From: Oleksandr Andrushchenko <oleksandr_andrushche...@epam.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.
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 ballooning in/out 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 other memory, e.g. system pages). 2. Extension of the gntdev driver to enable it to import/export dma-buf’s. The first two patches in this series solve #1 and the last one is for #2. 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. *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 Oleksandr Andrushchenko (3): xen/balloon: Allow allocating DMA buffers xen/grant-table: Extend API to work with DMA buffers xen/gntdev: Add support for Linux dma buffers drivers/xen/balloon.c | 214 +++++++-- drivers/xen/gntdev.c | 954 +++++++++++++++++++++++++++++++++++++- drivers/xen/grant-table.c | 49 ++ drivers/xen/xen-balloon.c | 2 + include/uapi/xen/gntdev.h | 101 ++++ include/xen/balloon.h | 11 +- include/xen/gntdev_exp.h | 23 + include/xen/grant_table.h | 7 + 8 files changed, 1310 insertions(+), 51 deletions(-) create mode 100644 include/xen/gntdev_exp.h -- 2.17.0
