[dpdk-dev] [PATCH v4] mk: fix scope of disabling AVX512F support
Varghese, Vipin
vipin.varghese at intel.com
Tue Jan 8 04:50:30 CET 2019
diff --git a/doc/guides/rel_notes/known_issues.rst b/doc/guides/rel_notes/known_issues.rst
index a1face9c1..b0057a2bc 100644
--- a/doc/guides/rel_notes/known_issues.rst
+++ b/doc/guides/rel_notes/known_issues.rst
@@ -826,3 +826,35 @@ Kernel crash when hot-unplug igb_uio device while DPDK application is running
**Driver/Module**:
``igb_uio`` module.
+
+
+AVX-512 support disabled
+------------------------
+
+**Description**:
+ ``AVX-512`` support has been disabled on some conditions.
+ This shouldn't be confused with ``CONFIG_RTE_ENABLE_AVX512`` config option which is already
+ disabled by default. This config option defines if ``AVX-512`` specific implementations of
+ some file to be used or not. What has been disabled is compiler feature to produce ``AVX-512``
+ instructions from any source code.
+
+ On DPDK v18.11 ``AVX-512`` disabled for all ``GCC`` builds which reported to cause a performance
+ drop.
+
+ On DPDK v19.02 ``AVX-512`` disable scope reduced to ``GCC`` and ``binutils version 2.30`` based
+ on information accured from the GCC community defect.
+
+**Reason**:
+ Generated ``AVX-512`` code cause crash:
+ https://bugs.dpdk.org/show_bug.cgi?id=97
+ https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88096
+
+**Resolution/Workaround**:
+ Update ``binutils`` to newer version than ``2.30``.
+ Use different compiler, like ``clang`` for this case.
+
+**Affected Environment/Platform**:
+ ``GCC`` and ``binutils version 2.30``.
+
+**Driver/Module**:
+ ALL.
diff --git a/doc/guides/rel_notes/release_19_02.rst b/doc/guides/rel_notes/release_19_02.rst
index 39c6ff826..b6322b282 100644
--- a/doc/guides/rel_notes/release_19_02.rst
+++ b/doc/guides/rel_notes/release_19_02.rst
@@ -276,6 +276,19 @@ Known Issues
Also, make sure to start the actual text at the margin.
=========================================================
+* ``AVX-512`` support has been disabled for ``GCC`` builds when
+``binutils 2.30``
+ is detected [1] because of a crash [2]. This can affect ``native``
+machine type
+ build targets on the platforms that support ``AVX512F`` like ``Intel
+Skylake``
+ processors, and can cause a possible performance drop. The immediate
+workaround
+ is to use ``clang`` compiler on these platforms.
+ Initial workaround in DPDK v18.11 was to disable ``AVX-512`` support
+for ``GCC``
+ completely, but based on information on defect submitted to GCC
+community [3],
+ issue has been identified as ``binutils 2.30`` issue. Since currently
+only GCC
+ generates ``AVX-512`` instructions, scope limited to ``GCC`` and
+``binutils 2.30``
+
+ - [1]: Commit ("mk: fix scope of disabling AVX512F support")
+ - [2]: https://bugs.dpdk.org/show_bug.cgi?id=97
+ - [3]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88096
Sharing ACK for documentation and update in release notes
Acked-by: Vipin Varghese <Vipin.varghese at intel.com>
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