================================
Coherent Accelerator (CXL) Flash
================================

Introduction
============

    The IBM Power architecture provides support for CAPI (Coherent
    Accelerator Power Interface), which is available to certain PCIe slots
    on Power 8 systems. CAPI can be thought of as a special tunneling
    protocol through PCIe that allow PCIe adapters to look like special
    purpose co-processors which can read or write an application's
    memory and generate page faults. As a result, the host interface to
    an adapter running in CAPI mode does not require the data buffers to
    be mapped to the device's memory (IOMMU bypass) nor does it require
    memory to be pinned.

    On Linux, Coherent Accelerator (CXL) kernel services present CAPI
    devices as a PCI device by implementing a virtual PCI host bridge.
    This abstraction simplifies the infrastructure and programming
    model, allowing for drivers to look similar to other native PCI
    device drivers.

    CXL provides a mechanism by which user space applications can
    directly talk to a device (network or storage) bypassing the typical
    kernel/device driver stack. The CXL Flash Adapter Driver enables a
    user space application direct access to Flash storage.

    The CXL Flash Adapter Driver is a kernel module that sits in the
    SCSI stack as a low level device driver (below the SCSI disk and
    protocol drivers) for the IBM CXL Flash Adapter. This driver is
    responsible for the initialization of the adapter, setting up the
    special path for user space access, and performing error recovery. It
    communicates directly the Flash Accelerator Functional Unit (AFU)
    as described in Documentation/powerpc/cxl.rst.

    The cxlflash driver supports two, mutually exclusive, modes of
    operation at the device (LUN) level:

        - Any flash device (LUN) can be configured to be accessed as a
          regular disk device (i.e.: /dev/sdc). This is the default mode.

        - Any flash device (LUN) can be configured to be accessed from
          user space with a special block library. This mode further
          specifies the means of accessing the device and provides for
          either raw access to the entire LUN (referred to as direct
          or physical LUN access) or access to a kernel/AFU-mediated
          partition of the LUN (referred to as virtual LUN access). The
          segmentation of a disk device into virtual LUNs is assisted
          by special translation services provided by the Flash AFU.

Overview
========

    The Coherent Accelerator Interface Architecture (CAIA) introduces a
    concept of a master context. A master typically has special privileges
    granted to it by the kernel or hypervisor allowing it to perform AFU
    wide management and control. The master may or may not be involved
    directly in each user I/O, but at the minimum is involved in the
    initial setup before the user application is allowed to send requests
    directly to the AFU.

    The CXL Flash Adapter Driver establishes a master context with the
    AFU. It uses memory mapped I/O (MMIO) for this control and setup. The
    Adapter Problem Space Memory Map looks like this::

                     +-------------------------------+
                     |    512 * 64 KB User MMIO      |
                     |        (per context)          |
                     |       User Accessible         |
                     +-------------------------------+
                     |    512 * 128 B per context    |
                     |    Provisioning and Control   |
                     |   Trusted Process accessible  |
                     +-------------------------------+
                     |         64 KB Global          |
                     |   Trusted Process accessible  |
                     +-------------------------------+

    This driver configures itself into the SCSI software stack as an
    adapter driver. The driver is the only entity that is considered a
    Trusted Process to program the Provisioning and Control and Global
    areas in the MMIO Space shown above.  The master context driver
    discovers all LUNs attached to the CXL Flash adapter and instantiates
    scsi block devices (/dev/sdb, /dev/sdc etc.) for each unique LUN
    seen from each path.

    Once these scsi block devices are instantiated, an application
    written to a specification provided by the block library may get
    access to the Flash from user space (without requiring a system call).

    This master context driver also provides a series of ioctls for this
    block library to enable this user space access.  The driver supports
    two modes for accessing the block device.

    The first mode is called a virtual mode. In this mode a single scsi
    block device (/dev/sdb) may be carved up into any number of distinct
    virtual LUNs. The virtual LUNs may be resized as long as the sum of
    the sizes of all the virtual LUNs, along with the meta-data associated
    with it does not exceed the physical capacity.

    The second mode is called the physical mode. In this mode a single
    block device (/dev/sdb) may be opened directly by the block library
    and the entire space for the LUN is available to the application.

    Only the physical mode provides persistence of the data.  i.e. The
    data written to the block device will survive application exit and
    restart and also reboot. The virtual LUNs do not persist (i.e. do
    not survive after the application terminates or the system reboots).


Block library API
=================

    Applications intending to get access to the CXL Flash from user
    space should use the block library, as it abstracts the details of
    interfacing directly with the cxlflash driver that are necessary for
    performing administrative actions (i.e.: setup, tear down, resize).
    The block library can be thought of as a 'user' of services,
    implemented as IOCTLs, that are provided by the cxlflash driver
    specifically for devices (LUNs) operating in user space access
    mode. While it is not a requirement that applications understand
    the interface between the block library and the cxlflash driver,
    a high-level overview of each supported service (IOCTL) is provided
    below.

    The block library can be found on GitHub:
    http://github.com/open-power/capiflash


CXL Flash Driver LUN IOCTLs
===========================

    Users, such as the block library, that wish to interface with a flash
    device (LUN) via user space access need to use the services provided
    by the cxlflash driver. As these services are implemented as ioctls,
    a file descriptor handle must first be obtained in order to establish
    the communication channel between a user and the kernel.  This file
    descriptor is obtained by opening the device special file associated
    with the scsi disk device (/dev/sdb) that was created during LUN
    discovery. As per the location of the cxlflash driver within the
    SCSI protocol stack, this open is actually not seen by the cxlflash
    driver. Upon successful open, the user receives a file descriptor
    (herein referred to as fd1) that should be used for issuing the
    subsequent ioctls listed below.

    The structure definitions for these IOCTLs are available in:
    uapi/scsi/cxlflash_ioctl.h

DK_CXLFLASH_ATTACH
------------------

    This ioctl obtains, initializes, and starts a context using the CXL
    kernel services. These services specify a context id (u16) by which
    to uniquely identify the context and its allocated resources. The
    services additionally provide a second file descriptor (herein
    referred to as fd2) that is used by the block library to initiate
    memory mapped I/O (via mmap()) to the CXL flash device and poll for
    completion events. This file descriptor is intentionally installed by
    this driver and not the CXL kernel services to allow for intermediary
    notification and access in the event of a non-user-initiated close(),
    such as a killed process. This design point is described in further
    detail in the description for the DK_CXLFLASH_DETACH ioctl.

    There are a few important aspects regarding the "tokens" (context id
    and fd2) that are provided back to the user:

        - These tokens are only valid for the process under which they
          were created. The child of a forked process cannot continue
          to use the context id or file descriptor created by its parent
          (see DK_CXLFLASH_VLUN_CLONE for further details).

        - These tokens are only valid for the lifetime of the context and
          the process under which they were created. Once either is
          destroyed, the tokens are to be considered stale and subsequent
          usage will result in errors.

        - A valid adapter file descriptor (fd2 >= 0) is only returned on
          the initial attach for a context. Subsequent attaches to an
          existing context (DK_CXLFLASH_ATTACH_REUSE_CONTEXT flag present)
          do not provide the adapter file descriptor as it was previously
          made known to the application.

        - When a context is no longer needed, the user shall detach from
          the context via the DK_CXLFLASH_DETACH ioctl. When this ioctl
          returns with a valid adapter file descriptor and the return flag
          DK_CXLFLASH_APP_CLOSE_ADAP_FD is present, the application _must_
          close the adapter file descriptor following a successful detach.

        - When this ioctl returns with a valid fd2 and the return flag
          DK_CXLFLASH_APP_CLOSE_ADAP_FD is present, the application _must_
          close fd2 in the following circumstances:

                + Following a successful detach of the last user of the context
                + Following a successful recovery on the context's original fd2
                + In the child process of a fork(), following a clone ioctl,
                  on the fd2 associated with the source context

        - At any time, a close on fd2 will invalidate the tokens. Applications
          should exercise caution to only close fd2 when appropriate (outlined
          in the previous bullet) to avoid premature loss of I/O.

DK_CXLFLASH_USER_DIRECT
-----------------------
    This ioctl is responsible for transitioning the LUN to direct
    (physical) mode access and configuring the AFU for direct access from
    user space on a per-context basis. Additionally, the block size and
    last logical block address (LBA) are returned to the user.

    As mentioned previously, when operating in user space access mode,
    LUNs may be accessed in whole or in part. Only one mode is allowed
    at a time and if one mode is active (outstanding references exist),
    requests to use the LUN in a different mode are denied.

    The AFU is configured for direct access from user space by adding an
    entry to the AFU's resource handle table. The index of the entry is
    treated as a resource handle that is returned to the user. The user
    is then able to use the handle to reference the LUN during I/O.

DK_CXLFLASH_USER_VIRTUAL
------------------------
    This ioctl is responsible for transitioning the LUN to virtual mode
    of access and configuring the AFU for virtual access from user space
    on a per-context basis. Additionally, the block size and last logical
    block address (LBA) are returned to the user.

    As mentioned previously, when operating in user space access mode,
    LUNs may be accessed in whole or in part. Only one mode is allowed
    at a time and if one mode is active (outstanding references exist),
    requests to use the LUN in a different mode are denied.

    The AFU is configured for virtual access from user space by adding
    an entry to the AFU's resource handle table. The index of the entry
    is treated as a resource handle that is returned to the user. The
    user is then able to use the handle to reference the LUN during I/O.

    By default, the virtual LUN is created with a size of 0. The user
    would need to use the DK_CXLFLASH_VLUN_RESIZE ioctl to adjust the grow
    the virtual LUN to a desired size. To avoid having to perform this
    resize for the initial creation of the virtual LUN, the user has the
    option of specifying a size as part of the DK_CXLFLASH_USER_VIRTUAL
    ioctl, such that when success is returned to the user, the
    resource handle that is provided is already referencing provisioned
    storage. This is reflected by the last LBA being a non-zero value.

    When a LUN is accessible from more than one port, this ioctl will
    return with the DK_CXLFLASH_ALL_PORTS_ACTIVE return flag set. This
    provides the user with a hint that I/O can be retried in the event
    of an I/O error as the LUN can be reached over multiple paths.

DK_CXLFLASH_VLUN_RESIZE
-----------------------
    This ioctl is responsible for resizing a previously created virtual
    LUN and will fail if invoked upon a LUN that is not in virtual
    mode. Upon success, an updated last LBA is returned to the user
    indicating the new size of the virtual LUN associated with the
    resource handle.

    The partitioning of virtual LUNs is jointly mediated by the cxlflash
    driver and the AFU. An allocation table is kept for each LUN that is
    operating in the virtual mode and used to program a LUN translation
    table that the AFU references when provided with a resource handle.

    This ioctl can return -EAGAIN if an AFU sync operation takes too long.
    In addition to returning a failure to user, cxlflash will also schedule
    an asynchronous AFU reset. Should the user choose to retry the operation,
    it is expected to succeed. If this ioctl fails with -EAGAIN, the user
    can either retry the operation or treat it as a failure.

DK_CXLFLASH_RELEASE
-------------------
    This ioctl is responsible for releasing a previously obtained
    reference to either a physical or virtual LUN. This can be
    thought of as the inverse of the DK_CXLFLASH_USER_DIRECT or
    DK_CXLFLASH_USER_VIRTUAL ioctls. Upon success, the resource handle
    is no longer valid and the entry in the resource handle table is
    made available to be used again.

    As part of the release process for virtual LUNs, the virtual LUN
    is first resized to 0 to clear out and free the translation tables
    associated with the virtual LUN reference.

DK_CXLFLASH_DETACH
------------------
    This ioctl is responsible for unregistering a context with the
    cxlflash driver and release outstanding resources that were
    not explicitly released via the DK_CXLFLASH_RELEASE ioctl. Upon
    success, all "tokens" which had been provided to the user from the
    DK_CXLFLASH_ATTACH onward are no longer valid.

    When the DK_CXLFLASH_APP_CLOSE_ADAP_FD flag was returned on a successful
    attach, the application _must_ close the fd2 associated with the context
    following the detach of the final user of the context.

DK_CXLFLASH_VLUN_CLONE
----------------------
    This ioctl is responsible for cloning a previously created
    context to a more recently created context. It exists solely to
    support maintaining user space access to storage after a process
    forks. Upon success, the child process (which invoked the ioctl)
    will have access to the same LUNs via the same resource handle(s)
    as the parent, but under a different context.

    Context sharing across processes is not supported with CXL and
    therefore each fork must be met with establishing a new context
    for the child process. This ioctl simplifies the state management
    and playback required by a user in such a scenario. When a process
    forks, child process can clone the parents context by first creating
    a context (via DK_CXLFLASH_ATTACH) and then using this ioctl to
    perform the clone from the parent to the child.

    The clone itself is fairly simple. The resource handle and lun
    translation tables are copied from the parent context to the child's
    and then synced with the AFU.

    When the DK_CXLFLASH_APP_CLOSE_ADAP_FD flag was returned on a successful
    attach, the application _must_ close the fd2 associated with the source
    context (still resident/accessible in the parent process) following the
    clone. This is to avoid a stale entry in the file descriptor table of the
    child process.

    This ioctl can return -EAGAIN if an AFU sync operation takes too long.
    In addition to returning a failure to user, cxlflash will also schedule
    an asynchronous AFU reset. Should the user choose to retry the operation,
    it is expected to succeed. If this ioctl fails with -EAGAIN, the user
    can either retry the operation or treat it as a failure.

DK_CXLFLASH_VERIFY
------------------
    This ioctl is used to detect various changes such as the capacity of
    the disk changing, the number of LUNs visible changing, etc. In cases
    where the changes affect the application (such as a LUN resize), the
    cxlflash driver will report the changed state to the application.

    The user calls in when they want to validate that a LUN hasn't been
    changed in response to a check condition. As the user is operating out
    of band from the kernel, they will see these types of events without
    the kernel's knowledge. When encountered, the user's architected
    behavior is to call in to this ioctl, indicating what they want to
    verify and passing along any appropriate information. For now, only
    verifying a LUN change (ie: size different) with sense data is
    supported.

DK_CXLFLASH_RECOVER_AFU
-----------------------
    This ioctl is used to drive recovery (if such an action is warranted)
    of a specified user context. Any state associated with the user context
    is re-established upon successful recovery.

    User contexts are put into an error condition when the device needs to
    be reset or is terminating. Users are notified of this error condition
    by seeing all 0xF's on an MMIO read. Upon encountering this, the
    architected behavior for a user is to call into this ioctl to recover
    their context. A user may also call into this ioctl at any time to
    check if the device is operating normally. If a failure is returned
    from this ioctl, the user is expected to gracefully clean up their
    context via release/detach ioctls. Until they do, the context they
    hold is not relinquished. The user may also optionally exit the process
    at which time the context/resources they held will be freed as part of
    the release fop.

    When the DK_CXLFLASH_APP_CLOSE_ADAP_FD flag was returned on a successful
    attach, the application _must_ unmap and close the fd2 associated with the
    original context following this ioctl returning success and indicating that
    the context was recovered (DK_CXLFLASH_RECOVER_AFU_CONTEXT_RESET).

DK_CXLFLASH_MANAGE_LUN
----------------------
    This ioctl is used to switch a LUN from a mode where it is available
    for file-system access (legacy), to a mode where it is set aside for
    exclusive user space access (superpipe). In case a LUN is visible
    across multiple ports and adapters, this ioctl is used to uniquely
    identify each LUN by its World Wide Node Name (WWNN).


CXL Flash Driver Host IOCTLs
============================

    Each host adapter instance that is supported by the cxlflash driver
    has a special character device associated with it to enable a set of
    host management function. These character devices are hosted in a
    class dedicated for cxlflash and can be accessed via `/dev/cxlflash/*`.

    Applications can be written to perform various functions using the
    host ioctl APIs below.

    The structure definitions for these IOCTLs are available in:
    uapi/scsi/cxlflash_ioctl.h

HT_CXLFLASH_LUN_PROVISION
-------------------------
    This ioctl is used to create and delete persistent LUNs on cxlflash
    devices that lack an external LUN management interface. It is only
    valid when used with AFUs that support the LUN provision capability.

    When sufficient space is available, LUNs can be created by specifying
    the target port to host the LUN and a desired size in 4K blocks. Upon
    success, the LUN ID and WWID of the created LUN will be returned and
    the SCSI bus can be scanned to detect the change in LUN topology. Note
    that partial allocations are not supported. Should a creation fail due
    to a space issue, the target port can be queried for its current LUN
    geometry.

    To remove a LUN, the device must first be disassociated from the Linux
    SCSI subsystem. The LUN deletion can then be initiated by specifying a
    target port and LUN ID. Upon success, the LUN geometry associated with
    the port will be updated to reflect new number of provisioned LUNs and
    available capacity.

    To query the LUN geometry of a port, the target port is specified and
    upon success, the following information is presented:

        - Maximum number of provisioned LUNs allowed for the port
        - Current number of provisioned LUNs for the port
        - Maximum total capacity of provisioned LUNs for the port (4K blocks)
        - Current total capacity of provisioned LUNs for the port (4K blocks)

    With this information, the number of available LUNs and capacity can be
    can be calculated.

HT_CXLFLASH_AFU_DEBUG
---------------------
    This ioctl is used to debug AFUs by supporting a command pass-through
    interface. It is only valid when used with AFUs that support the AFU
    debug capability.

    With exception of buffer management, AFU debug commands are opaque to
    cxlflash and treated as pass-through. For debug commands that do require
    data transfer, the user supplies an adequately sized data buffer and must
    specify the data transfer direction with respect to the host. There is a
    maximum transfer size of 256K imposed. Note that partial read completions
    are not supported - when errors are experienced with a host read data
    transfer, the data buffer is not copied back to the user.