Sharing the backing store with KMD

Starting in Windows 11 version 22H2 (WDDM 3.1), WDDM was extended to allow access to a graphics allocation backing store from the kernel-mode driver (KMD). A backing store refers to a committed memory buffer that holds the contents of a graphics allocation when it's not resident in video memory.

This feature allows both the user-mode driver (UMD) and KMD the ability to access the same allocation memory. This feature can be used when UMD is running on the host or in a virtual machine using GPU paravirtualization (GPU-PV).

This feature was back-ported to Windows 10 version 20H1. The DDI is available for WDDM 3.1 drivers or newer.

WDDM graphics allocations and backing stores

Every graphics allocation in the WDDM model has a backing store. A graphics allocation is created when UMD calls Dxgkrnl's D3DKMTCreateAllocation function. UMD passes private data for this allocation, which Dxgkrnl passes to KMD through a call to DxgkddiCreateAllocation. KMD returns the desired allocation flags to Dxgkrnl.

Checking for feature availability

To check whether the backing store sharing feature is available, KMD must first call one of the following callbacks with FeatureId set to DXGK_FEATURE_SHARE_BACKING_STORE_WITH_KMD:

• DXGKCB_QUERYFEATURESUPPORT (available starting in WDDM 2.9)
• DXGKCB_ISFEATUREENABLED

The feature can only be used if the callback succeeds and Enable is set to TRUE.

Feature flow

Once KMD successfully determines that the feature is enabled, UMD calls D3DKMTCreateAllocation to create a shared CPU-visible allocation and instructs KMD via private data that the allocation has to be shared with KMD. In the course of this call, the following occurs:

• KMD sets DXGK_ALLOCATIONINFOFLAGS2's ShareBackingStoreWithKmd flag when the OS calls KMD's DxgkddiCreateAllocation callback. If the feature isn't enabled, KMD must not set the ShareBackingStoreWithKmd flag.

• Dxgkrnl calls the DXGKDDI_SETALLOCATIONBACKINGSTORE DDI to give KMD a kernel-mode address to the allocation backing store.

• UMD calls D3DKMTLock2 to get the allocation's user-mode address.

Properties of the allocation

The allocation created in this way must have the following properties:

• The allocation is allowed to be only in the system memory segment.
• The allocation must be created as shared.
• The allocation can't use existing system memory as backing store.
• UMD can do any operation as for a regular allocation.
  • UMD can call D3DKMTLock2 to get a pointer to the allocation.
  • UMD can call D3DKMTMakeResident to make the allocation accessible by the GPU.