forked from mirror/qemu
You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
446 lines
21 KiB
ReStructuredText
446 lines
21 KiB
ReStructuredText
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.
|
|
|
|
TCG VCPU Features
|
|
=================
|
|
|
|
TCG VCPU features are CPU features that are specific to TCG.
|
|
Below is the list of TCG VCPU features and their descriptions.
|
|
|
|
pauth-impdef When ``FEAT_Pauth`` is enabled, either the
|
|
*impdef* (Implementation Defined) algorithm
|
|
is enabled or the *architected* QARMA algorithm
|
|
is enabled. By default the impdef algorithm
|
|
is disabled, and QARMA is enabled.
|
|
|
|
The architected QARMA algorithm has good
|
|
cryptographic properties, but can be quite slow
|
|
to emulate. The impdef algorithm used by QEMU
|
|
is non-cryptographic but significantly faster.
|
|
|
|
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
|
|
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.
|
|
|
|
SME CPU Property Examples
|
|
-------------------------
|
|
|
|
1) Disable SME::
|
|
|
|
$ qemu-system-aarch64 -M virt -cpu max,sme=off
|
|
|
|
2) Implicitly enable all vector lengths for the ``max`` CPU type::
|
|
|
|
$ qemu-system-aarch64 -M virt -cpu max
|
|
|
|
3) Only enable the 256-bit vector length::
|
|
|
|
$ qemu-system-aarch64 -M virt -cpu max,sme256=on
|
|
|
|
3) Enable the 256-bit and 1024-bit vector lengths::
|
|
|
|
$ qemu-system-aarch64 -M virt -cpu max,sme256=on,sme1024=on
|
|
|
|
4) Disable the 512-bit vector length. This results in all the other
|
|
lengths supported by ``max`` defaulting to enabled
|
|
(128, 256, 1024 and 2048)::
|
|
|
|
$ qemu-system-aarch64 -M virt -cpu max,sve512=off
|
|
|
|
SVE User-mode Default Vector Length Property
|
|
--------------------------------------------
|
|
|
|
For qemu-aarch64, the cpu property ``sve-default-vector-length=N`` is
|
|
defined to mirror the Linux kernel parameter file
|
|
``/proc/sys/abi/sve_default_vector_length``. The default length, ``N``,
|
|
is in units of bytes and must be between 16 and 8192.
|
|
If not specified, the default vector length is 64.
|
|
|
|
If the default length is larger than the maximum vector length enabled,
|
|
the actual vector length will be reduced. Note that the maximum vector
|
|
length supported by QEMU is 256.
|
|
|
|
If this property is set to ``-1`` then the default vector length
|
|
is set to the maximum possible length.
|
|
|
|
SME CPU Properties
|
|
==================
|
|
|
|
The SME CPU properties are much like the SVE properties: ``sme`` is
|
|
used to enable or disable the entire SME feature, and ``sme<N>`` is
|
|
used to enable or disable specific vector lengths. Finally,
|
|
``sme_fa64`` is used to enable or disable ``FEAT_SME_FA64``, which
|
|
allows execution of the "full a64" instruction set while Streaming
|
|
SVE mode is enabled.
|
|
|
|
SME is not supported by KVM at this time.
|
|
|
|
At least one vector length must be enabled when ``sme`` is enabled,
|
|
and all vector lengths must be powers of 2. The maximum vector
|
|
length supported by qemu is 2048 bits. Otherwise, there are no
|
|
additional constraints on the set of vector lengths supported by SME.
|
|
|
|
SME User-mode Default Vector Length Property
|
|
--------------------------------------------
|
|
|
|
For qemu-aarch64, the cpu property ``sme-default-vector-length=N`` is
|
|
defined to mirror the Linux kernel parameter file
|
|
``/proc/sys/abi/sme_default_vector_length``. The default length, ``N``,
|
|
is in units of bytes and must be between 16 and 8192.
|
|
If not specified, the default vector length is 32.
|
|
|
|
As with ``sve-default-vector-length``, if the default length is larger
|
|
than the maximum vector length enabled, the actual vector length will
|
|
be reduced. If this property is set to ``-1`` then the default vector
|
|
length is set to the maximum possible length.
|