cpu-features.rst (19323B)
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 By default kvm-no-adjvtime is disabled. This 181 means that by default the virtual time 182 adjustment is enabled (vtime is not *not* 183 adjusted). 184 185 When virtual time adjustment is enabled each 186 time the VM transitions back to running state 187 the VCPU's virtual counter is updated to ensure 188 stopped time is not counted. This avoids time 189 jumps surprising guest OSes and applications, 190 as long as they use the virtual counter for 191 timekeeping. However it has the side effect of 192 the virtual and physical counters diverging. 193 All timekeeping based on the virtual counter 194 will appear to lag behind any timekeeping that 195 does not subtract VM stopped time. The guest 196 may resynchronize its virtual counter with 197 other time sources as needed. 198 199 Enable kvm-no-adjvtime to disable virtual time 200 adjustment, also restoring the legacy (pre-5.0) 201 behavior. 202 203 kvm-steal-time Since v5.2, kvm-steal-time is enabled by 204 default when KVM is enabled, the feature is 205 supported, and the guest is 64-bit. 206 207 When kvm-steal-time is enabled a 64-bit guest 208 can account for time its CPUs were not running 209 due to the host not scheduling the corresponding 210 VCPU threads. The accounting statistics may 211 influence the guest scheduler behavior and/or be 212 exposed to the guest userspace. 213 214TCG VCPU Features 215================= 216 217TCG VCPU features are CPU features that are specific to TCG. 218Below is the list of TCG VCPU features and their descriptions. 219 220 pauth Enable or disable ``FEAT_Pauth``, pointer 221 authentication. By default, the feature is 222 enabled with ``-cpu max``. 223 224 pauth-impdef When ``FEAT_Pauth`` is enabled, either the 225 *impdef* (Implementation Defined) algorithm 226 is enabled or the *architected* QARMA algorithm 227 is enabled. By default the impdef algorithm 228 is disabled, and QARMA is enabled. 229 230 The architected QARMA algorithm has good 231 cryptographic properties, but can be quite slow 232 to emulate. The impdef algorithm used by QEMU 233 is non-cryptographic but significantly faster. 234 235SVE CPU Properties 236================== 237 238There are two types of SVE CPU properties: ``sve`` and ``sve<N>``. The first 239is used to enable or disable the entire SVE feature, just as the ``pmu`` 240CPU property completely enables or disables the PMU. The second type 241is used to enable or disable specific vector lengths, where ``N`` is the 242number of bits of the length. The ``sve<N>`` CPU properties have special 243dependencies and constraints, see "SVE CPU Property Dependencies and 244Constraints" below. Additionally, as we want all supported vector lengths 245to be enabled by default, then, in order to avoid overly verbose command 246lines (command lines full of ``sve<N>=off``, for all ``N`` not wanted), we 247provide the parsing semantics listed in "SVE CPU Property Parsing 248Semantics". 249 250SVE CPU Property Dependencies and Constraints 251--------------------------------------------- 252 253 1) At least one vector length must be enabled when ``sve`` is enabled. 254 255 2) If a vector length ``N`` is enabled, then, when KVM is enabled, all 256 smaller, host supported vector lengths must also be enabled. If 257 KVM is not enabled, then only all the smaller, power-of-two vector 258 lengths must be enabled. E.g. with KVM if the host supports all 259 vector lengths up to 512-bits (128, 256, 384, 512), then if ``sve512`` 260 is enabled, the 128-bit vector length, 256-bit vector length, and 261 384-bit vector length must also be enabled. Without KVM, the 384-bit 262 vector length would not be required. 263 264 3) If KVM is enabled then only vector lengths that the host CPU type 265 support may be enabled. If SVE is not supported by the host, then 266 no ``sve*`` properties may be enabled. 267 268SVE CPU Property Parsing Semantics 269---------------------------------- 270 271 1) If SVE is disabled (``sve=off``), then which SVE vector lengths 272 are enabled or disabled is irrelevant to the guest, as the entire 273 SVE feature is disabled and that disables all vector lengths for 274 the guest. However QEMU will still track any ``sve<N>`` CPU 275 properties provided by the user. If later an ``sve=on`` is provided, 276 then the guest will get only the enabled lengths. If no ``sve=on`` 277 is provided and there are explicitly enabled vector lengths, then 278 an error is generated. 279 280 2) If SVE is enabled (``sve=on``), but no ``sve<N>`` CPU properties are 281 provided, then all supported vector lengths are enabled, which when 282 KVM is not in use means including the non-power-of-two lengths, and, 283 when KVM is in use, it means all vector lengths supported by the host 284 processor. 285 286 3) If SVE is enabled, then an error is generated when attempting to 287 disable the last enabled vector length (see constraint (1) of "SVE 288 CPU Property Dependencies and Constraints"). 289 290 4) If one or more vector lengths have been explicitly enabled and at 291 at least one of the dependency lengths of the maximum enabled length 292 has been explicitly disabled, then an error is generated (see 293 constraint (2) of "SVE CPU Property Dependencies and Constraints"). 294 295 5) When KVM is enabled, if the host does not support SVE, then an error 296 is generated when attempting to enable any ``sve*`` properties (see 297 constraint (3) of "SVE CPU Property Dependencies and Constraints"). 298 299 6) When KVM is enabled, if the host does support SVE, then an error is 300 generated when attempting to enable any vector lengths not supported 301 by the host (see constraint (3) of "SVE CPU Property Dependencies and 302 Constraints"). 303 304 7) If one or more ``sve<N>`` CPU properties are set ``off``, but no ``sve<N>``, 305 CPU properties are set ``on``, then the specified vector lengths are 306 disabled but the default for any unspecified lengths remains enabled. 307 When KVM is not enabled, disabling a power-of-two vector length also 308 disables all vector lengths larger than the power-of-two length. 309 When KVM is enabled, then disabling any supported vector length also 310 disables all larger vector lengths (see constraint (2) of "SVE CPU 311 Property Dependencies and Constraints"). 312 313 8) If one or more ``sve<N>`` CPU properties are set to ``on``, then they 314 are enabled and all unspecified lengths default to disabled, except 315 for the required lengths per constraint (2) of "SVE CPU Property 316 Dependencies and Constraints", which will even be auto-enabled if 317 they were not explicitly enabled. 318 319 9) If SVE was disabled (``sve=off``), allowing all vector lengths to be 320 explicitly disabled (i.e. avoiding the error specified in (3) of 321 "SVE CPU Property Parsing Semantics"), then if later an ``sve=on`` is 322 provided an error will be generated. To avoid this error, one must 323 enable at least one vector length prior to enabling SVE. 324 325SVE CPU Property Examples 326------------------------- 327 328 1) Disable SVE:: 329 330 $ qemu-system-aarch64 -M virt -cpu max,sve=off 331 332 2) Implicitly enable all vector lengths for the ``max`` CPU type:: 333 334 $ qemu-system-aarch64 -M virt -cpu max 335 336 3) When KVM is enabled, implicitly enable all host CPU supported vector 337 lengths with the ``host`` CPU type:: 338 339 $ qemu-system-aarch64 -M virt,accel=kvm -cpu host 340 341 4) Only enable the 128-bit vector length:: 342 343 $ qemu-system-aarch64 -M virt -cpu max,sve128=on 344 345 5) Disable the 512-bit vector length and all larger vector lengths, 346 since 512 is a power-of-two. This results in all the smaller, 347 uninitialized lengths (128, 256, and 384) defaulting to enabled:: 348 349 $ qemu-system-aarch64 -M virt -cpu max,sve512=off 350 351 6) Enable the 128-bit, 256-bit, and 512-bit vector lengths:: 352 353 $ qemu-system-aarch64 -M virt -cpu max,sve128=on,sve256=on,sve512=on 354 355 7) The same as (6), but since the 128-bit and 256-bit vector 356 lengths are required for the 512-bit vector length to be enabled, 357 then allow them to be auto-enabled:: 358 359 $ qemu-system-aarch64 -M virt -cpu max,sve512=on 360 361 8) Do the same as (7), but by first disabling SVE and then re-enabling it:: 362 363 $ qemu-system-aarch64 -M virt -cpu max,sve=off,sve512=on,sve=on 364 365 9) Force errors regarding the last vector length:: 366 367 $ qemu-system-aarch64 -M virt -cpu max,sve128=off 368 $ qemu-system-aarch64 -M virt -cpu max,sve=off,sve128=off,sve=on 369 370SVE CPU Property Recommendations 371-------------------------------- 372 373The examples in "SVE CPU Property Examples" exhibit many ways to select 374vector lengths which developers may find useful in order to avoid overly 375verbose command lines. However, the recommended way to select vector 376lengths is to explicitly enable each desired length. Therefore only 377example's (1), (4), and (6) exhibit recommended uses of the properties. 378 379SVE User-mode Default Vector Length Property 380-------------------------------------------- 381 382For qemu-aarch64, the cpu property ``sve-default-vector-length=N`` is 383defined to mirror the Linux kernel parameter file 384``/proc/sys/abi/sve_default_vector_length``. The default length, ``N``, 385is in units of bytes and must be between 16 and 8192. 386If not specified, the default vector length is 64. 387 388If the default length is larger than the maximum vector length enabled, 389the actual vector length will be reduced. Note that the maximum vector 390length supported by QEMU is 256. 391 392If this property is set to ``-1`` then the default vector length 393is set to the maximum possible length.