Code Assumptions#

These are notes on contracts and assumptions in the xNVMe codebase. These are written down separate from the code for more convenient reading. Thus, whenever code-changes are made that alter assumptions and contracts described herein, then this document must be updated as well as part of the change-set.

The device handle contract#

Device handles are opaque pointers of type xnvme_dev. Device handles are obtained via the public xNVMe API call xnvme_dev_open(), and released via a call to xnvme_dev_close().

The entities involved in the implementation of this and their individual responsibility in setting up the data-structure is described in this section here.

These functions are implemented in the library-private device module (xnvme_dev.c). The device module is responsible for allocating and initializing the data-structure associated with and encapsulating the device handle upon xnvme_dev_open() and freeing these resources upon xnvme_dev_close().

Device handles are system interface specific; file-descriptors on UNIX-like systems, opaque pointers on Windows, typed pointers in user-space drivers such as SPDK, etc. Thus, the device module is only responsible for generic initialization, it delagates these system specifics to the backend module (xnvme_be.c).

The library-private backend module in turn, defines a backend interface (xnvme_be.h) and implements a backend factory (xnvme_be_factory()). The backend factory knows of all backends implementing the backend interface.

Thus, to obtain a system specific handle it delegates to each backend, in turn, the implementation of obtaining a system interface specific device handle, for a given device identifier, is handled by the first backend implementation (xnvme_be_*_dev.c) capable of producing a handle.

In other words, the backend factory chooses the first backend capable of returning a system interface specific device handle and the system interface specific handle produced by the backend.

The entities described above are those involved in the setup of a xNVMe device handle. The xNVMe device handle is opaque, that is the details of its representation is hidden from the user. Thus, functions are provided to retrieve data about a given device handle.

However, described here are the details on how the representation is constructed by the library-private: device module, backend module, and instances of the backend interface implementation aka backends (xnvme_be_{spdk,vfio,linux,fbsd,windows}_*.c`).

Identification and Geometry#

The structure encapsulating the device handle is library-private, however, for contributions to backends, device module, and related, then a bit of info on how it is setup is useful, it is provided below.

# Assigned by xnvme_be_factory()

# The backend-factory copies the opts and adjusts them to the options
# describing the backend, thus showing which "configuration" it ended up with

# Initialized by xnvme_dev_open() using helper xnvme_ident_from_uri(), then
# backends further manipulate the structure

# These are setup by the backend as it is "opening" the device, here it might
# do simple things such as fstat() on Linux to determine whether the device
# handle is a block-device or a char-device to indicate the ident.dtype

# The device module assigns this in the identification-helper _dev_idfy()

# Each backend-implementation sets these up after they have created a
# system-interface-specific device handle

dev.is_derived # setup by xnvme_dev_get_*() access after calling xnvme_dev_derive_geometry()

# These are assigned by the device module at "identification-time"

# Populated by the device module using the helper xnvme_dev_derive_geometry()

# Set by xnvme_dev_derive_geometry()