Integrated GPU passthrough support

Design document
Revision v3
Status released (6.5 SP1)
Review #33


Passthrough of discrete GPUs has been available since XenServer 6.0. With some extensions, we will also be able to support passthrough of integrated GPUs.

  • Whether an integrated GPU will be accessible to dom0 or available to passthrough to guests must be configurable via XenAPI.
  • Passthrough of an integrated GPU requires an extra flag to be sent to qemu.

Host Configuration

New fields will be added (both read-only):

  • PGPU.dom0_access enum(enabled|disable_on_reboot|disabled|enable_on_reboot)
  • host.display enum(enabled|disable_on_reboot|disabled|enable_on_reboot)

as well as new API calls used to modify the state of these fields:

  • PGPU.enable_dom0_access
  • PGPU.disable_dom0_access
  • host.enable_display
  • host.disable_display

Each of these API calls will return the new state of the field e.g. calling host.disable_display on a host with display = enabled will return disable_on_reboot.

Disabling dom0 access will modify the xen commandline (using the xen-cmdline tool) such that dom0 will not be able to access the GPU on next boot.

Calling host.disable_display will modify the xen and dom0 commandlines such that neither will attempt to send console output to the system display device.

A state diagram for the fields PGPU.dom0_access and host.display is shown below:

host.integrated_GPU_passthrough flow diagram

While it is possible for these two fields to be modified independently, a client must disable both the host display and dom0 access to the system display device before that device can be passed through to a guest.

Note that when a client enables or disables either of these fields, the change can be cancelled until the host is rebooted.

Handling vga_arbiter

Currently, xapi will not create a PGPU object for the PCI device with address reported by /dev/vga_arbiter. This is to prevent a GPU in use by dom0 from from being passed through to a guest. This behaviour will be changed - instead of not creating a PGPU object at all, xapi will create a PGPU, but its supported_VGPU_types field will be empty.

However, the PGPU’s supported_VGPU_types will be populated as normal if:

  1. dom0 access to the GPU is disabled.
  2. The host’s display is disabled.
  3. The vendor ID of the device is contained in a whitelist provided by xapi’s config file.

A read-only field will be added:

  • PGPU.is_system_display_device bool

This will be true for a PGPU iff /dev/vga_arbiter reports the PGPU as the system display device for the host on which the PGPU is installed.

Interfacing with xenopsd

When starting a VM attached to an integrated GPU, the VM config sent to xenopsd will contain a video_card of type IGD_passthrough. This will override the type determined from VM.platform:vga. xapi will consider a GPU to be integrated if both:

  1. It resides on bus 0.
  2. The vendor ID of the device is contained in a whitelist provided by xapi’s config file.

When xenopsd starts qemu for a VM with a video_card of type IGD_passthrough, it will pass the flags “-std-vga” AND “-gfx_passthru”.