DMA

tags : Memory Hierarchy, Virtual Memory, O, Copy on Write, Computer Bus

FAQ §

What basic things to know? §

• Not all physical memory addresses are “DMA-able”.
• Userspace can’t DMA atm. Security issues.
• Not all devices support DMA

IS DMA a security issue? §

• Yes, a security problem in a way
• IOMMU provides protection by giving permissions to devices on what memory address spaces they can access.

What? §

• An overview of direct memory access | The Infinite Loop
• DMA means devices can access physical memory independently of the CPU
• DMA is rather a concept than a specific technology. There is no specification which describes in detail how DMA transfers work.

Uses §

• Typical application is communicating with peripheral devices plugged into a bus system like ATA, SATA, PCI or PCI Express. (the one we care about atm)
• intra-core communication in micro processors
• Copy data from the memory of one computer into the memory of another computer over the network via remote DMA

Implementation §

Third party DMA using ISA §

• Bus topology
• These used DMA controller
• The DMA controller could be programmed by the CPU to perform a number of memory transfers on behalf of the CPU. This way of accomplishing DMA transfers is also known as third party DMA.

PCI DMA §

• Introduced bus mustering. No more DMA controllers.
• i.e only one device at a time to access the bus.
• The CPU, the DMA controller and bus masters could all be competing for memory access. The memory arbiter typically favors the CPU
• Compared to DMA controller, with bus mustering devices must contain a DMA engine driving the memory transfers.

PCIe DMA §

• Star topology, Point-to-Point/Serial.
• Arbitration by the “root complex”
• PCIe supports full duplex DMA transfers of multiple devices at the same time.