system/arm/cpu-features.rst

1 Arm CPU Features
4 CPU features are optional features that a CPU of supporting type may
5 choose to implement or not.  In QEMU, optional CPU features have
10 Cortex-A15 and the Cortex-A57, which respectively implement Arm
11 architecture reference manuals ARMv7-A and ARMv8-A, may both optionally
12 implement PMUs.  For example, if a user wants to use a Cortex-A15 without
13 a PMU, then the ``-cpu`` parameter should contain ``pmu=off`` on the QEMU
14 command line, i.e. ``-cpu cortex-a15,pmu=off``.
16 As not all CPU types support all optional CPU features, then whether or
18 that implement the ARMv8-A architecture reference manual may optionally
20 ``aarch64`` CPU property.  A CPU type such as the Cortex-A15, which does
21 not implement ARMv8-A, will not have the ``aarch64`` CPU property.
23 QEMU's support may be limited for some CPU features, only partially
29 it could work with TCG.  CPU features that are specific to KVM are
30 prefixed with "kvm-" and are described in "KVM VCPU Features".
35 Determining which CPU features are available and functional for a given
36 CPU type is possible with the ``query-cpu-model-expansion`` QMP command.
37 Below are some examples where ``scripts/qmp/qmp-shell`` (see the top comment
40 1. 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)::
44       (QEMU) query-cpu-model-expansion type=full model={"name":"max"}
55 ``sve<N>`` CPU features.  We also see that all the CPU features are
56 enabled, as they are all ``true``.  (The ``sve<N>`` CPU features are all
64       (QEMU) query-cpu-model-expansion type=full model={"name":"max","props":{"pmu":false}}
78       (QEMU) query-cpu-model-expansion type=full model={"name":"max","props":{"aarch64":false}}
81        "'aarch64' feature cannot be disabled unless KVM is enabled and 32-bit EL1 is supported"
90       (QEMU) query-cpu-model-expansion type=full model={"name":"max","props":{"sve":false}}
102 (5) Let's try probing CPU features for the Cortex-A15 CPU type::
104       (QEMU) query-cpu-model-expansion type=full model={"name":"cortex-a15"}
105       {"return": {"model": {"name": "cortex-a15", "props": {"pmu": true}}}}
110 -------------------------------------
112 It's possible for features to have dependencies on other features. I.e.
113 it may be possible to change one feature at a time without error, but
114 when attempting to change all features at once an error could occur
117 are satisfied with other features, but the same feature cannot be changed
118 independently without error.  For these reasons callers should always
123 -------------------------------
126 that do work, e.g. using the named CPU model ``cortex-a57`` with KVM on a
128 used.  This means the guest is provided all the same CPU features as the
130 enable all CPU features that the host has by default.  Indeed it's even
131 a bit strange to allow disabling CPU features that the host has when using
133 do if we want to launch guests without all the host's CPU features enabled.
135 Enabling KVM also affects the ``query-cpu-model-expansion`` QMP command.  The
136 affect is not only limited to specific features, as pointed out in example
142 above, the ``cortex-a57`` CPU type is also valid when KVM is enabled.
144 then attempt to query ``cortex-a57``, however that query will fail with our
147 some time.  Additionally, if the KVM-enabled QEMU instance running on a
148 seattle host is using the ``cortex-a57`` CPU type, then querying ``cortex-a57``
151 Using CPU Features
154 After determining which CPU features are available and supported for a
158   $ qemu-system-aarch64 -M virt -cpu max,pmu=off,sve=on,sve128=on,sve256=on
163 type, ``sve`` is already on by default.  Also, based on our probe of
170 KVM VCPU Features
173 KVM VCPU features are CPU features that are specific to KVM, such as
174 paravirt features or features that enable CPU virtualization extensions.
175 The features' CPU properties are only available when KVM is enabled and
176 are named with the prefix "kvm-".  KVM VCPU features may be probed,
177 enabled, and disabled in the same way as other CPU features.  Below is
178 the list of KVM VCPU features and their descriptions.
180 ``kvm-no-adjvtime``
181   By default kvm-no-adjvtime is disabled.  This means that by default
194   Enable kvm-no-adjvtime to disable virtual time adjustment, also
195   restoring the legacy (pre-5.0) behavior.
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.
201   When kvm-steal-time is enabled a 64-bit guest can account for time
207 TCG VCPU Features
210 TCG VCPU features are CPU features that are specific to TCG.
211 Below is the list of TCG VCPU features and their descriptions.
216 ``pauth-impdef``
219 ``pauth-qarma3``
222 ``pauth-qarma5``
225 Without ``pauth-impdef``, ``pauth-qarma3`` or ``pauth-qarma5`` enabled,
229 non-cryptographic but significantly faster.
241 to be enabled by default, then, in order to avoid overly verbose command
247 ---------------------------------------------
253      KVM is not enabled, then only all the smaller, power-of-two vector
255      vector lengths up to 512-bits (128, 256, 384, 512), then if ``sve512``
256      is enabled, the 128-bit vector length, 256-bit vector length, and
257      384-bit vector length must also be enabled. Without KVM, the 384-bit
265 ----------------------------------
278      KVM is not in use means including the non-power-of-two lengths, and,
302      disabled but the default for any unspecified lengths remains enabled.
303      When KVM is not enabled, disabling a power-of-two vector length also
304      disables all vector lengths larger than the power-of-two length.
310      are enabled and all unspecified lengths default to disabled, except
312      Dependencies and Constraints", which will even be auto-enabled if
322 -------------------------
326      $ qemu-system-aarch64 -M virt -cpu max,sve=off
330      $ qemu-system-aarch64 -M virt -cpu max
335      $ qemu-system-aarch64 -M virt,accel=kvm -cpu host
337   4) Only enable the 128-bit vector length::
339      $ qemu-system-aarch64 -M virt -cpu max,sve128=on
341   5) Disable the 512-bit vector length and all larger vector lengths,
342      since 512 is a power-of-two.  This results in all the smaller,
345      $ qemu-system-aarch64 -M virt -cpu max,sve512=off
347   6) Enable the 128-bit, 256-bit, and 512-bit vector lengths::
349      $ qemu-system-aarch64 -M virt -cpu max,sve128=on,sve256=on,sve512=on
351   7) The same as (6), but since the 128-bit and 256-bit vector
352      lengths are required for the 512-bit vector length to be enabled,
353      then allow them to be auto-enabled::
355      $ qemu-system-aarch64 -M virt -cpu max,sve512=on
357   8) Do the same as (7), but by first disabling SVE and then re-enabling it::
359      $ qemu-system-aarch64 -M virt -cpu max,sve=off,sve512=on,sve=on
363      $ qemu-system-aarch64 -M virt -cpu max,sve128=off
364      $ qemu-system-aarch64 -M virt -cpu max,sve=off,sve128=off,sve=on
367 --------------------------------
376 -------------------------
380      $ qemu-system-aarch64 -M virt -cpu max,sme=off
384      $ qemu-system-aarch64 -M virt -cpu max
386   3) Only enable the 256-bit vector length::
388      $ qemu-system-aarch64 -M virt -cpu max,sme256=on
390   3) Enable the 256-bit and 1024-bit vector lengths::
392      $ qemu-system-aarch64 -M virt -cpu max,sme256=on,sme1024=on
394   4) Disable the 512-bit vector length.  This results in all the other
398      $ qemu-system-aarch64 -M virt -cpu max,sve512=off
400 SVE User-mode Default Vector Length Property
401 --------------------------------------------
403 For qemu-aarch64, the cpu property ``sve-default-vector-length=N`` is
405 ``/proc/sys/abi/sve_default_vector_length``.  The default length, ``N``,
407 If not specified, the default vector length is 64.
409 If the default length is larger than the maximum vector length enabled,
413 If this property is set to ``-1`` then the default vector length
433 SME User-mode Default Vector Length Property
434 --------------------------------------------
436 For qemu-aarch64, the cpu property ``sme-default-vector-length=N`` is
438 ``/proc/sys/abi/sme_default_vector_length``.  The default length, ``N``,
440 If not specified, the default vector length is 32.
442 As with ``sve-default-vector-length``, if the default length is larger
444 be reduced.  If this property is set to ``-1`` then the default vector
452 The feature is enabled by the CPU property ``x-rme``, with the ``x-``
456 without notice or backward compatibility.
459 -----------------------------
462 configurations, ``x-l0gptsz=S`` may be used to specify the value
463 to encode into ``GPCCR_EL3.L0GPTSZ``, a read-only field that
465 Legal values for ``S`` are 30, 34, 36, and 39; the default is 30.
467 As with ``x-rme``, the ``x-l0gptsz`` property may be renamed or