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1 | Coherent Accelerator Interface (CXL) |
2 | ==================================== | |
3 | ||
4 | Introduction | |
5 | ============ | |
6 | ||
7 | The coherent accelerator interface is designed to allow the | |
8 | coherent connection of accelerators (FPGAs and other devices) to a | |
9 | POWER system. These devices need to adhere to the Coherent | |
10 | Accelerator Interface Architecture (CAIA). | |
11 | ||
12 | IBM refers to this as the Coherent Accelerator Processor Interface | |
13 | or CAPI. In the kernel it's referred to by the name CXL to avoid | |
14 | confusion with the ISDN CAPI subsystem. | |
15 | ||
16 | Coherent in this context means that the accelerator and CPUs can | |
17 | both access system memory directly and with the same effective | |
18 | addresses. | |
19 | ||
20 | ||
21 | Hardware overview | |
22 | ================= | |
23 | ||
24 | POWER8 FPGA | |
25 | +----------+ +---------+ | |
26 | | | | | | |
27 | | CPU | | AFU | | |
28 | | | | | | |
29 | | | | | | |
30 | | | | | | |
31 | +----------+ +---------+ | |
32 | | PHB | | | | |
33 | | +------+ | PSL | | |
34 | | | CAPP |<------>| | | |
35 | +---+------+ PCIE +---------+ | |
36 | ||
37 | The POWER8 chip has a Coherently Attached Processor Proxy (CAPP) | |
38 | unit which is part of the PCIe Host Bridge (PHB). This is managed | |
39 | by Linux by calls into OPAL. Linux doesn't directly program the | |
40 | CAPP. | |
41 | ||
42 | The FPGA (or coherently attached device) consists of two parts. | |
43 | The POWER Service Layer (PSL) and the Accelerator Function Unit | |
44 | (AFU). The AFU is used to implement specific functionality behind | |
45 | the PSL. The PSL, among other things, provides memory address | |
46 | translation services to allow each AFU direct access to userspace | |
47 | memory. | |
48 | ||
49 | The AFU is the core part of the accelerator (eg. the compression, | |
50 | crypto etc function). The kernel has no knowledge of the function | |
51 | of the AFU. Only userspace interacts directly with the AFU. | |
52 | ||
53 | The PSL provides the translation and interrupt services that the | |
54 | AFU needs. This is what the kernel interacts with. For example, if | |
55 | the AFU needs to read a particular effective address, it sends | |
56 | that address to the PSL, the PSL then translates it, fetches the | |
57 | data from memory and returns it to the AFU. If the PSL has a | |
58 | translation miss, it interrupts the kernel and the kernel services | |
59 | the fault. The context to which this fault is serviced is based on | |
60 | who owns that acceleration function. | |
61 | ||
62 | ||
63 | AFU Modes | |
64 | ========= | |
65 | ||
66 | There are two programming modes supported by the AFU. Dedicated | |
67 | and AFU directed. AFU may support one or both modes. | |
68 | ||
69 | When using dedicated mode only one MMU context is supported. In | |
70 | this mode, only one userspace process can use the accelerator at | |
71 | time. | |
72 | ||
73 | When using AFU directed mode, up to 16K simultaneous contexts can | |
74 | be supported. This means up to 16K simultaneous userspace | |
75 | applications may use the accelerator (although specific AFUs may | |
76 | support fewer). In this mode, the AFU sends a 16 bit context ID | |
77 | with each of its requests. This tells the PSL which context is | |
78 | associated with each operation. If the PSL can't translate an | |
79 | operation, the ID can also be accessed by the kernel so it can | |
80 | determine the userspace context associated with an operation. | |
81 | ||
82 | ||
83 | MMIO space | |
84 | ========== | |
85 | ||
86 | A portion of the accelerator MMIO space can be directly mapped | |
87 | from the AFU to userspace. Either the whole space can be mapped or | |
88 | just a per context portion. The hardware is self describing, hence | |
89 | the kernel can determine the offset and size of the per context | |
90 | portion. | |
91 | ||
92 | ||
93 | Interrupts | |
94 | ========== | |
95 | ||
96 | AFUs may generate interrupts that are destined for userspace. These | |
97 | are received by the kernel as hardware interrupts and passed onto | |
98 | userspace by a read syscall documented below. | |
99 | ||
100 | Data storage faults and error interrupts are handled by the kernel | |
101 | driver. | |
102 | ||
103 | ||
104 | Work Element Descriptor (WED) | |
105 | ============================= | |
106 | ||
107 | The WED is a 64-bit parameter passed to the AFU when a context is | |
108 | started. Its format is up to the AFU hence the kernel has no | |
109 | knowledge of what it represents. Typically it will be the | |
110 | effective address of a work queue or status block where the AFU | |
111 | and userspace can share control and status information. | |
112 | ||
113 | ||
114 | ||
115 | ||
116 | User API | |
117 | ======== | |
118 | ||
119 | For AFUs operating in AFU directed mode, two character device | |
120 | files will be created. /dev/cxl/afu0.0m will correspond to a | |
121 | master context and /dev/cxl/afu0.0s will correspond to a slave | |
122 | context. Master contexts have access to the full MMIO space an | |
123 | AFU provides. Slave contexts have access to only the per process | |
124 | MMIO space an AFU provides. | |
125 | ||
126 | For AFUs operating in dedicated process mode, the driver will | |
127 | only create a single character device per AFU called | |
128 | /dev/cxl/afu0.0d. This will have access to the entire MMIO space | |
129 | that the AFU provides (like master contexts in AFU directed). | |
130 | ||
131 | The types described below are defined in include/uapi/misc/cxl.h | |
132 | ||
133 | The following file operations are supported on both slave and | |
134 | master devices. | |
135 | ||
dc12f20b | 136 | A userspace library libcxl is available here: |
aee85fb6 MN |
137 | https://github.com/ibm-capi/libcxl |
138 | This provides a C interface to this kernel API. | |
a9282d01 IM |
139 | |
140 | open | |
141 | ---- | |
142 | ||
143 | Opens the device and allocates a file descriptor to be used with | |
144 | the rest of the API. | |
145 | ||
146 | A dedicated mode AFU only has one context and only allows the | |
147 | device to be opened once. | |
148 | ||
149 | An AFU directed mode AFU can have many contexts, the device can be | |
150 | opened once for each context that is available. | |
151 | ||
152 | When all available contexts are allocated the open call will fail | |
153 | and return -ENOSPC. | |
154 | ||
155 | Note: IRQs need to be allocated for each context, which may limit | |
156 | the number of contexts that can be created, and therefore | |
157 | how many times the device can be opened. The POWER8 CAPP | |
158 | supports 2040 IRQs and 3 are used by the kernel, so 2037 are | |
159 | left. If 1 IRQ is needed per context, then only 2037 | |
160 | contexts can be allocated. If 4 IRQs are needed per context, | |
161 | then only 2037/4 = 509 contexts can be allocated. | |
162 | ||
163 | ||
164 | ioctl | |
165 | ----- | |
166 | ||
167 | CXL_IOCTL_START_WORK: | |
168 | Starts the AFU context and associates it with the current | |
169 | process. Once this ioctl is successfully executed, all memory | |
170 | mapped into this process is accessible to this AFU context | |
171 | using the same effective addresses. No additional calls are | |
172 | required to map/unmap memory. The AFU memory context will be | |
173 | updated as userspace allocates and frees memory. This ioctl | |
174 | returns once the AFU context is started. | |
175 | ||
176 | Takes a pointer to a struct cxl_ioctl_start_work: | |
177 | ||
178 | struct cxl_ioctl_start_work { | |
179 | __u64 flags; | |
180 | __u64 work_element_descriptor; | |
181 | __u64 amr; | |
182 | __s16 num_interrupts; | |
183 | __s16 reserved1; | |
184 | __s32 reserved2; | |
185 | __u64 reserved3; | |
186 | __u64 reserved4; | |
187 | __u64 reserved5; | |
188 | __u64 reserved6; | |
189 | }; | |
190 | ||
191 | flags: | |
192 | Indicates which optional fields in the structure are | |
193 | valid. | |
194 | ||
195 | work_element_descriptor: | |
196 | The Work Element Descriptor (WED) is a 64-bit argument | |
197 | defined by the AFU. Typically this is an effective | |
198 | address pointing to an AFU specific structure | |
199 | describing what work to perform. | |
200 | ||
201 | amr: | |
202 | Authority Mask Register (AMR), same as the powerpc | |
203 | AMR. This field is only used by the kernel when the | |
204 | corresponding CXL_START_WORK_AMR value is specified in | |
205 | flags. If not specified the kernel will use a default | |
206 | value of 0. | |
207 | ||
208 | num_interrupts: | |
209 | Number of userspace interrupts to request. This field | |
210 | is only used by the kernel when the corresponding | |
211 | CXL_START_WORK_NUM_IRQS value is specified in flags. | |
212 | If not specified the minimum number required by the | |
213 | AFU will be allocated. The min and max number can be | |
214 | obtained from sysfs. | |
215 | ||
216 | reserved fields: | |
217 | For ABI padding and future extensions | |
218 | ||
219 | CXL_IOCTL_GET_PROCESS_ELEMENT: | |
220 | Get the current context id, also known as the process element. | |
221 | The value is returned from the kernel as a __u32. | |
222 | ||
223 | ||
224 | mmap | |
225 | ---- | |
226 | ||
227 | An AFU may have an MMIO space to facilitate communication with the | |
228 | AFU. If it does, the MMIO space can be accessed via mmap. The size | |
229 | and contents of this area are specific to the particular AFU. The | |
230 | size can be discovered via sysfs. | |
231 | ||
232 | In AFU directed mode, master contexts are allowed to map all of | |
233 | the MMIO space and slave contexts are allowed to only map the per | |
234 | process MMIO space associated with the context. In dedicated | |
235 | process mode the entire MMIO space can always be mapped. | |
236 | ||
237 | This mmap call must be done after the START_WORK ioctl. | |
238 | ||
239 | Care should be taken when accessing MMIO space. Only 32 and 64-bit | |
240 | accesses are supported by POWER8. Also, the AFU will be designed | |
241 | with a specific endianness, so all MMIO accesses should consider | |
242 | endianness (recommend endian(3) variants like: le64toh(), | |
243 | be64toh() etc). These endian issues equally apply to shared memory | |
244 | queues the WED may describe. | |
245 | ||
246 | ||
247 | read | |
248 | ---- | |
249 | ||
250 | Reads events from the AFU. Blocks if no events are pending | |
251 | (unless O_NONBLOCK is supplied). Returns -EIO in the case of an | |
252 | unrecoverable error or if the card is removed. | |
253 | ||
254 | read() will always return an integral number of events. | |
255 | ||
256 | The buffer passed to read() must be at least 4K bytes. | |
257 | ||
258 | The result of the read will be a buffer of one or more events, | |
259 | each event is of type struct cxl_event, of varying size. | |
260 | ||
261 | struct cxl_event { | |
262 | struct cxl_event_header header; | |
263 | union { | |
264 | struct cxl_event_afu_interrupt irq; | |
265 | struct cxl_event_data_storage fault; | |
266 | struct cxl_event_afu_error afu_error; | |
267 | }; | |
268 | }; | |
269 | ||
270 | The struct cxl_event_header is defined as: | |
271 | ||
272 | struct cxl_event_header { | |
273 | __u16 type; | |
274 | __u16 size; | |
275 | __u16 process_element; | |
276 | __u16 reserved1; | |
277 | }; | |
278 | ||
279 | type: | |
280 | This defines the type of event. The type determines how | |
281 | the rest of the event is structured. These types are | |
282 | described below and defined by enum cxl_event_type. | |
283 | ||
284 | size: | |
285 | This is the size of the event in bytes including the | |
286 | struct cxl_event_header. The start of the next event can | |
287 | be found at this offset from the start of the current | |
288 | event. | |
289 | ||
290 | process_element: | |
291 | Context ID of the event. | |
292 | ||
293 | reserved field: | |
294 | For future extensions and padding. | |
295 | ||
296 | If the event type is CXL_EVENT_AFU_INTERRUPT then the event | |
297 | structure is defined as: | |
298 | ||
299 | struct cxl_event_afu_interrupt { | |
300 | __u16 flags; | |
301 | __u16 irq; /* Raised AFU interrupt number */ | |
302 | __u32 reserved1; | |
303 | }; | |
304 | ||
305 | flags: | |
306 | These flags indicate which optional fields are present | |
307 | in this struct. Currently all fields are mandatory. | |
308 | ||
309 | irq: | |
310 | The IRQ number sent by the AFU. | |
311 | ||
312 | reserved field: | |
313 | For future extensions and padding. | |
314 | ||
315 | If the event type is CXL_EVENT_DATA_STORAGE then the event | |
316 | structure is defined as: | |
317 | ||
318 | struct cxl_event_data_storage { | |
319 | __u16 flags; | |
320 | __u16 reserved1; | |
321 | __u32 reserved2; | |
322 | __u64 addr; | |
323 | __u64 dsisr; | |
324 | __u64 reserved3; | |
325 | }; | |
326 | ||
327 | flags: | |
328 | These flags indicate which optional fields are present in | |
329 | this struct. Currently all fields are mandatory. | |
330 | ||
331 | address: | |
332 | The address that the AFU unsuccessfully attempted to | |
333 | access. Valid accesses will be handled transparently by the | |
334 | kernel but invalid accesses will generate this event. | |
335 | ||
336 | dsisr: | |
337 | This field gives information on the type of fault. It is a | |
338 | copy of the DSISR from the PSL hardware when the address | |
339 | fault occurred. The form of the DSISR is as defined in the | |
340 | CAIA. | |
341 | ||
342 | reserved fields: | |
343 | For future extensions | |
344 | ||
345 | If the event type is CXL_EVENT_AFU_ERROR then the event structure | |
346 | is defined as: | |
347 | ||
348 | struct cxl_event_afu_error { | |
349 | __u16 flags; | |
350 | __u16 reserved1; | |
351 | __u32 reserved2; | |
352 | __u64 error; | |
353 | }; | |
354 | ||
355 | flags: | |
356 | These flags indicate which optional fields are present in | |
357 | this struct. Currently all fields are Mandatory. | |
358 | ||
359 | error: | |
360 | Error status from the AFU. Defined by the AFU. | |
361 | ||
362 | reserved fields: | |
363 | For future extensions and padding | |
364 | ||
365 | Sysfs Class | |
366 | =========== | |
367 | ||
368 | A cxl sysfs class is added under /sys/class/cxl to facilitate | |
369 | enumeration and tuning of the accelerators. Its layout is | |
370 | described in Documentation/ABI/testing/sysfs-class-cxl | |
371 | ||
aee85fb6 | 372 | |
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373 | Udev rules |
374 | ========== | |
375 | ||
376 | The following udev rules could be used to create a symlink to the | |
377 | most logical chardev to use in any programming mode (afuX.Yd for | |
378 | dedicated, afuX.Ys for afu directed), since the API is virtually | |
379 | identical for each: | |
380 | ||
381 | SUBSYSTEM=="cxl", ATTRS{mode}=="dedicated_process", SYMLINK="cxl/%b" | |
382 | SUBSYSTEM=="cxl", ATTRS{mode}=="afu_directed", \ | |
383 | KERNEL=="afu[0-9]*.[0-9]*s", SYMLINK="cxl/%b" |