# GPUs in virtual machines: passthrough, DDA and what Windows guests can use

> The three ways to give a Windows VM a GPU, what each one supports, and the IOMMU/vBIOS problems that decide whether it boots.

**Tested on**

- Build: 26300.9457 / 26100.33650
- Edition: Pro (26H2) / Server
- Architecture: x64
- Evidence: E1
- Verified: 2026-10-29

## The short answer

There are three different things people call "GPU in a VM", and they are not interchangeable:

| Approach | What the guest gets | Typical use |
|---|---|---|
| **Discrete Device Assignment (DDA)** | one physical GPU, exclusively | Hyper-V, workstation VMs |
| **Full PCIe passthrough** | one physical GPU, exclusively | Proxmox/KVM, gaming VMs |
| **Virtual GPU (vGPU/GPU-P)** | a slice of a supported GPU | VDI, many sessions |

In all three cases the guest sees a real graphics driver, and the host loses the card (except vGPU, which is licensed and platform-specific).

## What decides whether it works

1. **IOMMU** enabled in firmware and the hypervisor. Without it, passthrough is not possible.
2. **The GPU must sit in its own IOMMU group** — if it shares a group with the chipset or a NIC, you cannot isolate it.
3. **The guest needs a driver for that card** — the same driver as bare metal.
4. **The vBIOS** — some consumer cards refuse to initialise in a VM without a vBIOS dump or a vendor-specific flag.
5. **The card must not be the host's only display output** while you configure it.

```powershell
# in the guest: what did the VM actually get?
Get-PnpDevice -Class Display | Select-Object FriendlyName,Status,InstanceId
Get-CimInstance Win32_VideoController | Select-Object Name,DriverVersion,AdapterRAM
```

In the guest, `InstanceId` still looks like a PCI device — that is expected with passthrough. A `Status : Error` device usually means the driver bound but the card did not initialise.

## Windows Server guests: DDA specifics

DDA is the supported path on Hyper-V:

- the GPU is **dismounted from the host** and assigned to one VM;
- the VM must be **stopped** to assign or remove;
- the guest needs the vendor driver installed **inside** the VM;
- some server GPUs support partitioning (GPU-P) instead, which requires vendor support and licensing.

DDA is not "GPU sharing": one card, one VM at a time.

## Desktop guests: what to expect

- **Gaming**: works well enough in KVM with a vBIOS and a second GPU for the host; latency depends on the display path (looking-glass, HDMI passthrough, or a physical monitor on the passed-through card).
- **Compute (CUDA/ROCm)**: usually the easiest case — no display path needed.
- **Video encode**: needs the vendor's encoder support inside the guest; some consumer drivers disable encode in VMs.

> **Caution: Nested virtualisation and GPU do not combine well**
>
> A GPU passed into a VM that itself runs Hyper-V/WSL2 will usually fail: the inner hypervisor needs features the outer one does not expose. If you need both, expect to choose.

## Common failures and what they mean

| Symptom | Usual cause |
|---|---|
| VM will not start with the device attached | IOMMU off, or device in a shared group |
| Guest boots, GPU shows error 43/31 | vBIOS or driver refuses the VM |
| Works until reboot | host reclaimed the device, or a driver update |
| Host loses display | the only GPU was assigned away |
| Very low performance | link width/speed downgraded, or software rendering is active |

Error 43 on a passed-through GPU is a driver policy refusal, not a hardware fault — the fix is vendor/driver-specific, and on some consumer cards it is simply not supported.

## Check before you invest an evening

```powershell
# host: IOMMU enabled and device grouping (Proxmox example)
# dmesg | grep -e DMAR -e IOMMU
# for d in /sys/kernel/iommu_groups/*; do echo "IOMMU group ${d##*/}"; done
```

On Windows hosts, Device Manager → **View → Devices by connection** shows which PCIe root port the GPU sits under; that maps to the grouping you will fight with.

## Related

- [Virtualisation flags](https://winbook.fyi/en/hardware/cpu/virtualization-flags/)
- [Windows VM on Proxmox](https://winbook.fyi/en/windows/dev/pve-windows-vm/)
- [WDDM, HAGS and MPO](https://winbook.fyi/en/hardware/gpu/wddm-hags-mpo/)

## Lab boundary

E1. DDA and GPU-P requirements are Microsoft-documented. The lab host has no discrete GPU assigned to a VM, so passthrough was not attempted — this page deliberately publishes no performance numbers.

## Sources

- [Microsoft Learn — Deploy devices using Discrete Device Assignment](https://learn.microsoft.com/en-us/windows-server/virtualization/hyper-v/plan/plan-for-deploying-devices-using-discrete-device-assignment)
- [Microsoft Learn — Plan for GPU acceleration in Windows Server](https://learn.microsoft.com/en-us/windows-server/virtualization/hyper-v/plan/plan-for-gpu-acceleration-in-windows-server)
- [Microsoft Learn — Windows 11 requirements](https://learn.microsoft.com/en-us/windows/whats-new/windows-11-requirements)

---

Source: https://winbook.fyi/en/hardware/gpu/passthrough/