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 214SVE CPU Properties 215================== 216 217There are two types of SVE CPU properties: `sve` and `sve<N>`. The first 218is used to enable or disable the entire SVE feature, just as the `pmu` 219CPU property completely enables or disables the PMU. The second type 220is used to enable or disable specific vector lengths, where `N` is the 221number of bits of the length. The `sve<N>` CPU properties have special 222dependencies and constraints, see "SVE CPU Property Dependencies and 223Constraints" below. Additionally, as we want all supported vector lengths 224to be enabled by default, then, in order to avoid overly verbose command 225lines (command lines full of `sve<N>=off`, for all `N` not wanted), we 226provide the parsing semantics listed in "SVE CPU Property Parsing 227Semantics". 228 229SVE CPU Property Dependencies and Constraints 230--------------------------------------------- 231 232 1) At least one vector length must be enabled when `sve` is enabled. 233 234 2) If a vector length `N` is enabled, then, when KVM is enabled, all 235 smaller, host supported vector lengths must also be enabled. If 236 KVM is not enabled, then only all the smaller, power-of-two vector 237 lengths must be enabled. E.g. with KVM if the host supports all 238 vector lengths up to 512-bits (128, 256, 384, 512), then if `sve512` 239 is enabled, the 128-bit vector length, 256-bit vector length, and 240 384-bit vector length must also be enabled. Without KVM, the 384-bit 241 vector length would not be required. 242 243 3) If KVM is enabled then only vector lengths that the host CPU type 244 support may be enabled. If SVE is not supported by the host, then 245 no `sve*` properties may be enabled. 246 247SVE CPU Property Parsing Semantics 248---------------------------------- 249 250 1) If SVE is disabled (`sve=off`), then which SVE vector lengths 251 are enabled or disabled is irrelevant to the guest, as the entire 252 SVE feature is disabled and that disables all vector lengths for 253 the guest. However QEMU will still track any `sve<N>` CPU 254 properties provided by the user. If later an `sve=on` is provided, 255 then the guest will get only the enabled lengths. If no `sve=on` 256 is provided and there are explicitly enabled vector lengths, then 257 an error is generated. 258 259 2) If SVE is enabled (`sve=on`), but no `sve<N>` CPU properties are 260 provided, then all supported vector lengths are enabled, which when 261 KVM is not in use means including the non-power-of-two lengths, and, 262 when KVM is in use, it means all vector lengths supported by the host 263 processor. 264 265 3) If SVE is enabled, then an error is generated when attempting to 266 disable the last enabled vector length (see constraint (1) of "SVE 267 CPU Property Dependencies and Constraints"). 268 269 4) If one or more vector lengths have been explicitly enabled and at 270 at least one of the dependency lengths of the maximum enabled length 271 has been explicitly disabled, then an error is generated (see 272 constraint (2) of "SVE CPU Property Dependencies and Constraints"). 273 274 5) When KVM is enabled, if the host does not support SVE, then an error 275 is generated when attempting to enable any `sve*` properties (see 276 constraint (3) of "SVE CPU Property Dependencies and Constraints"). 277 278 6) When KVM is enabled, if the host does support SVE, then an error is 279 generated when attempting to enable any vector lengths not supported 280 by the host (see constraint (3) of "SVE CPU Property Dependencies and 281 Constraints"). 282 283 7) If one or more `sve<N>` CPU properties are set `off`, but no `sve<N>`, 284 CPU properties are set `on`, then the specified vector lengths are 285 disabled but the default for any unspecified lengths remains enabled. 286 When KVM is not enabled, disabling a power-of-two vector length also 287 disables all vector lengths larger than the power-of-two length. 288 When KVM is enabled, then disabling any supported vector length also 289 disables all larger vector lengths (see constraint (2) of "SVE CPU 290 Property Dependencies and Constraints"). 291 292 8) If one or more `sve<N>` CPU properties are set to `on`, then they 293 are enabled and all unspecified lengths default to disabled, except 294 for the required lengths per constraint (2) of "SVE CPU Property 295 Dependencies and Constraints", which will even be auto-enabled if 296 they were not explicitly enabled. 297 298 9) If SVE was disabled (`sve=off`), allowing all vector lengths to be 299 explicitly disabled (i.e. avoiding the error specified in (3) of 300 "SVE CPU Property Parsing Semantics"), then if later an `sve=on` is 301 provided an error will be generated. To avoid this error, one must 302 enable at least one vector length prior to enabling SVE. 303 304SVE CPU Property Examples 305------------------------- 306 307 1) Disable SVE:: 308 309 $ qemu-system-aarch64 -M virt -cpu max,sve=off 310 311 2) Implicitly enable all vector lengths for the `max` CPU type:: 312 313 $ qemu-system-aarch64 -M virt -cpu max 314 315 3) When KVM is enabled, implicitly enable all host CPU supported vector 316 lengths with the `host` CPU type:: 317 318 $ qemu-system-aarch64 -M virt,accel=kvm -cpu host 319 320 4) Only enable the 128-bit vector length:: 321 322 $ qemu-system-aarch64 -M virt -cpu max,sve128=on 323 324 5) Disable the 512-bit vector length and all larger vector lengths, 325 since 512 is a power-of-two. This results in all the smaller, 326 uninitialized lengths (128, 256, and 384) defaulting to enabled:: 327 328 $ qemu-system-aarch64 -M virt -cpu max,sve512=off 329 330 6) Enable the 128-bit, 256-bit, and 512-bit vector lengths:: 331 332 $ qemu-system-aarch64 -M virt -cpu max,sve128=on,sve256=on,sve512=on 333 334 7) The same as (6), but since the 128-bit and 256-bit vector 335 lengths are required for the 512-bit vector length to be enabled, 336 then allow them to be auto-enabled:: 337 338 $ qemu-system-aarch64 -M virt -cpu max,sve512=on 339 340 8) Do the same as (7), but by first disabling SVE and then re-enabling it:: 341 342 $ qemu-system-aarch64 -M virt -cpu max,sve=off,sve512=on,sve=on 343 344 9) Force errors regarding the last vector length:: 345 346 $ qemu-system-aarch64 -M virt -cpu max,sve128=off 347 $ qemu-system-aarch64 -M virt -cpu max,sve=off,sve128=off,sve=on 348 349SVE CPU Property Recommendations 350-------------------------------- 351 352The examples in "SVE CPU Property Examples" exhibit many ways to select 353vector lengths which developers may find useful in order to avoid overly 354verbose command lines. However, the recommended way to select vector 355lengths is to explicitly enable each desired length. Therefore only 356example's (1), (4), and (6) exhibit recommended uses of the properties. 357 358