Direct Cache Access (DCA)

Note  The NetDMA interface is not supported in Windows 8 and later.

Direct cache access (DCA) provides a mechanism for NetDMA clients to indicate that destination data is targeted for a CPU cache. DCA support is not present for NetDMA providers before version 2.0 and it is optional for NetDMA 2.0 and later providers. A NetDMA provider that supports DCA sets the NET_DMA_PROVIDER_CHARACTERISTICS_DCA_SUPPORTED bit in the Flags member of the NET_DMA_PROVIDER_CHARACTERISTICS structure at the ProviderCharacteristics parameter of the NetDmaRegisterProvider function.

During NetDMA channel allocation, a NetDMA provider that supports DCA sets the CPU affinity of the Status Completion Write register (that is, the memory location that is specified in the CompletionVirtualAddress and CompletionPhysicalAddress members in the NET_DMA_CHANNEL_PARAMETERS structure) to the target processor that it specifies in the CpuNumber member of NET_DMA_CHANNEL_PARAMETERS structure. When the DMA engine completes a DMA transfer, if the NetDMA interface requests a status update (by setting the NET_DMA_STATUS_UPDATE_ON_COMPLETION bit), the DMA engine sends a DCA hint to the target processor that is associated with the NetDMA channel.

If a NetDMA provider supports DCA, the NetDMA interface sets the processor affinity of the destination data of DMA transfers on a particular channel by submitting a "Context Change" descriptor to the NetDMA provider on that channel right after allocating the channel and in other cases where the hardware context might have been lost.

A DCA-capable NetDMA engine provides an interface to control sending the DCA hint on a per-transfer basis. A DCA-capable NetDMA provider checks the NET_DMA_DESTINATION_DCA_ENABLE bit in the ControlFlags member of each submitted NET_DMA_DESCRIPTOR structure to determine if it should send a DCA hint for the destination address to the target processor. Drivers that are not DCA-capable should ignore the NET_DMA_DESTINATION_DCA_ENABLE bit.

When the NET_DMA_OP_TYPE_CONTEXT_CHANGE bit in the ControlFlags member is set, the DMA engine should identify the descriptor as a "Context Change" descriptor. The NetDMA interface submits a "Context Change" descriptor to a DCA-capable NetDMA provider to set the DCA target processor of the destination data for all the DMA transfers on a NetDMA channel. The NetDMA interface uses the DCAContext8 member in a "Context Change" descriptor to specify the 8-bit advanced programmable interrupt controller (APIC) identifier of the target processor for the channel. A DCA-capable NetDMA provider must keep the DCA affinity of a DMA channel with a processor as long as it has not received a new "Context Change" descriptor. The NetDMA interface submits a "Context Change" descriptor one time after the channel is allocated and again when it detects that the DMA provider might have lost the hardware context (for example, after a suspend and resume operation).