1=================================================== 2Scalable Matrix Extension support for AArch64 Linux 3=================================================== 4 5This document outlines briefly the interface provided to userspace by Linux in 6order to support use of the ARM Scalable Matrix Extension (SME). 7 8This is an outline of the most important features and issues only and not 9intended to be exhaustive. It should be read in conjunction with the SVE 10documentation in sve.rst which provides details on the Streaming SVE mode 11included in SME. 12 13This document does not aim to describe the SME architecture or programmer's 14model. To aid understanding, a minimal description of relevant programmer's 15model features for SME is included in Appendix A. 16 17 181. General 19----------- 20 21* PSTATE.SM, PSTATE.ZA, the streaming mode vector length, the ZA and (when 22 present) ZTn register state and TPIDR2_EL0 are tracked per thread. 23 24* The presence of SME is reported to userspace via HWCAP2_SME in the aux vector 25 AT_HWCAP2 entry. Presence of this flag implies the presence of the SME 26 instructions and registers, and the Linux-specific system interfaces 27 described in this document. SME is reported in /proc/cpuinfo as "sme". 28 29* The presence of SME2 is reported to userspace via HWCAP2_SME2 in the 30 aux vector AT_HWCAP2 entry. Presence of this flag implies the presence of 31 the SME2 instructions and ZT0, and the Linux-specific system interfaces 32 described in this document. SME2 is reported in /proc/cpuinfo as "sme2". 33 34* Support for the execution of SME instructions in userspace can also be 35 detected by reading the CPU ID register ID_AA64PFR1_EL1 using an MRS 36 instruction, and checking that the value of the SME field is nonzero. [3] 37 38 It does not guarantee the presence of the system interfaces described in the 39 following sections: software that needs to verify that those interfaces are 40 present must check for HWCAP2_SME instead. 41 42* There are a number of optional SME features, presence of these is reported 43 through AT_HWCAP2 through: 44 45 HWCAP2_SME_I16I64 46 HWCAP2_SME_F64F64 47 HWCAP2_SME_I8I32 48 HWCAP2_SME_F16F32 49 HWCAP2_SME_B16F32 50 HWCAP2_SME_F32F32 51 HWCAP2_SME_FA64 52 HWCAP2_SME2 53 54 This list may be extended over time as the SME architecture evolves. 55 56 These extensions are also reported via the CPU ID register ID_AA64SMFR0_EL1, 57 which userspace can read using an MRS instruction. See elf_hwcaps.txt and 58 cpu-feature-registers.txt for details. 59 60* Debuggers should restrict themselves to interacting with the target via the 61 NT_ARM_SVE, NT_ARM_SSVE, NT_ARM_ZA and NT_ARM_ZT regsets. The recommended 62 way of detecting support for these regsets is to connect to a target process 63 first and then attempt a 64 65 ptrace(PTRACE_GETREGSET, pid, NT_ARM_<regset>, &iov). 66 67* Whenever ZA register values are exchanged in memory between userspace and 68 the kernel, the register value is encoded in memory as a series of horizontal 69 vectors from 0 to VL/8-1 stored in the same endianness invariant format as is 70 used for SVE vectors. 71 72* On thread creation TPIDR2_EL0 is preserved unless CLONE_SETTLS is specified, 73 in which case it is set to 0. 74 752. Vector lengths 76------------------ 77 78SME defines a second vector length similar to the SVE vector length which is 79controls the size of the streaming mode SVE vectors and the ZA matrix array. 80The ZA matrix is square with each side having as many bytes as a streaming 81mode SVE vector. 82 83 843. Sharing of streaming and non-streaming mode SVE state 85--------------------------------------------------------- 86 87It is implementation defined which if any parts of the SVE state are shared 88between streaming and non-streaming modes. When switching between modes 89via software interfaces such as ptrace if no register content is provided as 90part of switching no state will be assumed to be shared and everything will 91be zeroed. 92 93 944. System call behaviour 95------------------------- 96 97* On syscall PSTATE.ZA is preserved, if PSTATE.ZA==1 then the contents of the 98 ZA matrix and ZTn (if present) are preserved. 99 100* On syscall PSTATE.SM will be cleared and the SVE registers will be handled 101 as per the standard SVE ABI. 102 103* None of the SVE registers, ZA or ZTn are used to pass arguments to 104 or receive results from any syscall. 105 106* On process creation (eg, clone()) the newly created process will have 107 PSTATE.SM cleared. 108 109* All other SME state of a thread, including the currently configured vector 110 length, the state of the PR_SME_VL_INHERIT flag, and the deferred vector 111 length (if any), is preserved across all syscalls, subject to the specific 112 exceptions for execve() described in section 6. 113 114 1155. Signal handling 116------------------- 117 118* Signal handlers are invoked with streaming mode and ZA disabled. 119 120* A new signal frame record TPIDR2_MAGIC is added formatted as a struct 121 tpidr2_context to allow access to TPIDR2_EL0 from signal handlers. 122 123* A new signal frame record za_context encodes the ZA register contents on 124 signal delivery. [1] 125 126* The signal frame record for ZA always contains basic metadata, in particular 127 the thread's vector length (in za_context.vl). 128 129* The ZA matrix may or may not be included in the record, depending on 130 the value of PSTATE.ZA. The registers are present if and only if: 131 za_context.head.size >= ZA_SIG_CONTEXT_SIZE(sve_vq_from_vl(za_context.vl)) 132 in which case PSTATE.ZA == 1. 133 134* If matrix data is present, the remainder of the record has a vl-dependent 135 size and layout. Macros ZA_SIG_* are defined [1] to facilitate access to 136 them. 137 138* The matrix is stored as a series of horizontal vectors in the same format as 139 is used for SVE vectors. 140 141* If the ZA context is too big to fit in sigcontext.__reserved[], then extra 142 space is allocated on the stack, an extra_context record is written in 143 __reserved[] referencing this space. za_context is then written in the 144 extra space. Refer to [1] for further details about this mechanism. 145 146* If ZTn is supported and PSTATE.ZA==1 then a signal frame record for ZTn will 147 be generated. 148 149* The signal record for ZTn has magic ZT_MAGIC (0x5a544e01) and consists of a 150 standard signal frame header followed by a struct zt_context specifying 151 the number of ZTn registers supported by the system, then zt_context.nregs 152 blocks of 64 bytes of data per register. 153 154 1555. Signal return 156----------------- 157 158When returning from a signal handler: 159 160* If there is no za_context record in the signal frame, or if the record is 161 present but contains no register data as described in the previous section, 162 then ZA is disabled. 163 164* If za_context is present in the signal frame and contains matrix data then 165 PSTATE.ZA is set to 1 and ZA is populated with the specified data. 166 167* The vector length cannot be changed via signal return. If za_context.vl in 168 the signal frame does not match the current vector length, the signal return 169 attempt is treated as illegal, resulting in a forced SIGSEGV. 170 171* If ZTn is not supported or PSTATE.ZA==0 then it is illegal to have a 172 signal frame record for ZTn, resulting in a forced SIGSEGV. 173 174 1756. prctl extensions 176-------------------- 177 178Some new prctl() calls are added to allow programs to manage the SME vector 179length: 180 181prctl(PR_SME_SET_VL, unsigned long arg) 182 183 Sets the vector length of the calling thread and related flags, where 184 arg == vl | flags. Other threads of the calling process are unaffected. 185 186 vl is the desired vector length, where sve_vl_valid(vl) must be true. 187 188 flags: 189 190 PR_SME_VL_INHERIT 191 192 Inherit the current vector length across execve(). Otherwise, the 193 vector length is reset to the system default at execve(). (See 194 Section 9.) 195 196 PR_SME_SET_VL_ONEXEC 197 198 Defer the requested vector length change until the next execve() 199 performed by this thread. 200 201 The effect is equivalent to implicit execution of the following 202 call immediately after the next execve() (if any) by the thread: 203 204 prctl(PR_SME_SET_VL, arg & ~PR_SME_SET_VL_ONEXEC) 205 206 This allows launching of a new program with a different vector 207 length, while avoiding runtime side effects in the caller. 208 209 Without PR_SME_SET_VL_ONEXEC, the requested change takes effect 210 immediately. 211 212 213 Return value: a nonnegative on success, or a negative value on error: 214 EINVAL: SME not supported, invalid vector length requested, or 215 invalid flags. 216 217 218 On success: 219 220 * Either the calling thread's vector length or the deferred vector length 221 to be applied at the next execve() by the thread (dependent on whether 222 PR_SME_SET_VL_ONEXEC is present in arg), is set to the largest value 223 supported by the system that is less than or equal to vl. If vl == 224 SVE_VL_MAX, the value set will be the largest value supported by the 225 system. 226 227 * Any previously outstanding deferred vector length change in the calling 228 thread is cancelled. 229 230 * The returned value describes the resulting configuration, encoded as for 231 PR_SME_GET_VL. The vector length reported in this value is the new 232 current vector length for this thread if PR_SME_SET_VL_ONEXEC was not 233 present in arg; otherwise, the reported vector length is the deferred 234 vector length that will be applied at the next execve() by the calling 235 thread. 236 237 * Changing the vector length causes all of ZA, ZTn, P0..P15, FFR and all 238 bits of Z0..Z31 except for Z0 bits [127:0] .. Z31 bits [127:0] to become 239 unspecified, including both streaming and non-streaming SVE state. 240 Calling PR_SME_SET_VL with vl equal to the thread's current vector 241 length, or calling PR_SME_SET_VL with the PR_SVE_SET_VL_ONEXEC flag, 242 does not constitute a change to the vector length for this purpose. 243 244 * Changing the vector length causes PSTATE.ZA and PSTATE.SM to be cleared. 245 Calling PR_SME_SET_VL with vl equal to the thread's current vector 246 length, or calling PR_SME_SET_VL with the PR_SVE_SET_VL_ONEXEC flag, 247 does not constitute a change to the vector length for this purpose. 248 249 250prctl(PR_SME_GET_VL) 251 252 Gets the vector length of the calling thread. 253 254 The following flag may be OR-ed into the result: 255 256 PR_SME_VL_INHERIT 257 258 Vector length will be inherited across execve(). 259 260 There is no way to determine whether there is an outstanding deferred 261 vector length change (which would only normally be the case between a 262 fork() or vfork() and the corresponding execve() in typical use). 263 264 To extract the vector length from the result, bitwise and it with 265 PR_SME_VL_LEN_MASK. 266 267 Return value: a nonnegative value on success, or a negative value on error: 268 EINVAL: SME not supported. 269 270 2717. ptrace extensions 272--------------------- 273 274* A new regset NT_ARM_SSVE is defined for access to streaming mode SVE 275 state via PTRACE_GETREGSET and PTRACE_SETREGSET, this is documented in 276 sve.rst. 277 278* A new regset NT_ARM_ZA is defined for ZA state for access to ZA state via 279 PTRACE_GETREGSET and PTRACE_SETREGSET. 280 281 Refer to [2] for definitions. 282 283The regset data starts with struct user_za_header, containing: 284 285 size 286 287 Size of the complete regset, in bytes. 288 This depends on vl and possibly on other things in the future. 289 290 If a call to PTRACE_GETREGSET requests less data than the value of 291 size, the caller can allocate a larger buffer and retry in order to 292 read the complete regset. 293 294 max_size 295 296 Maximum size in bytes that the regset can grow to for the target 297 thread. The regset won't grow bigger than this even if the target 298 thread changes its vector length etc. 299 300 vl 301 302 Target thread's current streaming vector length, in bytes. 303 304 max_vl 305 306 Maximum possible streaming vector length for the target thread. 307 308 flags 309 310 Zero or more of the following flags, which have the same 311 meaning and behaviour as the corresponding PR_SET_VL_* flags: 312 313 SME_PT_VL_INHERIT 314 315 SME_PT_VL_ONEXEC (SETREGSET only). 316 317* The effects of changing the vector length and/or flags are equivalent to 318 those documented for PR_SME_SET_VL. 319 320 The caller must make a further GETREGSET call if it needs to know what VL is 321 actually set by SETREGSET, unless is it known in advance that the requested 322 VL is supported. 323 324* The size and layout of the payload depends on the header fields. The 325 ZA_PT_ZA*() macros are provided to facilitate access to the data. 326 327* In either case, for SETREGSET it is permissible to omit the payload, in which 328 case the vector length and flags are changed and PSTATE.ZA is set to 0 329 (along with any consequences of those changes). If a payload is provided 330 then PSTATE.ZA will be set to 1. 331 332* For SETREGSET, if the requested VL is not supported, the effect will be the 333 same as if the payload were omitted, except that an EIO error is reported. 334 No attempt is made to translate the payload data to the correct layout 335 for the vector length actually set. It is up to the caller to translate the 336 payload layout for the actual VL and retry. 337 338* The effect of writing a partial, incomplete payload is unspecified. 339 340* A new regset NT_ARM_ZT is defined for access to ZTn state via 341 PTRACE_GETREGSET and PTRACE_SETREGSET. 342 343* The NT_ARM_ZT regset consists of a single 512 bit register. 344 345* When PSTATE.ZA==0 reads of NT_ARM_ZT will report all bits of ZTn as 0. 346 347* Writes to NT_ARM_ZT will set PSTATE.ZA to 1. 348 349 3508. ELF coredump extensions 351--------------------------- 352 353* NT_ARM_SSVE notes will be added to each coredump for 354 each thread of the dumped process. The contents will be equivalent to the 355 data that would have been read if a PTRACE_GETREGSET of the corresponding 356 type were executed for each thread when the coredump was generated. 357 358* A NT_ARM_ZA note will be added to each coredump for each thread of the 359 dumped process. The contents will be equivalent to the data that would have 360 been read if a PTRACE_GETREGSET of NT_ARM_ZA were executed for each thread 361 when the coredump was generated. 362 363* A NT_ARM_ZT note will be added to each coredump for each thread of the 364 dumped process. The contents will be equivalent to the data that would have 365 been read if a PTRACE_GETREGSET of NT_ARM_ZT were executed for each thread 366 when the coredump was generated. 367 368* The NT_ARM_TLS note will be extended to two registers, the second register 369 will contain TPIDR2_EL0 on systems that support SME and will be read as 370 zero with writes ignored otherwise. 371 3729. System runtime configuration 373-------------------------------- 374 375* To mitigate the ABI impact of expansion of the signal frame, a policy 376 mechanism is provided for administrators, distro maintainers and developers 377 to set the default vector length for userspace processes: 378 379/proc/sys/abi/sme_default_vector_length 380 381 Writing the text representation of an integer to this file sets the system 382 default vector length to the specified value, unless the value is greater 383 than the maximum vector length supported by the system in which case the 384 default vector length is set to that maximum. 385 386 The result can be determined by reopening the file and reading its 387 contents. 388 389 At boot, the default vector length is initially set to 32 or the maximum 390 supported vector length, whichever is smaller and supported. This 391 determines the initial vector length of the init process (PID 1). 392 393 Reading this file returns the current system default vector length. 394 395* At every execve() call, the new vector length of the new process is set to 396 the system default vector length, unless 397 398 * PR_SME_VL_INHERIT (or equivalently SME_PT_VL_INHERIT) is set for the 399 calling thread, or 400 401 * a deferred vector length change is pending, established via the 402 PR_SME_SET_VL_ONEXEC flag (or SME_PT_VL_ONEXEC). 403 404* Modifying the system default vector length does not affect the vector length 405 of any existing process or thread that does not make an execve() call. 406 407 408Appendix A. SME programmer's model (informative) 409================================================= 410 411This section provides a minimal description of the additions made by SME to the 412ARMv8-A programmer's model that are relevant to this document. 413 414Note: This section is for information only and not intended to be complete or 415to replace any architectural specification. 416 417A.1. Registers 418--------------- 419 420In A64 state, SME adds the following: 421 422* A new mode, streaming mode, in which a subset of the normal FPSIMD and SVE 423 features are available. When supported EL0 software may enter and leave 424 streaming mode at any time. 425 426 For best system performance it is strongly encouraged for software to enable 427 streaming mode only when it is actively being used. 428 429* A new vector length controlling the size of ZA and the Z registers when in 430 streaming mode, separately to the vector length used for SVE when not in 431 streaming mode. There is no requirement that either the currently selected 432 vector length or the set of vector lengths supported for the two modes in 433 a given system have any relationship. The streaming mode vector length 434 is referred to as SVL. 435 436* A new ZA matrix register. This is a square matrix of SVLxSVL bits. Most 437 operations on ZA require that streaming mode be enabled but ZA can be 438 enabled without streaming mode in order to load, save and retain data. 439 440 For best system performance it is strongly encouraged for software to enable 441 ZA only when it is actively being used. 442 443* A new ZT0 register is introduced when SME2 is present. This is a 512 bit 444 register which is accessible when PSTATE.ZA is set, as ZA itself is. 445 446* Two new 1 bit fields in PSTATE which may be controlled via the SMSTART and 447 SMSTOP instructions or by access to the SVCR system register: 448 449 * PSTATE.ZA, if this is 1 then the ZA matrix is accessible and has valid 450 data while if it is 0 then ZA can not be accessed. When PSTATE.ZA is 451 changed from 0 to 1 all bits in ZA are cleared. 452 453 * PSTATE.SM, if this is 1 then the PE is in streaming mode. When the value 454 of PSTATE.SM is changed then it is implementation defined if the subset 455 of the floating point register bits valid in both modes may be retained. 456 Any other bits will be cleared. 457 458 459References 460========== 461 462[1] arch/arm64/include/uapi/asm/sigcontext.h 463 AArch64 Linux signal ABI definitions 464 465[2] arch/arm64/include/uapi/asm/ptrace.h 466 AArch64 Linux ptrace ABI definitions 467 468[3] Documentation/arch/arm64/cpu-feature-registers.rst 469