DXGKCB_CREATECONTEXTALLOCATION callback function (d3dkmddi.h)

Article
10/27/2021

DXGKCB_CREATECONTEXTALLOCATION allocates a GPU context or device-specific context.

Syntax

DXGKCB_CREATECONTEXTALLOCATION DxgkcbCreatecontextallocation;

NTSTATUS DxgkcbCreatecontextallocation(
  [in/out] INOUT_PDXGKARGCB_CREATECONTEXTALLOCATION unnamedParam1
)
{...}

Parameters

[in/out] unnamedParam1

A pointer to a DXGKARGCB_CREATECONTEXTALLOCATION structure that specifies the attributes of the context to be allocated.

Return value

DXGKCB_CREATECONTEXTALLOCATION returns STATUS_SUCCESS if it succeeds. Otherwise, it returns one of the error codes defined in Ntstatus.h.

Remarks

Starting with WDDM 1.2, display miniport drivers can allocate a GPU-specific context (GPU context allocation) or a device-specific context (device context allocation).

A GPU context allocation allows GPUs to store context state from DMA buffers that are preempted in the middle of their execution. Drivers create allocations associated with a GPU context to save its state when it is necessary. The operating system ensures that the context allocation is resident before a command from this context is placed in the GPU's hardware execution queue. The context will stay resident until a command from another context is placed in the hardware execution queue.

In addition, the operating system supports lazy GPU context switching by assuming that hardware context state is retained on the GPU after completing a command that belongs to the context. In this way, contexts are only switched on the GPU when a command from a different context is submitted to the hardware queue.

DXGKCB_XXX functions are implemented by Dxgkrnl. To use this callback function, set the members of DXGKARGCB_ALLOCATEPAGESFORMDL and then call DxgkCbCreateContextAllocation via the DXGKRNL_INTERFACE.

GPU context allocations

GPU context allocations can only be made for non-system contexts. The display miniport driver creates these contexts by calling DxgkDdiCreateContext. To create a non-system context, the driver sets the SystemContext member of a DXGK_CREATECONTEXTFLAGS structure to zero, and passes a pointer to this structure in the pCreateContext parameter.

A device context allocation follows a similar model, except that it will remain resident for any context that belongs to the device that it’s associated with. This model allows drivers to use GPU context allocations for storing GPU context save area (CSA) data and to use device context allocations for storing page table data.

Device context allocations

Device context allocations can only be made for non-system devices. The display miniport driver creates these devices by calling DxgkDdiCreateDevice. To create a non-system device, the driver sets the Flags.SystemDevice member of a DXGK_CREATEDEVICEFLAGS structure to zero, and passes a pointer to this structure in the pCreateDevice parameter.

The display miniport driver calls DXGKCB_DESTROYCONTEXTALLOCATION to free the context resources that were allocated through DxgkCbCreateContextAllocation.

Virtual addresses for destination context allocations

To ensure that the operating system sets a valid (non-NULL) virtual address for the destination context allocation (InitContextResource->Destination->VirtualAddress member of the DXGKARG_BUILDPAGINGBUFFER structure), when the display miniport driver calls DxgkCbCreateContextAllocation it must:

Set the CpuVisible and Protected members of the DXGK_ALLOCATIONINFOFLAGS structure.
Page in the allocation only to aperture segments by setting the SupportedSegmentSet member of the DXGKARGCB_CREATECONTEXTALLOCATION structure.

Requirements

Requirement	Value
Minimum supported client	Windows 8 (WDDM 1.2)
Minimum supported server	Windows Server 2012
Target Platform	Desktop
Header	d3dkmddi.h (include D3dkmddi.h)
IRQL	PASSIVE_LEVEL

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