Earlier this month, the first rev of the NVM Express (or NVMe) standard was published. The idea is to establish a uniform register and command set for solid-state memories that use PCIe. It’s an abstracted interface, and doesn’t get into such details as wear-leveling; it works at the read/write/erase level, and the memory subsystem itself takes care of implementing low-level algorithms in the appropriate manner.
The architecture defines pairs of transactions – submissions and completions – that are managed on queues configured as circular buffers. There can be multiple submission and completion queues; submission queues can be matched with completion queues, or multiple submission queues can share a completion queue. This allows, for example, each core in a multicore system to own a queue without needing locks to protect transactions from interference by other cores.
Quoting from the standard itself, key features are:
- “Does not require uncacheable / MMIO register reads in the command issue or completion path.
- A maximum of one MMIO register write is necessary in the command issue path.
- Support for up to 64K I/O queues, with each I/O queue supporting up to 64K commands.
- Priority associated with each I/O queue with well-defined arbitration mechanism.
- All information to complete a 4KB read request is included in the 64B command itself, ensuring
- efficient small I/O operation.
- Efficient and streamlined command set.
- Support for MSI/MSI-X and interrupt aggregation.
- Support for multiple namespaces.
- Efficient support for I/O virtualization architectures like SR-IOV.
- Robust error reporting and management capabilities.”
Key characteristics of the register set are:
- “Indication of controller capabilities
- Status for device failures (command status is processed via CQ directly)
- Admin Queue configuration (I/O Queue configuration processed via Admin commands)
- Doorbell registers for scalable number of Submission and Completion Queues• Efficient support for I/O virtualization architectures like SR-IOV.
- Robust error reporting and management capabilities.”
Both Synopsys and Cadence wasted little time in announcing their verification IP support (Cadence also announced 12-Mbps SAS VIP). You can find out more about their announcements in the Synopsys release and the Cadence release, respectively.
