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docs/system/arm/cpu-features.rst: Format literals correctly 

In rST markup, single backticks `like this` represent "interpreted
text", which can be handled as a bunch of different things if tagged
with a specific "role":
https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html#interpreted-text
(the most common one for us is "reference to a URL, which gets
hyperlinked").

The default "role" if none is specified is "title_reference",
intended for references to book or article titles, and it renders
into the HTML as <cite>...</cite> (usually comes out as italics).

To format a literal (generally rendered as fixed-width font),
double-backticks are required.

cpu-features.rst consistently uses single backticks when double backticks
are required; correct it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20210726142338.31872-8-peter.maydell@linaro.org
This commit is contained in:
Peter Maydell 2021-07-26 15:23:35 +01:00
parent 9c372ecfec
commit 8a48a7c2e0
1 changed files with 58 additions and 58 deletions
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docs/system/arm/cpu-features.rst
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@ -10,22 +10,22 @@ 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`.
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.
``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
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,
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".
@ -33,12 +33,12 @@ 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
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
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"}
@ -51,9 +51,9 @@ block in the script for usage) is used to issue the QMP commands.
"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
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
@ -71,9 +71,9 @@ all.)
"sve896": true, "sve1280": true, "sve2048": true
}}}}
We see it worked, as `pmu` is now `false`.
We see it worked, as ``pmu`` is now ``false``.
(3) Let's try to disable `aarch64`, which enables the AArch32 CPU feature::
(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": {
@ -84,7 +84,7 @@ We see it worked, as `pmu` is now `false`.
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
(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}}
@ -97,14 +97,14 @@ currently have.
"sve896": false, "sve1280": false, "sve2048": false
}}}}
As expected they are now all `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.
Only the ``pmu`` CPU feature is available.
A note about CPU feature dependencies
-------------------------------------
@ -123,29 +123,29 @@ 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
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
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
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
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
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
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
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`
seattle host is using the ``cortex-a57`` CPU type, then querying ``cortex-a57``
will work.
Using CPU Features
@ -158,12 +158,12 @@ 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
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
disabling many SVE vector lengths would be quite verbose, the ``sve<N>`` CPU
properties have special semantics (see "SVE CPU Property Parsing
Semantics").
@ -217,11 +217,11 @@ 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 Enable or disable `FEAT_Pauth`, pointer
pauth Enable or disable ``FEAT_Pauth``, pointer
authentication. By default, the feature is
enabled with `-cpu max`.
enabled with ``-cpu max``.
pauth-impdef When `FEAT_Pauth` is enabled, either the
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
@ -235,49 +235,49 @@ Below is the list of TCG VCPU features and their descriptions.
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`
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
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
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.
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
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`
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.
no ``sve*`` properties may be enabled.
SVE CPU Property Parsing Semantics
----------------------------------
1) If SVE is disabled (`sve=off`), then which SVE vector lengths
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`
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
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
@ -293,7 +293,7 @@ SVE CPU Property Parsing Semantics
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
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
@ -301,8 +301,8 @@ SVE CPU Property Parsing Semantics
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
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.
@ -310,15 +310,15 @@ SVE CPU Property Parsing Semantics
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
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
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
"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.
@ -329,12 +329,12 @@ SVE CPU Property Examples
$ qemu-system-aarch64 -M virt -cpu max,sve=off
2) Implicitly enable all vector lengths for the `max` CPU type::
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::
lengths with the ``host`` CPU type::
$ qemu-system-aarch64 -M virt,accel=kvm -cpu host