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1 Introduction
2 ============
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.
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.
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.
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.
33 The cxlflash driver supports two, mutually exclusive, modes of
34 operation at the device (LUN) level:
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.
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.
48 Overview
49 ========
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.
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:
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 +-------------------------------+
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.
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).
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.
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.
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.
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).
108 Block library API
109 =================
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.
123 The block library can be found on GitHub:
124 http://github.com/open-power/capiflash
127 CXL Flash Driver LUN IOCTLs
128 ===========================
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.
143 The structure definitions for these IOCTLs are available in:
144 uapi/scsi/cxlflash_ioctl.h
146 DK_CXLFLASH_ATTACH
147 ------------------
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.
161 There are a few important aspects regarding the "tokens" (context id
162 and fd2) that are provided back to the user:
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).
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.
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.
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.
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:
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
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.
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.
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.
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.
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.
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.
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.
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.
242 When a LUN is accessible from more than one port, this ioctl will
243 return with the DK_CXLFLASH_ALL_PORTS_ACTIVE return flag set. This
244 provides the user with a hint that I/O can be retried in the event
245 of an I/O error as the LUN can be reached over multiple paths.
247 DK_CXLFLASH_VLUN_RESIZE
248 -----------------------
249 This ioctl is responsible for resizing a previously created virtual
250 LUN and will fail if invoked upon a LUN that is not in virtual
251 mode. Upon success, an updated last LBA is returned to the user
252 indicating the new size of the virtual LUN associated with the
253 resource handle.
255 The partitioning of virtual LUNs is jointly mediated by the cxlflash
256 driver and the AFU. An allocation table is kept for each LUN that is
257 operating in the virtual mode and used to program a LUN translation
258 table that the AFU references when provided with a resource handle.
260 This ioctl can return -EAGAIN if an AFU sync operation takes too long.
261 In addition to returning a failure to user, cxlflash will also schedule
262 an asynchronous AFU reset. Should the user choose to retry the operation,
263 it is expected to succeed. If this ioctl fails with -EAGAIN, the user
264 can either retry the operation or treat it as a failure.
266 DK_CXLFLASH_RELEASE
267 -------------------
268 This ioctl is responsible for releasing a previously obtained
269 reference to either a physical or virtual LUN. This can be
270 thought of as the inverse of the DK_CXLFLASH_USER_DIRECT or
271 DK_CXLFLASH_USER_VIRTUAL ioctls. Upon success, the resource handle
272 is no longer valid and the entry in the resource handle table is
273 made available to be used again.
275 As part of the release process for virtual LUNs, the virtual LUN
276 is first resized to 0 to clear out and free the translation tables
277 associated with the virtual LUN reference.
279 DK_CXLFLASH_DETACH
280 ------------------
281 This ioctl is responsible for unregistering a context with the
282 cxlflash driver and release outstanding resources that were
283 not explicitly released via the DK_CXLFLASH_RELEASE ioctl. Upon
284 success, all "tokens" which had been provided to the user from the
285 DK_CXLFLASH_ATTACH onward are no longer valid.
287 When the DK_CXLFLASH_APP_CLOSE_ADAP_FD flag was returned on a successful
288 attach, the application _must_ close the fd2 associated with the context
289 following the detach of the final user of the context.
291 DK_CXLFLASH_VLUN_CLONE
292 ----------------------
293 This ioctl is responsible for cloning a previously created
294 context to a more recently created context. It exists solely to
295 support maintaining user space access to storage after a process
296 forks. Upon success, the child process (which invoked the ioctl)
297 will have access to the same LUNs via the same resource handle(s)
298 as the parent, but under a different context.
300 Context sharing across processes is not supported with CXL and
301 therefore each fork must be met with establishing a new context
302 for the child process. This ioctl simplifies the state management
303 and playback required by a user in such a scenario. When a process
304 forks, child process can clone the parents context by first creating
305 a context (via DK_CXLFLASH_ATTACH) and then using this ioctl to
306 perform the clone from the parent to the child.
308 The clone itself is fairly simple. The resource handle and lun
309 translation tables are copied from the parent context to the child's
310 and then synced with the AFU.
312 When the DK_CXLFLASH_APP_CLOSE_ADAP_FD flag was returned on a successful
313 attach, the application _must_ close the fd2 associated with the source
314 context (still resident/accessible in the parent process) following the
315 clone. This is to avoid a stale entry in the file descriptor table of the
316 child process.
318 This ioctl can return -EAGAIN if an AFU sync operation takes too long.
319 In addition to returning a failure to user, cxlflash will also schedule
320 an asynchronous AFU reset. Should the user choose to retry the operation,
321 it is expected to succeed. If this ioctl fails with -EAGAIN, the user
322 can either retry the operation or treat it as a failure.
324 DK_CXLFLASH_VERIFY
325 ------------------
326 This ioctl is used to detect various changes such as the capacity of
327 the disk changing, the number of LUNs visible changing, etc. In cases
328 where the changes affect the application (such as a LUN resize), the
329 cxlflash driver will report the changed state to the application.
331 The user calls in when they want to validate that a LUN hasn't been
332 changed in response to a check condition. As the user is operating out
333 of band from the kernel, they will see these types of events without
334 the kernel's knowledge. When encountered, the user's architected
335 behavior is to call in to this ioctl, indicating what they want to
336 verify and passing along any appropriate information. For now, only
337 verifying a LUN change (ie: size different) with sense data is
338 supported.
340 DK_CXLFLASH_RECOVER_AFU
341 -----------------------
342 This ioctl is used to drive recovery (if such an action is warranted)
343 of a specified user context. Any state associated with the user context
344 is re-established upon successful recovery.
346 User contexts are put into an error condition when the device needs to
347 be reset or is terminating. Users are notified of this error condition
348 by seeing all 0xF's on an MMIO read. Upon encountering this, the
349 architected behavior for a user is to call into this ioctl to recover
350 their context. A user may also call into this ioctl at any time to
351 check if the device is operating normally. If a failure is returned
352 from this ioctl, the user is expected to gracefully clean up their
353 context via release/detach ioctls. Until they do, the context they
354 hold is not relinquished. The user may also optionally exit the process
355 at which time the context/resources they held will be freed as part of
356 the release fop.
358 When the DK_CXLFLASH_APP_CLOSE_ADAP_FD flag was returned on a successful
359 attach, the application _must_ unmap and close the fd2 associated with the
360 original context following this ioctl returning success and indicating that
361 the context was recovered (DK_CXLFLASH_RECOVER_AFU_CONTEXT_RESET).
363 DK_CXLFLASH_MANAGE_LUN
364 ----------------------
365 This ioctl is used to switch a LUN from a mode where it is available
366 for file-system access (legacy), to a mode where it is set aside for
367 exclusive user space access (superpipe). In case a LUN is visible
368 across multiple ports and adapters, this ioctl is used to uniquely
369 identify each LUN by its World Wide Node Name (WWNN).
372 CXL Flash Driver Host IOCTLs
373 ============================
375 Each host adapter instance that is supported by the cxlflash driver
376 has a special character device associated with it to enable a set of
377 host management function. These character devices are hosted in a
378 class dedicated for cxlflash and can be accessed via /dev/cxlflash/*.
380 Applications can be written to perform various functions using the
381 host ioctl APIs below.
383 The structure definitions for these IOCTLs are available in:
384 uapi/scsi/cxlflash_ioctl.h
386 HT_CXLFLASH_LUN_PROVISION
387 -------------------------
388 This ioctl is used to create and delete persistent LUNs on cxlflash
389 devices that lack an external LUN management interface. It is only
390 valid when used with AFUs that support the LUN provision capability.
392 When sufficient space is available, LUNs can be created by specifying
393 the target port to host the LUN and a desired size in 4K blocks. Upon
394 success, the LUN ID and WWID of the created LUN will be returned and
395 the SCSI bus can be scanned to detect the change in LUN topology. Note
396 that partial allocations are not supported. Should a creation fail due
397 to a space issue, the target port can be queried for its current LUN
398 geometry.
400 To remove a LUN, the device must first be disassociated from the Linux
401 SCSI subsystem. The LUN deletion can then be initiated by specifying a
402 target port and LUN ID. Upon success, the LUN geometry associated with
403 the port will be updated to reflect new number of provisioned LUNs and
404 available capacity.
406 To query the LUN geometry of a port, the target port is specified and
407 upon success, the following information is presented:
409 - Maximum number of provisioned LUNs allowed for the port
410 - Current number of provisioned LUNs for the port
411 - Maximum total capacity of provisioned LUNs for the port (4K blocks)
412 - Current total capacity of provisioned LUNs for the port (4K blocks)
414 With this information, the number of available LUNs and capacity can be
415 can be calculated.
417 HT_CXLFLASH_AFU_DEBUG
418 ---------------------
419 This ioctl is used to debug AFUs by supporting a command pass-through
420 interface. It is only valid when used with AFUs that support the AFU
421 debug capability.
423 With exception of buffer management, AFU debug commands are opaque to
424 cxlflash and treated as pass-through. For debug commands that do require
425 data transfer, the user supplies an adequately sized data buffer and must
426 specify the data transfer direction with respect to the host. There is a
427 maximum transfer size of 256K imposed. Note that partial read completions
428 are not supported - when errors are experienced with a host read data
429 transfer, the data buffer is not copied back to the user.