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From: Ciara Power <ciara.power@intel.com>
To: dev@dpdk.org
Cc: Ciara Power <ciara.power@intel.com>,
Anatoly Burakov <anatoly.burakov@intel.com>,
John McNamara <john.mcnamara@intel.com>,
Marko Kovacevic <marko.kovacevic@intel.com>
Date: Thu, 27 Aug 2020 17:12:50 +0100
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Subject: [dpdk-dev] [PATCH v2 03/17] doc: add detail on using max SIMD
bitwidth
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This patch adds documentation on the usage of the max SIMD bitwidth EAL
setting, and how to use it to enable AVX-512 at runtime.
Cc: Anatoly Burakov <anatoly.burakov@intel.com>
Cc: John McNamara <john.mcnamara@intel.com>
Cc: Marko Kovacevic <marko.kovacevic@intel.com>
Signed-off-by: Ciara Power <ciara.power@intel.com>
---
doc/guides/howto/avx512.rst | 36 +++++++++++++++++++
doc/guides/linux_gsg/eal_args.include.rst | 12 +++++++
.../prog_guide/env_abstraction_layer.rst | 31 ++++++++++++++++
3 files changed, 79 insertions(+)
create mode 100644 doc/guides/howto/avx512.rst
diff --git a/doc/guides/howto/avx512.rst b/doc/guides/howto/avx512.rst
new file mode 100644
index 0000000000..ebae0f2b4f
--- /dev/null
+++ b/doc/guides/howto/avx512.rst
@@ -0,0 +1,36 @@
+.. SPDX-License-Identifier: BSD-3-Clause
+ Copyright(c) 2020 Intel Corporation.
+
+
+Using AVX-512 with DPDK
+=======================
+
+AVX-512 is not used by default in DPDK, but it can be selected at runtime by apps through the use of EAL API,
+and by the user with a commandline argument. DPDK has a setting for max SIMD bitwidth,
+which can be modified and will then limit the vector path taken by the code.
+
+
+Using the API in apps
+---------------------
+
+Apps can request DPDK uses AVX-512 at runtime, if it provides improved application performance.
+This can be done by modifying the EAL setting for max SIMD bitwidth to 512, as by default it is 256,
+which does not allow for AVX-512.
+
+.. code-block:: c
+
+ rte_set_max_simd_bitwidth(RTE_MAX_512_SIMD);
+
+This API should only be called once at initialization, before EAL init.
+For more information on the possible enum values to use as a parameter, go to :ref:`max_simd_bitwidth`:
+
+
+Using the command-line argument
+---------------------------------------------
+
+The user can select to use AVX-512 at runtime, using the following argument to set the max bitwidth::
+
+ ./app/dpdk-testpmd --force-max-simd-bitwidth=512
+
+This will override any further changes to the max SIMD bitwidth in DPDK,
+which is useful for testing purposes.
diff --git a/doc/guides/linux_gsg/eal_args.include.rst b/doc/guides/linux_gsg/eal_args.include.rst
index 0fe4457968..bab3e14e47 100644
--- a/doc/guides/linux_gsg/eal_args.include.rst
+++ b/doc/guides/linux_gsg/eal_args.include.rst
@@ -210,3 +210,15 @@ Other options
* ``--no-telemetry``:
Disable telemetry.
+
+* ``--force-max-simd-bitwidth=<val>``:
+
+ Specify the maximum SIMD bitwidth size to handle. This limits which vector paths,
+ if any, are taken, as any paths taken must use a bitwidth below the max bitwidth limit.
+ For example, to allow all SIMD bitwidths up to and including AVX-512::
+
+ --force-max-simd-bitwidth=512
+
+ The following example shows limiting the bitwidth to 64-bits to disable all vector code::
+
+ --force-max-simd-bitwidth=64
diff --git a/doc/guides/prog_guide/env_abstraction_layer.rst b/doc/guides/prog_guide/env_abstraction_layer.rst
index f64ae953d1..74f26ed6c9 100644
--- a/doc/guides/prog_guide/env_abstraction_layer.rst
+++ b/doc/guides/prog_guide/env_abstraction_layer.rst
@@ -486,6 +486,37 @@ the desired addressing mode when virtual devices that are not directly attached
To facilitate forcing the IOVA mode to a specific value the EAL command line option ``--iova-mode`` can
be used to select either physical addressing('pa') or virtual addressing('va').
+.. _max_simd_bitwidth:
+
+
+Max SIMD bitwidth
+~~~~~~~~~~~~~~~~~
+
+The EAL provides a single setting to limit the max SIMD bitwidth used by DPDK,
+which is used in determining the vector path, if any, chosen by a component.
+The value can be set at runtime by an application using the 'rte_set_max_simd_bitwidth(uint16_t bitwidth)' function,
+which should only be called once at initialization, before EAL init.
+The value can be overridden by the user using the EAL command-line option '--force-max-sim-bitwidth'.
+
+When choosing a vector path, along with checking the CPU feature support,
+the value of the max SIMD bitwidth must also be checked, and can be retrieved using the 'rte_get_max_simd_bitwidth()' function.
+The value should be compared against the enum values for accepted max SIMD bitwidths:
+
+.. code-block:: c
+
+ enum rte_max_simd_t {
+ RTE_NO_SIMD = 64,
+ RTE_MAX_128_SIMD = 128,
+ RTE_MAX_256_SIMD = 256,
+ RTE_MAX_512_SIMD = 512
+ };
+
+ if (rte_get_max_simd_bitwidth() >= RTE_MAX_512_SIMD)
+ /* Take AVX-512 vector path */
+ else if (rte_get_max_simd_bitwidth() >= RTE_MAX_256_SIMD)
+ /* Take AVX2 vector path */
+
+
Memory Segments and Memory Zones (memzone)
------------------------------------------
--
2.17.1