# Arm CPU Features

CPU features are optional features that a CPU of supporting type may choose to implement or not. In QEMU, optional CPU features have corresponding boolean CPU proprieties that, when enabled, indicate that the feature is implemented, and, conversely, when disabled, indicate that it is not implemented. An example of an Arm CPU feature is the Performance Monitoring Unit (PMU). CPU types such as the Cortex-A15 and the Cortex-A57, which respectively implement Arm architecture reference manuals ARMv7-A and ARMv8-A, may both optionally implement PMUs. For example, if a user wants to use a Cortex-A15 without a PMU, then the -cpu parameter should contain pmu=off on the QEMU command line, i.e. -cpu cortex-a15,pmu=off.

As not all CPU types support all optional CPU features, then whether or not a CPU property exists depends on the CPU type. For example, CPUs that implement the ARMv8-A architecture reference manual may optionally support the AArch32 CPU feature, which may be enabled by disabling the aarch64 CPU property. A CPU type such as the Cortex-A15, which does not implement ARMv8-A, will not have the aarch64 CPU property.

QEMU’s support may be limited for some CPU features, only partially supporting the feature or only supporting the feature under certain configurations. For example, the aarch64 CPU feature, which, when disabled, enables the optional AArch32 CPU feature, is only supported when using the KVM accelerator and when running on a host CPU type that supports the feature. While aarch64 currently only works with KVM, it could work with TCG. CPU features that are specific to KVM are prefixed with “kvm-” and are described in “KVM VCPU Features”.

# CPU Feature Probing

Determining which CPU features are available and functional for a given CPU type is possible with the query-cpu-model-expansion QMP command. Below are some examples where scripts/qmp/qmp-shell (see the top comment block in the script for usage) is used to issue the QMP commands.

1. Determine which CPU features are available for the max CPU type (Note, we started QEMU with qemu-system-aarch64, so max is implementing the ARMv8-A reference manual in this case):

       (QEMU) query-cpu-model-expansion type=full model={"name":"max"}
       { "return": {
         "model": { "name": "max", "props": {
         "sve1664": true, "pmu": true, "sve1792": true, "sve1920": true,
         "sve128": true, "aarch64": true, "sve1024": true, "sve": true,
         "sve640": true, "sve768": true, "sve1408": true, "sve256": true,
         "sve1152": true, "sve512": true, "sve384": true, "sve1536": true,
         "sve896": true, "sve1280": true, "sve2048": true
       }}}}

We see that the max CPU type has the pmu, aarch64, sve, and many sve<N> CPU features. We also see that all the CPU features are enabled, as they are all true. (The sve<N> CPU features are all optional SVE vector lengths (see “SVE CPU Properties”). While with TCG all SVE vector lengths can be supported, when KVM is in use it’s more likely that only a few lengths will be supported, if SVE is supported at all.)

2. Let’s try to disable the PMU:

       (QEMU) query-cpu-model-expansion type=full model={"name":"max","props":{"pmu":false}}
       { "return": {
         "model": { "name": "max", "props": {
         "sve1664": true, "pmu": false, "sve1792": true, "sve1920": true,
         "sve128": true, "aarch64": true, "sve1024": true, "sve": true,
         "sve640": true, "sve768": true, "sve1408": true, "sve256": true,
         "sve1152": true, "sve512": true, "sve384": true, "sve1536": true,
         "sve896": true, "sve1280": true, "sve2048": true
       }}}}

We see it worked, as pmu is now false.

3. Let’s try to disable aarch64, which enables the AArch32 CPU feature:

       (QEMU) query-cpu-model-expansion type=full model={"name":"max","props":{"aarch64":false}}
       {"error": {
        "class": "GenericError", "desc":
        "'aarch64' feature cannot be disabled unless KVM is enabled and 32-bit EL1 is supported"
       }}

It looks like this feature is limited to a configuration we do not currently have.

4. Let’s disable sve and see what happens to all the optional SVE vector lengths:

       (QEMU) query-cpu-model-expansion type=full model={"name":"max","props":{"sve":false}}
       { "return": {
         "model": { "name": "max", "props": {
         "sve1664": false, "pmu": true, "sve1792": false, "sve1920": false,
         "sve128": false, "aarch64": true, "sve1024": false, "sve": false,
         "sve640": false, "sve768": false, "sve1408": false, "sve256": false,
         "sve1152": false, "sve512": false, "sve384": false, "sve1536": false,
         "sve896": false, "sve1280": false, "sve2048": false
       }}}}

As expected they are now all false.

5. Let’s try probing CPU features for the Cortex-A15 CPU type:

       (QEMU) query-cpu-model-expansion type=full model={"name":"cortex-a15"}
       {"return": {"model": {"name": "cortex-a15", "props": {"pmu": true}}}}

Only the pmu CPU feature is available.

## A note about CPU feature dependencies

It’s possible for features to have dependencies on other features. I.e. it may be possible to change one feature at a time without error, but when attempting to change all features at once an error could occur depending on the order they are processed. It’s also possible changing all at once doesn’t generate an error, because a feature’s dependencies are satisfied with other features, but the same feature cannot be changed independently without error. For these reasons callers should always attempt to make their desired changes all at once in order to ensure the collection is valid.

## A note about CPU models and KVM

Named CPU models generally do not work with KVM. There are a few cases that do work, e.g. using the named CPU model cortex-a57 with KVM on a seattle host, but mostly if KVM is enabled the host CPU type must be used. This means the guest is provided all the same CPU features as the host CPU type has. And, for this reason, the host CPU type should enable all CPU features that the host has by default. Indeed it’s even a bit strange to allow disabling CPU features that the host has when using the host CPU type, but in the absence of CPU models it’s the best we can do if we want to launch guests without all the host’s CPU features enabled.

Enabling KVM also affects the query-cpu-model-expansion QMP command. The affect is not only limited to specific features, as pointed out in example (3) of “CPU Feature Probing”, but also to which CPU types may be expanded. When KVM is enabled, only the max, host, and current CPU type may be expanded. This restriction is necessary as it’s not possible to know all CPU types that may work with KVM, but it does impose a small risk of users experiencing unexpected errors. For example on a seattle, as mentioned above, the cortex-a57 CPU type is also valid when KVM is enabled. Therefore a user could use the host CPU type for the current type, but then attempt to query cortex-a57, however that query will fail with our restrictions. This shouldn’t be an issue though as management layers and users have been preferring the host CPU type for use with KVM for quite some time. Additionally, if the KVM-enabled QEMU instance running on a seattle host is using the cortex-a57 CPU type, then querying cortex-a57 will work.

# Using CPU Features

After determining which CPU features are available and supported for a given CPU type, then they may be selectively enabled or disabled on the QEMU command line with that CPU type:

    $ qemu-system-aarch64 -M virt -cpu max,pmu=off,sve=on,sve128=on,sve256=on

The example above disables the PMU and enables the first two SVE vector lengths for the max CPU type. Note, the sve=on isn’t actually necessary, because, as we observed above with our probe of the max CPU type, sve is already on by default. Also, based on our probe of defaults, it would seem we need to disable many SVE vector lengths, rather than only enabling the two we want. This isn’t the case, because, as disabling many SVE vector lengths would be quite verbose, the sve<N> CPU properties have special semantics (see “SVE CPU Property Parsing Semantics”).

# KVM VCPU Features

KVM VCPU features are CPU features that are specific to KVM, such as paravirt features or features that enable CPU virtualization extensions. The features’ CPU properties are only available when KVM is enabled and are named with the prefix “kvm-“. KVM VCPU features may be probed, enabled, and disabled in the same way as other CPU features. Below is the list of KVM VCPU features and their descriptions.

> kvm-no-adjvtime By default kvm-no-adjvtime is disabled. This

>   means that by default the virtual time adjustment is enabled (vtime is not not adjusted).

>   When virtual time adjustment is enabled each time the VM transitions back to running state the VCPU’s virtual counter is updated to ensure stopped time is not counted. This avoids time jumps surprising guest OSes and applications, as long as they use the virtual counter for timekeeping. However it has the side effect of the virtual and physical counters diverging. All timekeeping based on the virtual counter will appear to lag behind any timekeeping that does not subtract VM stopped time. The guest may resynchronize its virtual counter with other time sources as needed.

>   Enable kvm-no-adjvtime to disable virtual time adjustment, also restoring the legacy (pre-5.0) behavior.

> kvm-steal-time Since v5.2, kvm-steal-time is enabled by

>   default when KVM is enabled, the feature is supported, and the guest is 64-bit.

>   When kvm-steal-time is enabled a 64-bit guest can account for time its CPUs were not running due to the host not scheduling the corresponding VCPU threads. The accounting statistics may influence the guest scheduler behavior and/or be exposed to the guest userspace.

# SVE CPU Properties

There are two types of SVE CPU properties: sve and sve<N>. The first is used to enable or disable the entire SVE feature, just as the pmu CPU property completely enables or disables the PMU. The second type is used to enable or disable specific vector lengths, where N is the number of bits of the length. The sve<N> CPU properties have special dependencies and constraints, see “SVE CPU Property Dependencies and Constraints” below. Additionally, as we want all supported vector lengths to be enabled by default, then, in order to avoid overly verbose command lines (command lines full of sve<N>=off, for all N not wanted), we provide the parsing semantics listed in “SVE CPU Property Parsing Semantics”.

## SVE CPU Property Dependencies and Constraints

> 1. At least one vector length must be enabled when sve is enabled.

> 2. If a vector length N is enabled, then, when KVM is enabled, all smaller, host supported vector lengths must also be enabled. If KVM is not enabled, then only all the smaller, power-of-two vector lengths must be enabled. E.g. with KVM if the host supports all vector lengths up to 512-bits (128, 256, 384, 512), then if sve512 is enabled, the 128-bit vector length, 256-bit vector length, and 384-bit vector length must also be enabled. Without KVM, the 384-bit vector length would not be required.

> 3. If KVM is enabled then only vector lengths that the host CPU type support may be enabled. If SVE is not supported by the host, then no sve* properties may be enabled.

## SVE CPU Property Parsing Semantics

> 1. If SVE is disabled (sve=off), then which SVE vector lengths are enabled or disabled is irrelevant to the guest, as the entire SVE feature is disabled and that disables all vector lengths for the guest. However QEMU will still track any sve<N> CPU properties provided by the user. If later an sve=on is provided, then the guest will get only the enabled lengths. If no sve=on is provided and there are explicitly enabled vector lengths, then an error is generated.

> 2. If SVE is enabled (sve=on), but no sve<N> CPU properties are provided, then all supported vector lengths are enabled, which when KVM is not in use means including the non-power-of-two lengths, and, when KVM is in use, it means all vector lengths supported by the host processor.

> 3. If SVE is enabled, then an error is generated when attempting to disable the last enabled vector length (see constraint (1) of “SVE CPU Property Dependencies and Constraints”).

> 4. If one or more vector lengths have been explicitly enabled and at at least one of the dependency lengths of the maximum enabled length has been explicitly disabled, then an error is generated (see constraint (2) of “SVE CPU Property Dependencies and Constraints”).

> 5. When KVM is enabled, if the host does not support SVE, then an error is generated when attempting to enable any sve* properties (see constraint (3) of “SVE CPU Property Dependencies and Constraints”).

> 6. When KVM is enabled, if the host does support SVE, then an error is generated when attempting to enable any vector lengths not supported by the host (see constraint (3) of “SVE CPU Property Dependencies and Constraints”).

> 7. If one or more sve<N> CPU properties are set off, but no sve<N>, CPU properties are set on, then the specified vector lengths are disabled but the default for any unspecified lengths remains enabled. When KVM is not enabled, disabling a power-of-two vector length also disables all vector lengths larger than the power-of-two length. When KVM is enabled, then disabling any supported vector length also disables all larger vector lengths (see constraint (2) of “SVE CPU Property Dependencies and Constraints”).

> 8. If one or more sve<N> CPU properties are set to on, then they are enabled and all unspecified lengths default to disabled, except for the required lengths per constraint (2) of “SVE CPU Property Dependencies and Constraints”, which will even be auto-enabled if they were not explicitly enabled.

> 9. If SVE was disabled (sve=off), allowing all vector lengths to be explicitly disabled (i.e. avoiding the error specified in (3) of “SVE CPU Property Parsing Semantics”), then if later an sve=on is provided an error will be generated. To avoid this error, one must enable at least one vector length prior to enabling SVE.

## SVE CPU Property Examples

> 1. Disable SVE:

>        $ qemu-system-aarch64 -M virt -cpu max,sve=off

> 2. Implicitly enable all vector lengths for the max CPU type:

>        $ qemu-system-aarch64 -M virt -cpu max

> 3. When KVM is enabled, implicitly enable all host CPU supported vector lengths with the host CPU type:

>        $ qemu-system-aarch64 -M virt,accel=kvm -cpu host

> 4. Only enable the 128-bit vector length:

>        $ qemu-system-aarch64 -M virt -cpu max,sve128=on

> 5. Disable the 512-bit vector length and all larger vector lengths, since 512 is a power-of-two. This results in all the smaller, uninitialized lengths (128, 256, and 384) defaulting to enabled:

>        $ qemu-system-aarch64 -M virt -cpu max,sve512=off

> 6. Enable the 128-bit, 256-bit, and 512-bit vector lengths:

>        $ qemu-system-aarch64 -M virt -cpu max,sve128=on,sve256=on,sve512=on

> 7. The same as (6), but since the 128-bit and 256-bit vector lengths are required for the 512-bit vector length to be enabled, then allow them to be auto-enabled:

>        $ qemu-system-aarch64 -M virt -cpu max,sve512=on

> 8. Do the same as (7), but by first disabling SVE and then re-enabling it:

>        $ qemu-system-aarch64 -M virt -cpu max,sve=off,sve512=on,sve=on

> 9. Force errors regarding the last vector length:

>        $ qemu-system-aarch64 -M virt -cpu max,sve128=off
>        $ qemu-system-aarch64 -M virt -cpu max,sve=off,sve128=off,sve=on

## SVE CPU Property Recommendations

The examples in “SVE CPU Property Examples” exhibit many ways to select vector lengths which developers may find useful in order to avoid overly verbose command lines. However, the recommended way to select vector lengths is to explicitly enable each desired length. Therefore only example’s (1), (4), and (6) exhibit recommended uses of the properties.