drm/xen-front Xen para-virtualized frontend driver

This frontend driver implements Xen para-virtualized display according to the display protocol described at include/xen/interface/io/displif.h

Driver modes of operation in terms of display buffers used

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, display buffers can be allocated by either frontend driver or backend.

Buffers allocated by the frontend driver

In this mode of operation driver allocates 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.

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:

1. If guest domain dies then pages/grants received from the backend cannot be claimed back
2. Misbehaving guest may send too many requests to the backend exhausting its grant references and memory (consider this from security POV)

Driver limitations

1. Only primary plane without additional properties is supported.
2. Only one video mode per connector supported which is configured via XenStore.
3. All CRTCs operate at fixed frequency of 60Hz.