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    • Oleksandr Andrushchenko's avatar
    • Oleksandr Andrushchenko's avatar
      drm/xen-front: Add support for Xen PV display frontend · c575b7ee
      Oleksandr Andrushchenko authored
      Add support for Xen para-virtualized frontend display driver.
      Accompanying backend [1] is implemented as a user-space application
      and its helper library [2], capable of running as a Weston client
      or DRM master.
      Configuration of both backend and frontend is done via
      Xen guest domain configuration options [3].
      
      Driver limitations:
       1. Only primary plane without additional properties is supported.
       2. Only one video mode supported which resolution is configured
          via XenStore.
       3. All CRTCs operate at fixed frequency of 60Hz.
      
      1. Implement Xen bus state machine for the frontend driver according to
      the state diagram and recovery flow from display para-virtualized
      protocol: xen/interface/io/displif.h.
      
      2. Read configuration values from Xen store according
      to xen/interface/io/displif.h protocol:
        - read connector(s) configuration
        - read buffer allocation mode (backend/frontend)
      
      3. Handle Xen event channels:
        - create for all configured connectors and publish
          corresponding ring references and event channels in Xen store,
          so backend can connect
        - implement event channels interrupt handlers
        - create and destroy event channels with respect to Xen bus state
      
      4. Implement shared buffer handling according to the
      para-virtualized display device protocol at xen/interface/io/displif.h:
        - handle page directories according to displif protocol:
          - allocate and share page directories
          - grant references to the required set of pages for the
            page directory
        - allocate xen balllooned pages via Xen balloon driver
          with alloc_xenballooned_pages/free_xenballooned_pages
        - grant references to the required set of pages for the
          shared buffer itself
        - implement pages map/unmap for the buffers allocated by the
          backend (gnttab_map_refs/gnttab_unmap_refs)
      
      5. Implement kernel modesetiing/connector handling using
      DRM simple KMS helper pipeline:
      
      - implement KMS part of the driver with the help of DRM
        simple pipepline helper which is possible due to the fact
        that the para-virtualized driver only supports a single
        (primary) plane:
        - initialize connectors according to XenStore configuration
        - handle frame done events from the backend
        - create and destroy frame buffers and propagate those
          to the backend
        - propagate set/reset mode configuration to the backend on display
          enable/disable callbacks
        - send page flip request to the backend and implement logic for
          reporting backend IO errors on prepare fb callback
      
      - implement virtual connector handling:
        - support only pixel formats suitable for single plane modes
        - make sure the connector is always connected
        - support a single video mode as per para-virtualized driver
          configuration
      
      6. Implement GEM handling depending on driver mode of operation:
      depending on the requirements for the para-virtualized environment,
      namely requirements dictated by the accompanying DRM/(v)GPU drivers
      running in both host and guest environments, number of operating
      modes of para-virtualized display driver are supported:
       - display buffers can be allocated by either
         frontend driver or backend
       - display buffers can be allocated to be contiguous
         in memory or not
      
      Note! Frontend driver itself has no dependency on contiguous memory for
      its operation.
      
      6.1. Buffers allocated by the frontend driver.
      
      The below modes of operation are configured at compile-time via
      frontend driver's kernel configuration.
      
      6.1.1. Front driver configured to use GEM CMA helpers
           This use-case is useful when used with accompanying DRM/vGPU driver
           in guest domain which was designed to only work with contiguous
           buffers, e.g. DRM driver based on GEM CMA helpers: such drivers can
           only import contiguous PRIME buffers, thus requiring frontend driver
           to provide such. In order to implement this mode of operation
           para-virtualized frontend driver can be configured to use
           GEM CMA helpers.
      
      6.1.2. Front driver doesn't use GEM CMA
           If accompanying drivers can cope with non-contiguous memory then, to
           lower pressure on CMA subsystem of the kernel, driver can allocate
           buffers from system memory.
      
      Note! If used with accompanying DRM/(v)GPU drivers this mode of operation
      may require IOMMU support on the platform, so accompanying DRM/vGPU
      hardware can still reach display buffer memory while importing PRIME
      buffers from the frontend driver.
      
      6.2. Buffers allocated by the backend
      
      This mode of operation is run-time configured via guest domain
      configuration through XenStore entries.
      
      For systems which do not provide IOMMU support, but having specific
      requirements for display buffers it is possible to allocate such buffers
      at backend side and share those with the frontend.
      For example, if host domain is 1:1 mapped and has DRM/GPU hardware
      expecting physically contiguous memory, this allows implementing
      zero-copying use-cases.
      
      Note, while using this scenario the following should be considered:
        a) If guest domain dies then pages/grants received from the backend
           cannot be claimed back
        b) Misbehaving guest may send too many requests to the
           backend exhausting its grant references and memory
           (consider this from security POV).
      
      Note! Configuration options 1.1 (contiguous display buffers) and 2
      (backend allocated buffers) are not supported at the same time.
      
      7. Handle communication with the backend:
       - send requests and wait for the responses according
         to the displif protocol
       - serialize access to the communication channel
       - time-out used for backend communication is set to 3000 ms
       - manage display buffers shared with the backend
      
      [1] https://github.com/xen-troops/displ_be
      [2] https://github.com/xen-troops/libxenbe
      [3] https://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=docs/man/xl.cfg.pod.5.in;h=a699367779e2ae1212ff8f638eff0206ec1a1cc9;hb=refs/heads/master#l1257Signed-off-by: default avatarOleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
      Reviewed-by: default avatarBoris Ostrovsky <boris.ostrovsky@oracle.com>
      Reviewed-by: default avatarDaniel Vetter <daniel.vetter@ffwll.ch>
      Link: https://patchwork.freedesktop.org/patch/msgid/20180403112317.28751-2-andr2000@gmail.com
      c575b7ee
  10. 30 Mar, 2018 2 commits
  11. 29 Mar, 2018 6 commits