Message-ID: <c5ef52fe-3a71-7a90-0723-6dcb18c564c7@epam.com>
Date:   Mon, 18 Jun 2018 09:11:33 +0300
From:   Oleksandr Andrushchenko <Oleksandr_Andrushchenko@...m.com>
To:     jgross@...e.com, boris.ostrovsky@...cle.com
Cc:     Oleksandr Andrushchenko <andr2000@...il.com>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-media@...r.kernel.org,
        konrad.wilk@...cle.com, daniel.vetter@...el.com,
        dongwon.kim@...el.com, matthew.d.roper@...el.com
Subject: Re: [PATCH v4 0/9] xen: dma-buf support for grant device

Boris, Juergen!

Thank you so much for your comments and time spent on this
series. Appreciate that very much!

Thank you,
Oleksandr

On 06/15/2018 09:27 AM, Oleksandr Andrushchenko wrote:
> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>
>
> This work is in response to my previous attempt to introduce Xen/DRM
> zero-copy driver [1] to enable Linux dma-buf API [2] for Xen based
> frontends/backends. There is also an existing hyper_dmabuf approach
> available [3] which, if reworked to utilize the proposed solution,
> can greatly benefit as well.
>
> RFC for this series was published and discussed [9], comments addressed.
>
> The original rationale behind this work was to enable zero-copying
> use-cases while working with Xen para-virtual display driver [4]:
> 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
> second it may result in unneeded huge data bus occupation and
> performance loss.
>
> The helper driver [4] 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 (implemented on top of Linux dma-buf)
> 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
>
> Finally, it was discussed and decided ([1], [5]) that it is worth
> implementing such use-cases via extension of the existing Xen gntdev
> driver instead of introducing new DRM specific driver.
> Please note, that the support of dma-buf is Linux only,
> as dma-buf is a Linux only thing.
>
> Now to the proposed solution. The changes  to the existing Xen drivers
> in the Linux kernel fall into 2 categories:
> 1. DMA-able memory buffer allocation and increasing/decreasing memory
>     reservation of the pages of such a buffer.
>     This is required if we are about to share dma-buf with the hardware
>     that does require those to be allocated with dma_alloc_xxx API.
>     (It is still possible to allocate a dma-buf from any system memory,
>     e.g. system pages).
> 2. Extension of the gntdev driver to enable it to import/export dma-buf’s.
>
> The first six patches are in preparation for Xen dma-buf support,
> but I consider those usable regardless of the dma-buf use-case,
> e.g. other frontend/backend kernel modules may also benefit from these
> for better code reuse:
>      0001-xen-grant-table-Export-gnttab_-alloc-free-_pages-as-.patch
>      0002-xen-grant-table-Make-set-clear-page-private-code-sha.patch
>      0003-xen-balloon-Share-common-memory-reservation-routines.patch
>      0004-xen-grant-table-Allow-allocating-buffers-suitable-fo.patch
>      0005-xen-gntdev-Allow-mappings-for-DMA-buffers.patch
>      0006-xen-gntdev-Make-private-routines-structures-accessib.patch
>
> The next three patches are Xen implementation of dma-buf as part of
> the grant device:
>      0007-xen-gntdev-Add-initial-support-for-dma-buf-UAPI.patch
>      0008-xen-gntdev-Implement-dma-buf-export-functionality.patch
>      0009-xen-gntdev-Implement-dma-buf-import-functionality.patch
>
> The corresponding libxengnttab changes are available at [6].
>
> All the above was tested with display backend [7] and its accompanying
> helper library [8] on Renesas ARM64 based board.
> Basic balloon tests on x86.
>
> *To all the communities*: I would like to ask you to review the proposed
> solution and give feedback on it, so I can improve and send final
> patches for review (this is still work in progress, but enough to start
> discussing the implementation).
>
> Thank you in advance,
> Oleksandr Andrushchenko
>
> [1] https://lists.freedesktop.org/archives/dri-devel/2018-April/173163.html
> [2] https://elixir.bootlin.com/linux/v4.17-rc5/source/Documentation/driver-api/dma-buf.rst
> [3] https://lists.xenproject.org/archives/html/xen-devel/2018-02/msg01202.html
> [4] https://cgit.freedesktop.org/drm/drm-misc/tree/drivers/gpu/drm/xen
> [5] https://patchwork.kernel.org/patch/10279681/
> [6] https://github.com/andr2000/xen/tree/xen_dma_buf_v1
> [7] https://github.com/andr2000/displ_be/tree/xen_dma_buf_v1
> [8] https://github.com/andr2000/libxenbe/tree/xen_dma_buf_v1
> [9] https://lkml.org/lkml/2018/5/17/215
>
> Changes since v3:
> *****************
> - added r-b tags
> - minor fixes
> - removed gntdev_remove_map as it can be coded directly now
> - moved IOCTL code to gntdev-dmabuf.c
> - removed usless wait list walks and changed some walks to use
>    normal version of list iterators instead of safe ones as
>    we run under a lock anyways
> - cleaned up comments, descriptions, pr_debug messages
>
> Changes since v2:
> *****************
> - fixed missed break in dmabuf_exp_wait_obj_signal
> - re-worked debug and error messages, be less verbose
> - removed patch for making gntdev functions available to other drivers
> - removed WARN_ON's in dma-buf code
> - moved all dma-buf related code into gntdev-dmabuf
> - introduced gntdev-common.h with common structures and function prototypes
> - added additional checks for number of grants in IOCTLs
> - gnttab patch cleanup
> - made xenmem_reservation_scrub_page defined in the header as inline
> - fixed __pfn_to_mfn use to pfn_to_bfn
> - no changes to patches 1-2
>
> Changes since v1:
> *****************
> - Define GNTDEV_DMA_FLAG_XXX starting from bit 0
> - Rename mem_reservation.h to mem-reservation.h
> - Remove usless comments
> - Change licenses from GPLv2 OR MIT to GPLv2 only
> - Make xenmem_reservation_va_mapping_{update|clear} inline
> - Change EXPORT_SYMBOL to EXPORT_SYMBOL_GPL for new functions
> - Make gnttab_dma_{alloc|free}_pages to request frames array
>    be allocated outside
> - Fixe gnttab_dma_alloc_pages fail path (added xenmem_reservation_increase)
> - Move most of dma-buf from gntdev.c to gntdev-dmabuf.c
> - Add required dependencies to Kconfig
> - Rework "#ifdef CONFIG_XEN_XXX" for if/else
> - Export gnttab_{alloc|free}_pages as GPL symbols (patch 1)
>
> Oleksandr Andrushchenko (9):
>    xen/grant-table: Export gnttab_{alloc|free}_pages as GPL
>    xen/grant-table: Make set/clear page private code shared
>    xen/balloon: Share common memory reservation routines
>    xen/grant-table: Allow allocating buffers suitable for DMA
>    xen/gntdev: Allow mappings for DMA buffers
>    xen/gntdev: Make private routines/structures accessible
>    xen/gntdev: Add initial support for dma-buf UAPI
>    xen/gntdev: Implement dma-buf export functionality
>    xen/gntdev: Implement dma-buf import functionality
>
>   drivers/xen/Kconfig           |  24 +
>   drivers/xen/Makefile          |   2 +
>   drivers/xen/balloon.c         |  75 +--
>   drivers/xen/gntdev-common.h   |  94 ++++
>   drivers/xen/gntdev-dmabuf.c   | 870 ++++++++++++++++++++++++++++++++++
>   drivers/xen/gntdev-dmabuf.h   |  33 ++
>   drivers/xen/gntdev.c          | 220 ++++++---
>   drivers/xen/grant-table.c     | 153 +++++-
>   drivers/xen/mem-reservation.c | 118 +++++
>   include/uapi/xen/gntdev.h     | 106 +++++
>   include/xen/grant_table.h     |  21 +
>   include/xen/mem-reservation.h |  59 +++
>   12 files changed, 1615 insertions(+), 160 deletions(-)
>   create mode 100644 drivers/xen/gntdev-common.h
>   create mode 100644 drivers/xen/gntdev-dmabuf.c
>   create mode 100644 drivers/xen/gntdev-dmabuf.h
>   create mode 100644 drivers/xen/mem-reservation.c
>   create mode 100644 include/xen/mem-reservation.h
>

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