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SubjectRe: [PATCH 0/1] drm/xen-zcopy: Add Xen zero-copy helper DRM driver
Yeah, I definitely agree on the idea of expanding the use case to the 
general domain where dmabuf sharing is used. However, what you are
targetting with proposed changes is identical to the core design of
hyper_dmabuf.

On top of this basic functionalities, hyper_dmabuf has driver level
inter-domain communication, that is needed for dma-buf remote tracking
(no fence forwarding though), event triggering and event handling, extra
meta data exchange and hyper_dmabuf_id that represents grefs
(grefs are shared implicitly on driver level)

Also it is designed with frontend (common core framework) + backend
(hyper visor specific comm and memory sharing) structure for portability.
We just can't limit this feature to Xen because we want to use the same
uapis not only for Xen but also other applicable hypervisor, like ACORN.

So I am wondering we can start with this hyper_dmabuf then modify it for
your use-case if needed and polish and fix any glitches if we want to
to use this for all general dma-buf usecases.

Also, I still have one unresolved question regarding the export/import flow
in both of hyper_dmabuf and xen-zcopy.

@danvet: Would this flow (guest1->import existing dmabuf->share underlying
pages->guest2->map shared pages->create/export dmabuf) be acceptable now?

Regards,
DW

On Mon, Apr 16, 2018 at 05:33:46PM +0300, Oleksandr Andrushchenko wrote:
> Hello, all!
>
> After discussing xen-zcopy and hyper-dmabuf [1] approaches
>
> it seems that xen-zcopy can be made not depend on DRM core any more
>
> and be dma-buf centric (which it in fact is).
>
> The DRM code was mostly there for dma-buf's FD import/export
>
> with DRM PRIME UAPI and with DRM use-cases in mind, but it comes out that if
>
> the proposed 2 IOCTLs (DRM_XEN_ZCOPY_DUMB_FROM_REFS and
> DRM_XEN_ZCOPY_DUMB_TO_REFS)
>
> are extended to also provide a file descriptor of the corresponding dma-buf,
> then
>
> PRIME stuff in the driver is not needed anymore.
>
> That being said, xen-zcopy can safely be detached from DRM and moved from
>
> drivers/gpu/drm/xen into drivers/xen/dma-buf-backend(?).
>
> This driver then becomes a universal way to turn any shared buffer between
> Dom0/DomD
>
> and DomU(s) into a dma-buf, e.g. one can create a dma-buf from any grant
> references
>
> or represent a dma-buf as grant-references for export.
>
> This way the driver can be used not only for DRM use-cases, but also for
> other
>
> use-cases which may require zero copying between domains.
>
> For example, the use-cases we are about to work in the nearest future will
> use
>
> V4L, e.g. we plan to support cameras, codecs etc. and all these will benefit
>
> from zero copying much. Potentially, even block/net devices may benefit,
>
> but this needs some evaluation.
>
>
> I would love to hear comments for authors of the hyper-dmabuf
>
> and Xen community, as well as DRI-Devel and other interested parties.
>
>
> Thank you,
>
> Oleksandr
>
>
> On 03/29/2018 04:19 PM, Oleksandr Andrushchenko wrote:
> >From: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
> >
> >Hello!
> >
> >When using Xen PV DRM frontend driver then on backend side one will need
> >to do copying of display buffers' contents (filled by the
> >frontend's user-space) into buffers allocated at the backend side.
> >Taking into account the size of display buffers and frames per seconds
> >it may result in unneeded huge data bus occupation and performance loss.
> >
> >This helper driver allows implementing zero-copying use-cases
> >when using Xen para-virtualized frontend display driver by
> >implementing a DRM/KMS helper driver running on backend's side.
> >It utilizes PRIME buffers API to share frontend's buffers with
> >physical device drivers on backend's side:
> >
> > - a dumb buffer created on backend's side can be shared
> > with the Xen PV frontend driver, so it directly writes
> > into backend's domain memory (into the buffer exported from
> > DRM/KMS driver of a physical display device)
> > - a dumb buffer allocated by the frontend can be imported
> > into physical device DRM/KMS driver, thus allowing to
> > achieve no copying as well
> >
> >For that reason number of IOCTLs are introduced:
> > - DRM_XEN_ZCOPY_DUMB_FROM_REFS
> > This will create a DRM dumb buffer from grant references provided
> > by the frontend
> > - DRM_XEN_ZCOPY_DUMB_TO_REFS
> > This will grant references to a dumb/display buffer's memory provided
> > by the backend
> > - DRM_XEN_ZCOPY_DUMB_WAIT_FREE
> > This will block until the dumb buffer with the wait handle provided
> > be freed
> >
> >With this helper driver I was able to drop CPU usage from 17% to 3%
> >on Renesas R-Car M3 board.
> >
> >This was tested with Renesas' Wayland-KMS and backend running as DRM master.
> >
> >Thank you,
> >Oleksandr
> >
> >Oleksandr Andrushchenko (1):
> > drm/xen-zcopy: Add Xen zero-copy helper DRM driver
> >
> > Documentation/gpu/drivers.rst | 1 +
> > Documentation/gpu/xen-zcopy.rst | 32 +
> > drivers/gpu/drm/xen/Kconfig | 25 +
> > drivers/gpu/drm/xen/Makefile | 5 +
> > drivers/gpu/drm/xen/xen_drm_zcopy.c | 880 ++++++++++++++++++++++++++++
> > drivers/gpu/drm/xen/xen_drm_zcopy_balloon.c | 154 +++++
> > drivers/gpu/drm/xen/xen_drm_zcopy_balloon.h | 38 ++
> > include/uapi/drm/xen_zcopy_drm.h | 129 ++++
> > 8 files changed, 1264 insertions(+)
> > create mode 100644 Documentation/gpu/xen-zcopy.rst
> > create mode 100644 drivers/gpu/drm/xen/xen_drm_zcopy.c
> > create mode 100644 drivers/gpu/drm/xen/xen_drm_zcopy_balloon.c
> > create mode 100644 drivers/gpu/drm/xen/xen_drm_zcopy_balloon.h
> > create mode 100644 include/uapi/drm/xen_zcopy_drm.h
> >
> [1]
> https://lists.xenproject.org/archives/html/xen-devel/2018-02/msg01202.html

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