1Arm CPU Features 2================ 3 4CPU features are optional features that a CPU of supporting type may 5choose to implement or not. In QEMU, optional CPU features have 6corresponding boolean CPU proprieties that, when enabled, indicate 7that the feature is implemented, and, conversely, when disabled, 8indicate that it is not implemented. An example of an Arm CPU feature 9is the Performance Monitoring Unit (PMU). CPU types such as the 10Cortex-A15 and the Cortex-A57, which respectively implement Arm 11architecture reference manuals ARMv7-A and ARMv8-A, may both optionally 12implement PMUs. For example, if a user wants to use a Cortex-A15 without 13a PMU, then the ``-cpu`` parameter should contain ``pmu=off`` on the QEMU 14command line, i.e. ``-cpu cortex-a15,pmu=off``. 15 16As not all CPU types support all optional CPU features, then whether or 17not a CPU property exists depends on the CPU type. For example, CPUs 18that implement the ARMv8-A architecture reference manual may optionally 19support the AArch32 CPU feature, which may be enabled by disabling the 20``aarch64`` CPU property. A CPU type such as the Cortex-A15, which does 21not implement ARMv8-A, will not have the ``aarch64`` CPU property. 22 23QEMU's support may be limited for some CPU features, only partially 24supporting the feature or only supporting the feature under certain 25configurations. For example, the ``aarch64`` CPU feature, which, when 26disabled, enables the optional AArch32 CPU feature, is only supported 27when using the KVM accelerator and when running on a host CPU type that 28supports the feature. While ``aarch64`` currently only works with KVM, 29it could work with TCG. CPU features that are specific to KVM are 30prefixed with "kvm-" and are described in "KVM VCPU Features". 31 32CPU Feature Probing 33=================== 34 35Determining which CPU features are available and functional for a given 36CPU type is possible with the ``query-cpu-model-expansion`` QMP command. 37Below are some examples where ``scripts/qmp/qmp-shell`` (see the top comment 38block in the script for usage) is used to issue the QMP commands. 39 401. Determine which CPU features are available for the ``max`` CPU type 41 (Note, we started QEMU with qemu-system-aarch64, so ``max`` is 42 implementing the ARMv8-A reference manual in this case):: 43 44 (QEMU) query-cpu-model-expansion type=full model={"name":"max"} 45 { "return": { 46 "model": { "name": "max", "props": { 47 "sve1664": true, "pmu": true, "sve1792": true, "sve1920": true, 48 "sve128": true, "aarch64": true, "sve1024": true, "sve": true, 49 "sve640": true, "sve768": true, "sve1408": true, "sve256": true, 50 "sve1152": true, "sve512": true, "sve384": true, "sve1536": true, 51 "sve896": true, "sve1280": true, "sve2048": true 52 }}}} 53 54We see that the ``max`` CPU type has the ``pmu``, ``aarch64``, ``sve``, and many 55``sve<N>`` CPU features. We also see that all the CPU features are 56enabled, as they are all ``true``. (The ``sve<N>`` CPU features are all 57optional SVE vector lengths (see "SVE CPU Properties"). While with TCG 58all SVE vector lengths can be supported, when KVM is in use it's more 59likely that only a few lengths will be supported, if SVE is supported at 60all.) 61 62(2) Let's try to disable the PMU:: 63 64 (QEMU) query-cpu-model-expansion type=full model={"name":"max","props":{"pmu":false}} 65 { "return": { 66 "model": { "name": "max", "props": { 67 "sve1664": true, "pmu": false, "sve1792": true, "sve1920": true, 68 "sve128": true, "aarch64": true, "sve1024": true, "sve": true, 69 "sve640": true, "sve768": true, "sve1408": true, "sve256": true, 70 "sve1152": true, "sve512": true, "sve384": true, "sve1536": true, 71 "sve896": true, "sve1280": true, "sve2048": true 72 }}}} 73 74We see it worked, as ``pmu`` is now ``false``. 75 76(3) Let's try to disable ``aarch64``, which enables the AArch32 CPU feature:: 77 78 (QEMU) query-cpu-model-expansion type=full model={"name":"max","props":{"aarch64":false}} 79 {"error": { 80 "class": "GenericError", "desc": 81 "'aarch64' feature cannot be disabled unless KVM is enabled and 32-bit EL1 is supported" 82 }} 83 84It looks like this feature is limited to a configuration we do not 85currently have. 86 87(4) Let's disable ``sve`` and see what happens to all the optional SVE 88 vector lengths:: 89 90 (QEMU) query-cpu-model-expansion type=full model={"name":"max","props":{"sve":false}} 91 { "return": { 92 "model": { "name": "max", "props": { 93 "sve1664": false, "pmu": true, "sve1792": false, "sve1920": false, 94 "sve128": false, "aarch64": true, "sve1024": false, "sve": false, 95 "sve640": false, "sve768": false, "sve1408": false, "sve256": false, 96 "sve1152": false, "sve512": false, "sve384": false, "sve1536": false, 97 "sve896": false, "sve1280": false, "sve2048": false 98 }}}} 99 100As expected they are now all ``false``. 101 102(5) Let's try probing CPU features for the Cortex-A15 CPU type:: 103 104 (QEMU) query-cpu-model-expansion type=full model={"name":"cortex-a15"} 105 {"return": {"model": {"name": "cortex-a15", "props": {"pmu": true}}}} 106 107Only the ``pmu`` CPU feature is available. 108 109A note about CPU feature dependencies 110------------------------------------- 111 112It's possible for features to have dependencies on other features. I.e. 113it may be possible to change one feature at a time without error, but 114when attempting to change all features at once an error could occur 115depending on the order they are processed. It's also possible changing 116all at once doesn't generate an error, because a feature's dependencies 117are satisfied with other features, but the same feature cannot be changed 118independently without error. For these reasons callers should always 119attempt to make their desired changes all at once in order to ensure the 120collection is valid. 121 122A note about CPU models and KVM 123------------------------------- 124 125Named CPU models generally do not work with KVM. There are a few cases 126that do work, e.g. using the named CPU model ``cortex-a57`` with KVM on a 127seattle host, but mostly if KVM is enabled the ``host`` CPU type must be 128used. This means the guest is provided all the same CPU features as the 129host CPU type has. And, for this reason, the ``host`` CPU type should 130enable all CPU features that the host has by default. Indeed it's even 131a bit strange to allow disabling CPU features that the host has when using 132the ``host`` CPU type, but in the absence of CPU models it's the best we can 133do if we want to launch guests without all the host's CPU features enabled. 134 135Enabling KVM also affects the ``query-cpu-model-expansion`` QMP command. The 136affect is not only limited to specific features, as pointed out in example 137(3) of "CPU Feature Probing", but also to which CPU types may be expanded. 138When KVM is enabled, only the ``max``, ``host``, and current CPU type may be 139expanded. This restriction is necessary as it's not possible to know all 140CPU types that may work with KVM, but it does impose a small risk of users 141experiencing unexpected errors. For example on a seattle, as mentioned 142above, the ``cortex-a57`` CPU type is also valid when KVM is enabled. 143Therefore a user could use the ``host`` CPU type for the current type, but 144then attempt to query ``cortex-a57``, however that query will fail with our 145restrictions. This shouldn't be an issue though as management layers and 146users have been preferring the ``host`` CPU type for use with KVM for quite 147some time. Additionally, if the KVM-enabled QEMU instance running on a 148seattle host is using the ``cortex-a57`` CPU type, then querying ``cortex-a57`` 149will work. 150 151Using CPU Features 152================== 153 154After determining which CPU features are available and supported for a 155given CPU type, then they may be selectively enabled or disabled on the 156QEMU command line with that CPU type:: 157 158 $ qemu-system-aarch64 -M virt -cpu max,pmu=off,sve=on,sve128=on,sve256=on 159 160The example above disables the PMU and enables the first two SVE vector 161lengths for the ``max`` CPU type. Note, the ``sve=on`` isn't actually 162necessary, because, as we observed above with our probe of the ``max`` CPU 163type, ``sve`` is already on by default. Also, based on our probe of 164defaults, it would seem we need to disable many SVE vector lengths, rather 165than only enabling the two we want. This isn't the case, because, as 166disabling many SVE vector lengths would be quite verbose, the ``sve<N>`` CPU 167properties have special semantics (see "SVE CPU Property Parsing 168Semantics"). 169 170KVM VCPU Features 171================= 172 173KVM VCPU features are CPU features that are specific to KVM, such as 174paravirt features or features that enable CPU virtualization extensions. 175The features' CPU properties are only available when KVM is enabled and 176are named with the prefix "kvm-". KVM VCPU features may be probed, 177enabled, and disabled in the same way as other CPU features. Below is 178the list of KVM VCPU features and their descriptions. 179 180``kvm-no-adjvtime`` 181 By default kvm-no-adjvtime is disabled. This means that by default 182 the virtual time adjustment is enabled (vtime is not *not* adjusted). 183 184 When virtual time adjustment is enabled each time the VM transitions 185 back to running state the VCPU's virtual counter is updated to 186 ensure stopped time is not counted. This avoids time jumps 187 surprising guest OSes and applications, as long as they use the 188 virtual counter for timekeeping. However it has the side effect of 189 the virtual and physical counters diverging. All timekeeping based 190 on the virtual counter will appear to lag behind any timekeeping 191 that does not subtract VM stopped time. The guest may resynchronize 192 its virtual counter with other time sources as needed. 193 194 Enable kvm-no-adjvtime to disable virtual time adjustment, also 195 restoring the legacy (pre-5.0) behavior. 196 197``kvm-steal-time`` 198 Since v5.2, kvm-steal-time is enabled by default when KVM is 199 enabled, the feature is supported, and the guest is 64-bit. 200 201 When kvm-steal-time is enabled a 64-bit guest can account for time 202 its CPUs were not running due to the host not scheduling the 203 corresponding VCPU threads. The accounting statistics may influence 204 the guest scheduler behavior and/or be exposed to the guest 205 userspace. 206 207TCG VCPU Features 208================= 209 210TCG VCPU features are CPU features that are specific to TCG. 211Below is the list of TCG VCPU features and their descriptions. 212 213``pauth-impdef`` 214 When ``FEAT_Pauth`` is enabled, either the *impdef* (Implementation 215 Defined) algorithm is enabled or the *architected* QARMA algorithm 216 is enabled. By default the impdef algorithm is disabled, and QARMA 217 is enabled. 218 219 The architected QARMA algorithm has good cryptographic properties, 220 but can be quite slow to emulate. The impdef algorithm used by QEMU 221 is non-cryptographic but significantly faster. 222 223SVE CPU Properties 224================== 225 226There are two types of SVE CPU properties: ``sve`` and ``sve<N>``. The first 227is used to enable or disable the entire SVE feature, just as the ``pmu`` 228CPU property completely enables or disables the PMU. The second type 229is used to enable or disable specific vector lengths, where ``N`` is the 230number of bits of the length. The ``sve<N>`` CPU properties have special 231dependencies and constraints, see "SVE CPU Property Dependencies and 232Constraints" below. Additionally, as we want all supported vector lengths 233to be enabled by default, then, in order to avoid overly verbose command 234lines (command lines full of ``sve<N>=off``, for all ``N`` not wanted), we 235provide the parsing semantics listed in "SVE CPU Property Parsing 236Semantics". 237 238SVE CPU Property Dependencies and Constraints 239--------------------------------------------- 240 241 1) At least one vector length must be enabled when ``sve`` is enabled. 242 243 2) If a vector length ``N`` is enabled, then, when KVM is enabled, all 244 smaller, host supported vector lengths must also be enabled. If 245 KVM is not enabled, then only all the smaller, power-of-two vector 246 lengths must be enabled. E.g. with KVM if the host supports all 247 vector lengths up to 512-bits (128, 256, 384, 512), then if ``sve512`` 248 is enabled, the 128-bit vector length, 256-bit vector length, and 249 384-bit vector length must also be enabled. Without KVM, the 384-bit 250 vector length would not be required. 251 252 3) If KVM is enabled then only vector lengths that the host CPU type 253 support may be enabled. If SVE is not supported by the host, then 254 no ``sve*`` properties may be enabled. 255 256SVE CPU Property Parsing Semantics 257---------------------------------- 258 259 1) If SVE is disabled (``sve=off``), then which SVE vector lengths 260 are enabled or disabled is irrelevant to the guest, as the entire 261 SVE feature is disabled and that disables all vector lengths for 262 the guest. However QEMU will still track any ``sve<N>`` CPU 263 properties provided by the user. If later an ``sve=on`` is provided, 264 then the guest will get only the enabled lengths. If no ``sve=on`` 265 is provided and there are explicitly enabled vector lengths, then 266 an error is generated. 267 268 2) If SVE is enabled (``sve=on``), but no ``sve<N>`` CPU properties are 269 provided, then all supported vector lengths are enabled, which when 270 KVM is not in use means including the non-power-of-two lengths, and, 271 when KVM is in use, it means all vector lengths supported by the host 272 processor. 273 274 3) If SVE is enabled, then an error is generated when attempting to 275 disable the last enabled vector length (see constraint (1) of "SVE 276 CPU Property Dependencies and Constraints"). 277 278 4) If one or more vector lengths have been explicitly enabled and at 279 least one of the dependency lengths of the maximum enabled length 280 has been explicitly disabled, then an error is generated (see 281 constraint (2) of "SVE CPU Property Dependencies and Constraints"). 282 283 5) When KVM is enabled, if the host does not support SVE, then an error 284 is generated when attempting to enable any ``sve*`` properties (see 285 constraint (3) of "SVE CPU Property Dependencies and Constraints"). 286 287 6) When KVM is enabled, if the host does support SVE, then an error is 288 generated when attempting to enable any vector lengths not supported 289 by the host (see constraint (3) of "SVE CPU Property Dependencies and 290 Constraints"). 291 292 7) If one or more ``sve<N>`` CPU properties are set ``off``, but no ``sve<N>``, 293 CPU properties are set ``on``, then the specified vector lengths are 294 disabled but the default for any unspecified lengths remains enabled. 295 When KVM is not enabled, disabling a power-of-two vector length also 296 disables all vector lengths larger than the power-of-two length. 297 When KVM is enabled, then disabling any supported vector length also 298 disables all larger vector lengths (see constraint (2) of "SVE CPU 299 Property Dependencies and Constraints"). 300 301 8) If one or more ``sve<N>`` CPU properties are set to ``on``, then they 302 are enabled and all unspecified lengths default to disabled, except 303 for the required lengths per constraint (2) of "SVE CPU Property 304 Dependencies and Constraints", which will even be auto-enabled if 305 they were not explicitly enabled. 306 307 9) If SVE was disabled (``sve=off``), allowing all vector lengths to be 308 explicitly disabled (i.e. avoiding the error specified in (3) of 309 "SVE CPU Property Parsing Semantics"), then if later an ``sve=on`` is 310 provided an error will be generated. To avoid this error, one must 311 enable at least one vector length prior to enabling SVE. 312 313SVE CPU Property Examples 314------------------------- 315 316 1) Disable SVE:: 317 318 $ qemu-system-aarch64 -M virt -cpu max,sve=off 319 320 2) Implicitly enable all vector lengths for the ``max`` CPU type:: 321 322 $ qemu-system-aarch64 -M virt -cpu max 323 324 3) When KVM is enabled, implicitly enable all host CPU supported vector 325 lengths with the ``host`` CPU type:: 326 327 $ qemu-system-aarch64 -M virt,accel=kvm -cpu host 328 329 4) Only enable the 128-bit vector length:: 330 331 $ qemu-system-aarch64 -M virt -cpu max,sve128=on 332 333 5) Disable the 512-bit vector length and all larger vector lengths, 334 since 512 is a power-of-two. This results in all the smaller, 335 uninitialized lengths (128, 256, and 384) defaulting to enabled:: 336 337 $ qemu-system-aarch64 -M virt -cpu max,sve512=off 338 339 6) Enable the 128-bit, 256-bit, and 512-bit vector lengths:: 340 341 $ qemu-system-aarch64 -M virt -cpu max,sve128=on,sve256=on,sve512=on 342 343 7) The same as (6), but since the 128-bit and 256-bit vector 344 lengths are required for the 512-bit vector length to be enabled, 345 then allow them to be auto-enabled:: 346 347 $ qemu-system-aarch64 -M virt -cpu max,sve512=on 348 349 8) Do the same as (7), but by first disabling SVE and then re-enabling it:: 350 351 $ qemu-system-aarch64 -M virt -cpu max,sve=off,sve512=on,sve=on 352 353 9) Force errors regarding the last vector length:: 354 355 $ qemu-system-aarch64 -M virt -cpu max,sve128=off 356 $ qemu-system-aarch64 -M virt -cpu max,sve=off,sve128=off,sve=on 357 358SVE CPU Property Recommendations 359-------------------------------- 360 361The examples in "SVE CPU Property Examples" exhibit many ways to select 362vector lengths which developers may find useful in order to avoid overly 363verbose command lines. However, the recommended way to select vector 364lengths is to explicitly enable each desired length. Therefore only 365example's (1), (4), and (6) exhibit recommended uses of the properties. 366 367SME CPU Property Examples 368------------------------- 369 370 1) Disable SME:: 371 372 $ qemu-system-aarch64 -M virt -cpu max,sme=off 373 374 2) Implicitly enable all vector lengths for the ``max`` CPU type:: 375 376 $ qemu-system-aarch64 -M virt -cpu max 377 378 3) Only enable the 256-bit vector length:: 379 380 $ qemu-system-aarch64 -M virt -cpu max,sme256=on 381 382 3) Enable the 256-bit and 1024-bit vector lengths:: 383 384 $ qemu-system-aarch64 -M virt -cpu max,sme256=on,sme1024=on 385 386 4) Disable the 512-bit vector length. This results in all the other 387 lengths supported by ``max`` defaulting to enabled 388 (128, 256, 1024 and 2048):: 389 390 $ qemu-system-aarch64 -M virt -cpu max,sve512=off 391 392SVE User-mode Default Vector Length Property 393-------------------------------------------- 394 395For qemu-aarch64, the cpu property ``sve-default-vector-length=N`` is 396defined to mirror the Linux kernel parameter file 397``/proc/sys/abi/sve_default_vector_length``. The default length, ``N``, 398is in units of bytes and must be between 16 and 8192. 399If not specified, the default vector length is 64. 400 401If the default length is larger than the maximum vector length enabled, 402the actual vector length will be reduced. Note that the maximum vector 403length supported by QEMU is 256. 404 405If this property is set to ``-1`` then the default vector length 406is set to the maximum possible length. 407 408SME CPU Properties 409================== 410 411The SME CPU properties are much like the SVE properties: ``sme`` is 412used to enable or disable the entire SME feature, and ``sme<N>`` is 413used to enable or disable specific vector lengths. Finally, 414``sme_fa64`` is used to enable or disable ``FEAT_SME_FA64``, which 415allows execution of the "full a64" instruction set while Streaming 416SVE mode is enabled. 417 418SME is not supported by KVM at this time. 419 420At least one vector length must be enabled when ``sme`` is enabled, 421and all vector lengths must be powers of 2. The maximum vector 422length supported by qemu is 2048 bits. Otherwise, there are no 423additional constraints on the set of vector lengths supported by SME. 424 425SME User-mode Default Vector Length Property 426-------------------------------------------- 427 428For qemu-aarch64, the cpu property ``sme-default-vector-length=N`` is 429defined to mirror the Linux kernel parameter file 430``/proc/sys/abi/sme_default_vector_length``. The default length, ``N``, 431is in units of bytes and must be between 16 and 8192. 432If not specified, the default vector length is 32. 433 434As with ``sve-default-vector-length``, if the default length is larger 435than the maximum vector length enabled, the actual vector length will 436be reduced. If this property is set to ``-1`` then the default vector 437length is set to the maximum possible length. 438