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Virtual Function I/O Mediated devices (vfio-mdev) Create and Configure Virtual Function I/O Mediated devices (vfio-mdev) true 2025-03-31T16:08:44.405Z markdown 2022-07-21T21:10:41.046Z

vfio-mdev configuration

These instructions only cover Intel GPUs that are compatible with vfio-mdev (5th to 10th generation). Since generation 11th, vfio-mdev has been superseded by SR-IOV. {.is-warning}

Virtual Function I/O Mediated devices (vfio-mdev) allows a phyiscal GPU to be split into multiple virtual GPUs (vGPU). Such a vGPU can then be assigned to a virtual machine or a container.

Contrary to paravirtualized GPUs (e.g. virtio-gpu), vGPUs can use the same driver as their parent GPU (e.g. a guest compatible with an Intel GPUs will be able to leverage an Intel-based vGPUs)

Preparation

  • Install the Phyllome OS Desktop II

  • Make sure the GRUB has been updated after the first boot: # grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg

Procedure

Create a virtual GPU

List all available vGPUs types:

$ mdevctl types

0000:00:02.0
  i915-GVTg_V5_1
    Available instances: 1
    Device API: vfio-pci
    Description: low_gm_size: 512MB, high_gm_size: 2048MB, fence: 4, resolution: 1920x1200, weight: 16
  i915-GVTg_V5_2
    Available instances: 2
    Device API: vfio-pci
    Description: low_gm_size: 256MB, high_gm_size: 1024MB, fence: 4, resolution: 1920x1200, weight: 8
  i915-GVTg_V5_4
    Available instances: 5
    Device API: vfio-pci
    Description: low_gm_size: 128MB, high_gm_size: 512MB, fence: 4, resolution: 1920x1200, weight: 4
  i915-GVTg_V5_8
    Available instances: 7
    Device API: vfio-pci
    Description: low_gm_size: 64MB, high_gm_size: 384MB, fence: 4, resolution: 1024x768, weight: 2

Allocating more memory to the GPU in the platform firmware may increase the number of vGPUs one can create. See Troubleshooting section below. {.is-info}

In the example above, the i915-GVTg_V5_4 virtual type seems to offer the best trade-offs between the available resolution and the number of available instances.

  • Generate a universally unique identifier (UUID) with the following command:
$ uuidgen

$ 7686131b-b229-4768-a02c-35d1dbed7c66
  • Start a vGPU based on the kind i915-GVTg_V5_4 using the previously generated UUID
# mdevctl start -u 7686131b-b229-4768-a02c-35d1dbed7c66 -p 0000:00:02.0 --type i915-GVTg_V5_4
  • Define, or make this vGPU permanent:
# mdevctl define -u 7686131b-b229-4768-a02c-35d1dbed7c66
  • Set the vGPU to auto-start after the host boots up:
# mdevctl modify -u 7686131b-b229-4768-a02c-35d1dbed7c66 --auto
  • Finally, verify that the vGPU has successfully been created and is set to auto-start:
$ mdevctl list -d

$ 7686131b-b229-4768-a02c-35d1dbed7c66 0000:00:02.0 i915-GVTg_V5_4 auto (active)

Remove any video device or display devices

  • Remove any video device such as virtio-gpu and set the last one to the none.
<domain type="kvm">
[...]
	<device>
[...]
    <video>
    	<model type="none"/>
    </video>
[...]
	</device>
[...]
</domain>

Assign a vGPU to a virtual machine

  • Add that segment to a virtual machine's definition. Make sure the provided uuid matches the previously generated UUID.
<domain type="kvm">
[...]
	<device>
[...]
    <hostdev mode="subsystem" type="mdev" managed="no" model="vfio-pci" display="off" ramfb="off">
      <source>
        <address uuid="7686131b-b229-4768-a02c-35d1dbed7c66"/>
      </source>
    </hostdev>
[...]
	</device>
[...]
</domain>
  • Start the machine

Add a display device

It will only work locally {.is-info}

  • Add a spice display device
	<device>
[...]
		<graphics type="spice">
  			<listen type="none"/>
  		<gl enable="yes" rendernode="/dev/dri/by-path/pci-0000:00:02.0-render"/>
		</graphics>
[...]
	</device>
  • Modify the vGPU setting to allow for a display and for RAMFB
<domain type="kvm">
[...]
	<device>
[...]
    <hostdev mode="subsystem" type="mdev" managed="no" model="vfio-pci" display="on" ramfb="on">
      <source>
        <address uuid="7686131b-b229-4768-a02c-35d1dbed7c66"/>
      </source>
    </hostdev>
[...]
	</device>
[...]
</domain>

RAMFB is set to on, which activates Drect Memory Access Buffers (DMA-BUFs), making the output of a virtual monitor available before the guest operating system takes over {.is-info}

  • Then starts the domain

Troubleshooting

Low number of available vGPU instances

On some computers, it is possible to increase the system memory allocated to the integrated GPU. By doing so, you may be able to create more vGPUs.

  • Before the host operating system boots up, enter the BIOS/UEFI and look for a setting called GPU aperture size or GPU shared memory

  • Use the highest possible value, but not higher than the available system memory.

System memory will be reserved for the GPU, so make sure you have enough system memory to accomodate both the GPU and your operating system. For instance, if you have a total of 16GB of system memory, it is recommanded to not allocate more than 4GB to the GPU. {.is-warning}

Resources

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