NVMe Command Sets

The NVMe specification has since its first version defined the Admin Command Set providing essential administrative and management commands including: identify and get-log-page. For I/O, the NVMe specification has since its inception defined the NVM Command Set, which includes the most essential I/O commands: read and write.

The commands included in the two sets has grown since then, some commands being mandatory others optional.

With emerging technologies such as Zoned Namespaces, Key-Value SSDs, and computational storage then the set of commands continue to grow, to encapsulate the growth the NVMe specification now include a formal specification on how a NVMe device can communicate support for different I/O command sets.

This section contains notes specific to I/O Command Sets, how to probe a device for support and references to details on the specific command sets.

Note

Please see the NVMe specification for ground thruth, that is, the exact definitions and descriptions and use the information presented here as an informal introduction.

Concepts and Prelude

This expands on the concepts of NVMe Controllers, Controller-registers, Namespaces, and Commands.

I/O Command Set Identifier (csi), as the name suggests, then csi identifies an I/O Command Set. Note, that in the context of NVMe-MI, it is referred to as iocsi.

I/O Command Set Identifiers, are defined in their respective specifications, e.g. the Zoned Namespace I/O Command Set specification declares the csi value for Zoned I/O Command Set.

I/O Command Set Profile

I/O Command Set Combination Index (iocsci)

I/O Command Set Combination

Abbreviations

Abbreviation

Description

csi

Command Set Identifier

iocsci

I/O Command Set Combination Index

iocsi

I/O Command Set Identifier

Controller Command Set Support

I/O Command-set support is governed by the controller capabilities and its configuration. That is, which capability is enabled in the configuration.

Specifically, the read-only field CAP.CSS, defines what is supported, and the read-write field CC.CSS defines what is enabled.

Capability

The CAP register, controller capabilities, has a 8bit-wide field (bits 44-37) named CSS indicating I/O Command Set(s) that the controller supports

cap.css:

CAP-bits

Definition

37

Controller supports the NVM Command Set.

Controllers that support the NVM Command Set shall set this bit even if bit 43 is set to ‘1’.

42-38

Reserved

43

Controller supports one or more I/O Command Sets and supports the Identify I/O Command Set data structure (ref. ?).

Controllers that support I/O Command Sets other than the NVM Command Set shall set bit 43 to ‘1’.

Controllers that only support the NVM Command Set may set this bit to ‘1’ to indicate support for the Command Set Identifier field in commands that use the Command Set Identifier field.

From the spec you can see that only the NVM Command Set has a bit dedicated to indicate support. For any other I/O Command Set, supported is communicated in Identify I/O Command Set data structure.

So, what can you do with the information in the controller registers? Find a couple of examples below.

Determine whether the controller supports the NVM I/O Command Set:

  • Check that CAP-bit 37 is set

Determine whether a controller supports any other I/O Command Set:

  • Check whether CAP-bit 43 is set

  • Retrieve the Identify I/O Command Set data structure

How do one check if a controller support I/O Command Set xyz?

Configuration

The CC register, controller configuration, has a 3bit-wide field (bits 6-4) named CSS which shall be set prior to enabling the controller.

What can you enable here? Here are a couple of examples:

  • Enable only the Admin Command Set

  • Enable the Admin Command Set and the NVM Command Set

  • Enable the Admin Command Set, the NVM Command Set, and any other I/O Command Set

Identify: I/O Command Set data structure

The controller registers CAP.CSS and CC.CSS are insufficient beyond the NVM and Admin Command sets. For any I/O Command Set, other than NVM, then the NVMe commands: identify, get-feature and set-feature, shall be used, in addition to the controller registers, in order to determine capability and configuring I/O Command Sets.

Example, sending this command using the xNVMe CLI:

xnvme idfy-cs /dev/nvme0n1

Should yield output on the form:

xnvme_spec_idfy_cs:
  - { iocsci: 0, val: 0x5, nvm: 1, zns: 1 }

Feature: I/O Command Set Profile

The I/O Command Set Profile feature, identified by TBD, provides the I/O Command Set Combination Index (iocsci) of the currently selected I/O Command Set Combination. To change it the currently selected I/O Command Set Combination, then use set-feature using a valid iocsci.

Namespace Command Set Association

A namespace is associated with exactly one I/O command set. So, how do you determine which one it is?

FAQ

Q: Where are Command Set Identifiers defined? A: TODO: I guess they should be defined in their respective I/O Command Specifications? Looks like they are not defined in the base-spec.