From: Oleksandr Andrushchenko oleksandr_andrushchenko@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.
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/d... [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