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

[Oleksandr Andrushchenko <andr2000@gmail.com>]

On 04/18/2018 07:01 PM, Dongwon Kim wrote:
> On Wed, Apr 18, 2018 at 03:42:29PM +0300, Oleksandr Andrushchenko wrote:
>> On 04/18/2018 01:55 PM, 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?
> FYI,
>
> our use-case is "surface sharing" or "graphic obj sharing" where a client
> application in one guest renders and export this render target(e.g. EGL surface)
> as dma-buf. This dma-buf is then exported to another guest/host via hyper_dmabuf
> drv where a compositor is running. This importing domain creates a dmabuf with
> shared reference then it is imported as EGL image that later can be used as
> texture object via EGL api.

>   Mapping dmabuf to the userspace or vice versa
> might be possible with modifying user space drivers/applications but it is an
> unnecessary extra step from our perspective.
+1. I also feel like if it is implemented in the kernel space it
will be *much* more easier then inventing workarounds with
gntdev, user-space and helper dma-buf driver (which obviously can be
implemented). Of course, this approach is easier for Xen as we do not
touch its kernel code ;)
But there is a demand for changes as number of embedded/multimedia use-cases
is constantly growing and we have to react.
> Also, we want to keep all objects
> in the kernel level.
>
>>> 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.
>> There is a use-case I have: a display unit on my target has a DMA
>> controller which can't do scatter-gather, e.g. it only expects a
>> single starting address of the buffer.
>> In order to create a dma-buf from grefs in this case
>> I allocate memory with dma_alloc_xxx and then balloon pages of the
>> buffer and finally map grefs onto this DMA buffer.
>> This way I can give this shared buffer to the display unit as its bus
>> addresses are contiguous.
>>
>> With the proposed solution (gntdev + device-X) I won't be able to achieve
>> this,
>> as I have no control over from where gntdev/balloon drivers get the pages
>> (even more, those can easily be out of DMA address space of the display
>> unit).
>>
>> Thus, even if implemented, I can't use this approach.
>>> In any case I just knew about dma-bufs this morning, there might be
>>> things that I'm missing.
>>>
>>> Roger.