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Some devices, especially GPUs, require additional MMIO space to be allocated to the VM for the memory of that device to be accessible. By default, each VM starts off with 128MB of low MMIO space and 512MB of high MMIO space allocated to it. However, a device might require more MMIO space, or multiple devices may be passed through such that the combined requirements exceed these values. Changing MMIO Space is straight forward and can be performed in PowerShell using the following commands:

Device Driver.

Before using the script, please ensure you have the Hyper-V role installed and you run the script from a PowerShell command window that has Administrator privileges.

If the system is incorrectly configured to support Discrete Device Assignment, the tool will display an error message as to what is wrong. If the tool finds the system configured correctly, it will enumerate all the devices it can find on the PCIe Bus.

For a more in-depth look at MMIO space, see Discrete Device Assignment - GPUs on the TechCommunity blog.

To help ensure hardware the hardware is capable of Discrete Device Assignment, our engineers have put together a Machine Profile Script that you can run on an Hyper-V enabled host to test if your server is correctly setup and what devices are capable of Discrete Device Assignment.

If you would like to bypass the security checks for a device that does not have a Device Mitigation Driver, you will have to pass the -Force parameter to the Dismount-VMHostAssignableDevice cmdlet. Understand that by doing so, you have changed the security profile of that system and this is only recommended during prototyping or trusted environments.

To learn about other methods of GPU virtualization, see Plan for GPU acceleration in Windows Server. If you are ready to try out Discrete Device Assignment, you can jump over to Deploying Graphics Devices Using Discrete Device Assignment or Deploying Storage Devices using Discrete Device Assignment to get started.

When assigning MMIO space to a VM, the user needs to be sure set the MMIO space to the sum of the requested MMIO space for all desired assigned devices plus an additional buffer if there are other virtual devices that require a few MB of MMIO space. Use the default MMIO values described above as the buffer for low and high MMIO (128 MB and 512 MB, respectively).

Device manufactures can reach out to their Microsoft representative for more details.

System Requirements.

Discrete Device Assignment allows physical PCIe hardware to be directly accessible from within a virtual machine. This guide will discuss the type of devices that can use Discrete Device Assignment, host system requirements, limitations imposed on the virtual machines as well as security implications of Discrete Device Assignment.

Discrete Device Assignment is supported for Generation 1 or 2 VMs. Additionally, the guests supported include Windows 10, Windows Server 2019, Windows Server 2016, Windows Server 2012r2 with KB 3133690 applied, and various distributions of the Linux OS.

Machine Profile Script.

As Discrete Device Assignment passes the entire PCIe device into the Guest VM, a host driver is not required to be installed prior to the device being mounted within the VM. The only requirement on the host is that the device's PCIe Location Path can be determined. The device's driver can optionally be installed if this helps in identifying the device. For example, a GPU without its device driver installed on the host may appear as a Microsoft Basic Render Device. If the device driver is installed, its manufacturer and model will likely be displayed.

The easiest way to determine how much MMIO space to allocate is to use the Machine Profile Script. To download and run the machine profile script, run the following commands in a PowerShell console:

MMIO Space.

Device Requirements.

Once the device is mounted inside the guest, the Manufacturer's device driver can now be installed like normal inside the guest virtual machine.

Getting the Location Path by Using Device Manager.

In addition to the System Requirements for Windows Server and the System Requirements for Hyper-V, Discrete Device Assignment requires server class hardware that is capable of granting the operating system control over configuring the PCIe fabric (Native PCI Express Control). In addition, the PCIe Root Complex has to support "Access Control Services" (ACS), which enables Hyper-V to force all PCIe traffic through the I/O MMU.

If a user were to assign a single K520 GPU as in the example above, they must set the MMIO space of the VM to the value outputted by the machine profile script plus a buffer--176 MB + 512 MB. If a user were to assign three K520 GPUs, they must set the MMIO space to three times 176 MB plus a buffer, or 528 MB + 512 MB.

Due to the nature of how Discrete Device Assignment is implemented, some features of a virtual machine are restricted while a device is attached. The following features are not available:

For devices that are able to be assigned, the script will display the MMIO requirements of a given device like the example below:

Not every PCIe device can be used with Discrete Device Assignment. For example, older devices that leverage legacy (INTx) PCI Interrupts are not supported. Jake Oshin's blog posts go into more detail - however, for the consumer, running the Machine Profile Script will display which devices are capable of being used for Discrete Device Assignment.


If the admin desires to use a device with an untrusted tenant, we have provided device manufactures with the ability to create a Device Mitigation driver that can be installed on the host. Please contact the device manufacturer for details on whether they provide a Device Mitigation Driver.

For Discrete Device Assignment's initial release, we have focused on two device classes to be formally supported by Microsoft: Graphics Adapters and NVMe Storage devices. Other devices are likely to work and hardware vendors are able to offer statements of support for those devices. For these other devices, please reach out to those hardware vendors for support.

VM Save/Restore Live migration of a VM The use of dynamic memory Adding the VM to a high availability (HA) cluster.

Virtual Machine Limitations.

The low MMIO space is used only by 32-bit operating systems and devices that use 32-bit addresses. In most circumstances, setting the high MMIO space of a VM will be enough since 32-bit configurations aren't very common.

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