
> 
> https://www.kraxel.org/cgit/linux/commit/?h=gvt-dmabuf-v14&id=c0b0c407e
> 33904e749dec1ef44ec01099c16d39f
> 
> > > > Do you think the fd interface is enough for all kinds of buffer
> > > > exposed by Mdev?
> > >
> > > What kind of buffers do you have in mind which might not be covered?
> >
> > I thinking about the case that would like to postpone the buffers
> > releasing operation, after user space has closed all the fd.
> 
> Work fine.  qemu can import the dma-buf as opengl texture, which creates a
> extra reference.  Then close the fd.  dma-buf continues to exist as long as the
> texture referencing it exists.
> 
> > Later these buffers may be used to expose to other kinds of fd to user
> > space.
> 
> Sorry, I don't understand that sentence.
> 
> cheers,
>   Gerd