Documentation/powerpc/cxlflash.txt

   1 Introduction
   2 ============
   3
   4     The IBM Power architecture provides support for CAPI (Coherent
   5     Accelerator Power Interface), which is available to certain PCIe slots
   6     on Power 8 systems. CAPI can be thought of as a special tunneling
   7     protocol through PCIe that allow PCIe adapters to look like special
   8     purpose co-processors which can read or write an application's
   9     memory and generate page faults. As a result, the host interface to
  10     an adapter running in CAPI mode does not require the data buffers to
  11     be mapped to the device's memory (IOMMU bypass) nor does it require
  12     memory to be pinned.
  13
  14     On Linux, Coherent Accelerator (CXL) kernel services present CAPI
  15     devices as a PCI device by implementing a virtual PCI host bridge.
  16     This abstraction simplifies the infrastructure and programming
  17     model, allowing for drivers to look similar to other native PCI
  18     device drivers.
  19
  20     CXL provides a mechanism by which user space applications can
  21     directly talk to a device (network or storage) bypassing the typical
  22     kernel/device driver stack. The CXL Flash Adapter Driver enables a
  23     user space application direct access to Flash storage.
  24
  25     The CXL Flash Adapter Driver is a kernel module that sits in the
  26     SCSI stack as a low level device driver (below the SCSI disk and
  27     protocol drivers) for the IBM CXL Flash Adapter. This driver is
  28     responsible for the initialization of the adapter, setting up the
  29     special path for user space access, and performing error recovery. It
  30     communicates directly the Flash Accelerator Functional Unit (AFU)
  31     as described in Documentation/powerpc/cxl.txt.
  32
  33     The cxlflash driver supports two, mutually exclusive, modes of
  34     operation at the device (LUN) level:
  35
  36         - Any flash device (LUN) can be configured to be accessed as a
  37           regular disk device (i.e.: /dev/sdc). This is the default mode.
  38
  39         - Any flash device (LUN) can be configured to be accessed from
  40           user space with a special block library. This mode further
  41           specifies the means of accessing the device and provides for
  42           either raw access to the entire LUN (referred to as direct
  43           or physical LUN access) or access to a kernel/AFU-mediated
  44           partition of the LUN (referred to as virtual LUN access). The
  45           segmentation of a disk device into virtual LUNs is assisted
  46           by special translation services provided by the Flash AFU.
  47
  48 Overview
  49 ========
  50
  51     The Coherent Accelerator Interface Architecture (CAIA) introduces a
  52     concept of a master context. A master typically has special privileges
  53     granted to it by the kernel or hypervisor allowing it to perform AFU
  54     wide management and control. The master may or may not be involved
  55     directly in each user I/O, but at the minimum is involved in the
  56     initial setup before the user application is allowed to send requests
  57     directly to the AFU.
  58
  59     The CXL Flash Adapter Driver establishes a master context with the
  60     AFU. It uses memory mapped I/O (MMIO) for this control and setup. The
  61     Adapter Problem Space Memory Map looks like this:
  62
  63                      +-------------------------------+
  64                      |    512 * 64 KB User MMIO      |
  65                      |        (per context)          |
  66                      |       User Accessible         |
  67                      +-------------------------------+
  68                      |    512 * 128 B per context    |
  69                      |    Provisioning and Control   |
  70                      |   Trusted Process accessible  |
  71                      +-------------------------------+
  72                      |         64 KB Global          |
  73                      |   Trusted Process accessible  |
  74                      +-------------------------------+
  75
  76     This driver configures itself into the SCSI software stack as an
  77     adapter driver. The driver is the only entity that is considered a
  78     Trusted Process to program the Provisioning and Control and Global
  79     areas in the MMIO Space shown above.  The master context driver
  80     discovers all LUNs attached to the CXL Flash adapter and instantiates
  81     scsi block devices (/dev/sdb, /dev/sdc etc.) for each unique LUN
  82     seen from each path.
  83
  84     Once these scsi block devices are instantiated, an application
  85     written to a specification provided by the block library may get
  86     access to the Flash from user space (without requiring a system call).
  87
  88     This master context driver also provides a series of ioctls for this
  89     block library to enable this user space access.  The driver supports
  90     two modes for accessing the block device.
  91
  92     The first mode is called a virtual mode. In this mode a single scsi
  93     block device (/dev/sdb) may be carved up into any number of distinct
  94     virtual LUNs. The virtual LUNs may be resized as long as the sum of
  95     the sizes of all the virtual LUNs, along with the meta-data associated
  96     with it does not exceed the physical capacity.
  97
  98     The second mode is called the physical mode. In this mode a single
  99     block device (/dev/sdb) may be opened directly by the block library
 100     and the entire space for the LUN is available to the application.
 101
 102     Only the physical mode provides persistence of the data.  i.e. The
 103     data written to the block device will survive application exit and
 104     restart and also reboot. The virtual LUNs do not persist (i.e. do
 105     not survive after the application terminates or the system reboots).
 106
 107
 108 Block library API
 109 =================
 110
 111     Applications intending to get access to the CXL Flash from user
 112     space should use the block library, as it abstracts the details of
 113     interfacing directly with the cxlflash driver that are necessary for
 114     performing administrative actions (i.e.: setup, tear down, resize).
 115     The block library can be thought of as a 'user' of services,
 116     implemented as IOCTLs, that are provided by the cxlflash driver
 117     specifically for devices (LUNs) operating in user space access
 118     mode. While it is not a requirement that applications understand
 119     the interface between the block library and the cxlflash driver,
 120     a high-level overview of each supported service (IOCTL) is provided
 121     below.
 122
 123     The block library can be found on GitHub:
 124     http://github.com/open-power/capiflash
 125
 126
 127 CXL Flash Driver IOCTLs
 128 =======================
 129
 130     Users, such as the block library, that wish to interface with a flash
 131     device (LUN) via user space access need to use the services provided
 132     by the cxlflash driver. As these services are implemented as ioctls,
 133     a file descriptor handle must first be obtained in order to establish
 134     the communication channel between a user and the kernel.  This file
 135     descriptor is obtained by opening the device special file associated
 136     with the scsi disk device (/dev/sdb) that was created during LUN
 137     discovery. As per the location of the cxlflash driver within the
 138     SCSI protocol stack, this open is actually not seen by the cxlflash
 139     driver. Upon successful open, the user receives a file descriptor
 140     (herein referred to as fd1) that should be used for issuing the
 141     subsequent ioctls listed below.
 142
 143     The structure definitions for these IOCTLs are available in:
 144     uapi/scsi/cxlflash_ioctl.h
 145
 146 DK_CXLFLASH_ATTACH
 147 ------------------
 148
 149     This ioctl obtains, initializes, and starts a context using the CXL
 150     kernel services. These services specify a context id (u16) by which
 151     to uniquely identify the context and its allocated resources. The
 152     services additionally provide a second file descriptor (herein
 153     referred to as fd2) that is used by the block library to initiate
 154     memory mapped I/O (via mmap()) to the CXL flash device and poll for
 155     completion events. This file descriptor is intentionally installed by
 156     this driver and not the CXL kernel services to allow for intermediary
 157     notification and access in the event of a non-user-initiated close(),
 158     such as a killed process. This design point is described in further
 159     detail in the description for the DK_CXLFLASH_DETACH ioctl.
 160
 161     There are a few important aspects regarding the "tokens" (context id
 162     and fd2) that are provided back to the user:
 163
 164         - These tokens are only valid for the process under which they
 165           were created. The child of a forked process cannot continue
 166           to use the context id or file descriptor created by its parent
 167           (see DK_CXLFLASH_VLUN_CLONE for further details).
 168
 169         - These tokens are only valid for the lifetime of the context and
 170           the process under which they were created. Once either is
 171           destroyed, the tokens are to be considered stale and subsequent
 172           usage will result in errors.
 173
 174         - A valid adapter file descriptor (fd2 >= 0) is only returned on
 175           the initial attach for a context. Subsequent attaches to an
 176           existing context (DK_CXLFLASH_ATTACH_REUSE_CONTEXT flag present)
 177           do not provide the adapter file descriptor as it was previously
 178           made known to the application.
 179
 180         - When a context is no longer needed, the user shall detach from
 181           the context via the DK_CXLFLASH_DETACH ioctl. When this ioctl
 182           returns with a valid adapter file descriptor and the return flag
 183           DK_CXLFLASH_APP_CLOSE_ADAP_FD is present, the application _must_
 184           close the adapter file descriptor following a successful detach.
 185
 186         - When this ioctl returns with a valid fd2 and the return flag
 187           DK_CXLFLASH_APP_CLOSE_ADAP_FD is present, the application _must_
 188           close fd2 in the following circumstances:
 189
 190                 + Following a successful detach of the last user of the context
 191                 + Following a successful recovery on the context's original fd2
 192                 + In the child process of a fork(), following a clone ioctl,
 193                   on the fd2 associated with the source context
 194
 195         - At any time, a close on fd2 will invalidate the tokens. Applications
 196           should exercise caution to only close fd2 when appropriate (outlined
 197           in the previous bullet) to avoid premature loss of I/O.
 198
 199 DK_CXLFLASH_USER_DIRECT
 200 -----------------------
 201     This ioctl is responsible for transitioning the LUN to direct
 202     (physical) mode access and configuring the AFU for direct access from
 203     user space on a per-context basis. Additionally, the block size and
 204     last logical block address (LBA) are returned to the user.
 205
 206     As mentioned previously, when operating in user space access mode,
 207     LUNs may be accessed in whole or in part. Only one mode is allowed
 208     at a time and if one mode is active (outstanding references exist),
 209     requests to use the LUN in a different mode are denied.
 210
 211     The AFU is configured for direct access from user space by adding an
 212     entry to the AFU's resource handle table. The index of the entry is
 213     treated as a resource handle that is returned to the user. The user
 214     is then able to use the handle to reference the LUN during I/O.
 215
 216 DK_CXLFLASH_USER_VIRTUAL
 217 ------------------------
 218     This ioctl is responsible for transitioning the LUN to virtual mode
 219     of access and configuring the AFU for virtual access from user space
 220     on a per-context basis. Additionally, the block size and last logical
 221     block address (LBA) are returned to the user.
 222
 223     As mentioned previously, when operating in user space access mode,
 224     LUNs may be accessed in whole or in part. Only one mode is allowed
 225     at a time and if one mode is active (outstanding references exist),
 226     requests to use the LUN in a different mode are denied.
 227
 228     The AFU is configured for virtual access from user space by adding
 229     an entry to the AFU's resource handle table. The index of the entry
 230     is treated as a resource handle that is returned to the user. The
 231     user is then able to use the handle to reference the LUN during I/O.
 232
 233     By default, the virtual LUN is created with a size of 0. The user
 234     would need to use the DK_CXLFLASH_VLUN_RESIZE ioctl to adjust the grow
 235     the virtual LUN to a desired size. To avoid having to perform this
 236     resize for the initial creation of the virtual LUN, the user has the
 237     option of specifying a size as part of the DK_CXLFLASH_USER_VIRTUAL
 238     ioctl, such that when success is returned to the user, the
 239     resource handle that is provided is already referencing provisioned
 240     storage. This is reflected by the last LBA being a non-zero value.
 241
 242 DK_CXLFLASH_VLUN_RESIZE
 243 -----------------------
 244     This ioctl is responsible for resizing a previously created virtual
 245     LUN and will fail if invoked upon a LUN that is not in virtual
 246     mode. Upon success, an updated last LBA is returned to the user
 247     indicating the new size of the virtual LUN associated with the
 248     resource handle.
 249
 250     The partitioning of virtual LUNs is jointly mediated by the cxlflash
 251     driver and the AFU. An allocation table is kept for each LUN that is
 252     operating in the virtual mode and used to program a LUN translation
 253     table that the AFU references when provided with a resource handle.
 254
 255 DK_CXLFLASH_RELEASE
 256 -------------------
 257     This ioctl is responsible for releasing a previously obtained
 258     reference to either a physical or virtual LUN. This can be
 259     thought of as the inverse of the DK_CXLFLASH_USER_DIRECT or
 260     DK_CXLFLASH_USER_VIRTUAL ioctls. Upon success, the resource handle
 261     is no longer valid and the entry in the resource handle table is
 262     made available to be used again.
 263
 264     As part of the release process for virtual LUNs, the virtual LUN
 265     is first resized to 0 to clear out and free the translation tables
 266     associated with the virtual LUN reference.
 267
 268 DK_CXLFLASH_DETACH
 269 ------------------
 270     This ioctl is responsible for unregistering a context with the
 271     cxlflash driver and release outstanding resources that were
 272     not explicitly released via the DK_CXLFLASH_RELEASE ioctl. Upon
 273     success, all "tokens" which had been provided to the user from the
 274     DK_CXLFLASH_ATTACH onward are no longer valid.
 275
 276     When the DK_CXLFLASH_APP_CLOSE_ADAP_FD flag was returned on a successful
 277     attach, the application _must_ close the fd2 associated with the context
 278     following the detach of the final user of the context.
 279
 280 DK_CXLFLASH_VLUN_CLONE
 281 ----------------------
 282     This ioctl is responsible for cloning a previously created
 283     context to a more recently created context. It exists solely to
 284     support maintaining user space access to storage after a process
 285     forks. Upon success, the child process (which invoked the ioctl)
 286     will have access to the same LUNs via the same resource handle(s)
 287     as the parent, but under a different context.
 288
 289     Context sharing across processes is not supported with CXL and
 290     therefore each fork must be met with establishing a new context
 291     for the child process. This ioctl simplifies the state management
 292     and playback required by a user in such a scenario. When a process
 293     forks, child process can clone the parents context by first creating
 294     a context (via DK_CXLFLASH_ATTACH) and then using this ioctl to
 295     perform the clone from the parent to the child.
 296
 297     The clone itself is fairly simple. The resource handle and lun
 298     translation tables are copied from the parent context to the child's
 299     and then synced with the AFU.
 300
 301     When the DK_CXLFLASH_APP_CLOSE_ADAP_FD flag was returned on a successful
 302     attach, the application _must_ close the fd2 associated with the source
 303     context (still resident/accessible in the parent process) following the
 304     clone. This is to avoid a stale entry in the file descriptor table of the
 305     child process.
 306
 307 DK_CXLFLASH_VERIFY
 308 ------------------
 309     This ioctl is used to detect various changes such as the capacity of
 310     the disk changing, the number of LUNs visible changing, etc. In cases
 311     where the changes affect the application (such as a LUN resize), the
 312     cxlflash driver will report the changed state to the application.
 313
 314     The user calls in when they want to validate that a LUN hasn't been
 315     changed in response to a check condition. As the user is operating out
 316     of band from the kernel, they will see these types of events without
 317     the kernel's knowledge. When encountered, the user's architected
 318     behavior is to call in to this ioctl, indicating what they want to
 319     verify and passing along any appropriate information. For now, only
 320     verifying a LUN change (ie: size different) with sense data is
 321     supported.
 322
 323 DK_CXLFLASH_RECOVER_AFU
 324 -----------------------
 325     This ioctl is used to drive recovery (if such an action is warranted)
 326     of a specified user context. Any state associated with the user context
 327     is re-established upon successful recovery.
 328
 329     User contexts are put into an error condition when the device needs to
 330     be reset or is terminating. Users are notified of this error condition
 331     by seeing all 0xF's on an MMIO read. Upon encountering this, the
 332     architected behavior for a user is to call into this ioctl to recover
 333     their context. A user may also call into this ioctl at any time to
 334     check if the device is operating normally. If a failure is returned
 335     from this ioctl, the user is expected to gracefully clean up their
 336     context via release/detach ioctls. Until they do, the context they
 337     hold is not relinquished. The user may also optionally exit the process
 338     at which time the context/resources they held will be freed as part of
 339     the release fop.
 340
 341     When the DK_CXLFLASH_APP_CLOSE_ADAP_FD flag was returned on a successful
 342     attach, the application _must_ unmap and close the fd2 associated with the
 343     original context following this ioctl returning success and indicating that
 344     the context was recovered (DK_CXLFLASH_RECOVER_AFU_CONTEXT_RESET).
 345
 346 DK_CXLFLASH_MANAGE_LUN
 347 ----------------------
 348     This ioctl is used to switch a LUN from a mode where it is available
 349     for file-system access (legacy), to a mode where it is set aside for
 350     exclusive user space access (superpipe). In case a LUN is visible
 351     across multiple ports and adapters, this ioctl is used to uniquely
 352     identify each LUN by its World Wide Node Name (WWNN).