The QDMA provides an optional feature to support Single Root I/O Virtualization (SR-IOV). The PCI-SIG® Single Root I/O Virtualization and Sharing (SR-IOV) specification (available from PCI-SIG Specifications (www.pcisig.com/specifications) standardizes the method for bypassing the VMM involvement in datapath transactions and allows a single endpoint to appear as multiple separate endpoints. SR-IOV classifies the functions as:
- Physical Functions (PF): Full featured PCIe® functions which include SR-IOV capabilities among others.
- Virtual Functions (VF): PCIe functions featuring configuration space with Base Address Registers (BARs) but lacking the full configuration resources and controlled by the PF configuration. The main role of the VF is data transfer.
Apart from PCIe defined configuration space, QDMA Subsystem for PCI Express virtualizes data path operations, such as pointer updates for queues, and interrupts. The rest of the management and configuration functionality is deferred to the physical function driver. The Drivers that do not have sufficient privilege must communicate with the privileged Driver through the mailbox interface which is provided in part of the QDMA Subsystem for PCI Express.
Security is an important aspect of virtualization. The QDMA Subsystem for PCI Express offers the following security functionality:
- QDMA allows only privileged PF to configure the per queue context and registers. VFs inform the corresponding PFs of any queue context programming.
- Drivers are allowed to do pointer updates only for the queue allocated to them.
- The system IOMMU can be turned on to check that the DMA access is being requested by PFs or VFs. The ARID comes from queue context programmed by a privileged function.
Any PF or VF can communicate to a PF (not itself) through mailbox. Each function implements one 128B inbox and 128B outbox. These mailboxes are visible to the driver in the DMA BAR (typically BAR0) of its own function. At any given time, any function can have one outgoing mailbox and one incoming mailbox message outstanding per function.
The diagram below shows how a typical system can use QDMA with different functions and operating systems. Different Queues can be allocated to different functions, and each function can transfer DMA packets independent of each other.