Re: [Xen-devel] [PATCH 0/1] drm/xen-zcopy: Add Xen zero-copy helper DRM driver

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Apr 18, 2018 at 11:55:26AM +0100, Roger Pau Monné wrote:
> On Wed, Apr 18, 2018 at 01:39:35PM +0300, Oleksandr Andrushchenko wrote:
> > On 04/18/2018 01:18 PM, Paul Durrant wrote:
> > > > -----Original Message-----
> > > > From: Xen-devel [mailto:xen-devel-bounces@lists.xenproject.org] On Behalf
> > > > Of Roger Pau Monné
> > > > Sent: 18 April 2018 11:11
> > > > To: Oleksandr Andrushchenko <andr2000@gmail.com>
> > > > Cc: jgross@suse.com; Artem Mygaiev <Artem_Mygaiev@epam.com>;
> > > > Dongwon Kim <dongwon.kim@intel.com>; airlied@linux.ie;
> > > > Oleksandr_Andrushchenko@epam.com; linux-kernel@vger.kernel.org; dri-
> > > > devel@lists.freedesktop.org; Potrola, MateuszX
> > > > <mateuszx.potrola@intel.com>; xen-devel@lists.xenproject.org;
> > > > daniel.vetter@intel.com; boris.ostrovsky@oracle.com; Matt Roper
> > > > <matthew.d.roper@intel.com>
> > > > Subject: Re: [Xen-devel] [PATCH 0/1] drm/xen-zcopy: Add Xen zero-copy
> > > > helper DRM driver
> > > > 
> > > > On Wed, Apr 18, 2018 at 11:01:12AM +0300, Oleksandr Andrushchenko
> > > > wrote:
> > > > > On 04/18/2018 10:35 AM, Roger Pau Monné wrote:
> > > > After speaking with Oleksandr on IRC, I think the main usage of the
> > > > gntdev extension is to:
> > > > 
> > > > 1. Create a dma-buf from a set of grant references.
> > > > 2. Share dma-buf and get a list of grant references.
> > > > 
> > > > I think this set of operations could be broken into:
> > > > 
> > > > 1.1 Map grant references into user-space using the gntdev.
> > > > 1.2 Create a dma-buf out of a set of user-space virtual addresses.
> > > > 
> > > > 2.1 Map a dma-buf into user-space.
> > > > 2.2 Get grefs out of the user-space addresses where the dma-buf is
> > > >      mapped.
> > > > 
> > > > So it seems like what's actually missing is a way to:
> > > > 
> > > >   - Create a dma-buf from a list of user-space virtual addresses.
> > > >   - Allow to map a dma-buf into user-space, so it can then be used with
> > > >     the gntdev.
> > > > 
> > > > I think this is generic enough that it could be implemented by a
> > > > device not tied to Xen. AFAICT the hyper_dma guys also wanted
> > > > something similar to this.
> > Ok, so just to summarize, xen-zcopy/hyper-dmabuf as they are now,
> > are no go from your POV?
> 
> My opinion is that there seems to be a more generic way to implement
> this, and thus I would prefer that one.
> 
> > Instead, we have to make all that fancy stuff
> > with VAs <-> device-X and have that device-X driver live out of drivers/xen
> > as it is not a Xen specific driver?
> 
> That would be my preference if feasible, simply because it can be
> reused by other use-cases that need to create dma-bufs in user-space.
> 
> In any case I just knew about dma-bufs this morning, there might be
> things that I'm missing.

See my other reply, I really don't want a generic userspace memory ->
dma-buf thing, it doesn't work. Having a xen-specific grant references <->
dma-buf conversion thing (and I'm honestly still not sure whether the
dma-buf -> grant renferences is a good idea) seems much more reasonable to
me.

Imo simplest way forward would be to start out with a grant refs ->
dma-buf ioctl, and see how far that gets us.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch