target/arm/cpu64: max cpu: Introduce sve<N> properties

Introduce cpu properties to give fine control over SVE vector lengths.
We introduce a property for each valid length up to the current
maximum supported, which is 2048-bits. The properties are named, e.g.
sve128, sve256, sve384, sve512, ..., where the number is the number of
bits. See the updates to docs/arm-cpu-features.rst for a description
of the semantics and for example uses.

Note, as sve-max-vq is still present and we'd like to be able to
support qmp_query_cpu_model_expansion with guests launched with e.g.
-cpu max,sve-max-vq=8 on their command lines, then we do allow
sve-max-vq and sve<N> properties to be provided at the same time, but
this is not recommended, and is why sve-max-vq is not mentioned in the
document.  If sve-max-vq is provided then it enables all lengths smaller
than and including the max and disables all lengths larger. It also has
the side-effect that no larger lengths may be enabled and that the max
itself cannot be disabled. Smaller non-power-of-two lengths may,
however, be disabled, e.g. -cpu max,sve-max-vq=4,sve384=off provides a
guest the vector lengths 128, 256, and 512 bits.

This patch has been co-authored with Richard Henderson, who reworked
the target/arm/cpu64.c changes in order to push all the validation and
auto-enabling/disabling steps into the finalizer, resulting in a nice
LOC reduction.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
Reviewed-by: Beata Michalska <beata.michalska@linaro.org>
Message-id: 20191031142734.8590-5-drjones@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
This commit is contained in:
Andrew Jones 2019-10-31 15:27:29 +01:00 committed by Peter Maydell
parent 73234775ad
commit 0df9142d27
8 changed files with 606 additions and 9 deletions

View file

@ -48,18 +48,31 @@ block in the script for usage) is used to issue the QMP commands.
(QEMU) query-cpu-model-expansion type=full model={"name":"max"}
{ "return": {
"model": { "name": "max", "props": {
"pmu": true, "aarch64": true
"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` and `aarch64` CPU features.
We also see that the CPU features are enabled, as they are all `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": {
"pmu": false, "aarch64": true
"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`.
@ -75,7 +88,22 @@ 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 try probing CPU features for the Cortex-A15 CPU type::
(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}}}}
@ -131,7 +159,133 @@ 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
$ qemu-system-aarch64 -M virt -cpu max,pmu=off,sve=on,sve128=on,sve256=on
The example above disables the PMU for the `max` CPU type.
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").
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 all power-of-two vector
lengths smaller than `N` must also be enabled. E.g. if `sve512`
is enabled, then the 128-bit and 256-bit vector lengths must also
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, including
the non-power-of-two lengths.
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) 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.
Disabling a power-of-two vector length also disables all vector
lengths larger than the power-of-two length (see constraint (2) of
"SVE CPU Property Dependencies and Constraints").
6) 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.
7) 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) Only enable the 128-bit vector length::
$ qemu-system-aarch64 -M virt -cpu max,sve128=on
4) 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
5) Enable the 128-bit, 256-bit, and 512-bit vector lengths::
$ qemu-system-aarch64 -M virt -cpu max,sve128=on,sve256=on,sve512=on
6) The same as (5), 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
7) Do the same as (6), but by first disabling SVE and then re-enabling it::
$ qemu-system-aarch64 -M virt -cpu max,sve=off,sve512=on,sve=on
8) 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), (3), and (5) exhibit recommended uses of the properties.