dpif-lookup: add avx512 gather implementation.

This commit adds an AVX-512 dpcls lookup implementation.
It uses the AVX-512 SIMD ISA to perform multiple miniflow
operations in parallel.

To run this implementation, the "avx512f" and "bmi2" ISAs are
required. These ISA checks are performed at runtime while
probing the subtable implementation. If a CPU does not provide
both "avx512f" and "bmi2", then this code does not execute.

The avx512 code is built as a separate static library, with added
CFLAGS to enable the required ISA features. By building only this
static library with avx512 enabled, it is ensured that the main OVS
core library is *not* using avx512, and that OVS continues to run
as before on CPUs that do not support avx512.

The approach taken in this implementation is to use the
gather instruction to access the packet miniflow, allowing
any miniflow blocks to be loaded into an AVX-512 register.
This maximizes the usefulness of the register, and hence this
implementation handles any subtable with up to miniflow 8 bits.

Note that specialization of these avx512 lookup routines
still provides performance value, as the hashing of the
resulting data is performed in scalar code, and compile-time
loop unrolling occurs when specialized to miniflow bits.

This commit checks at configure time if the assembling in use
has a known bug in assembling AVX512 code. If this bug is present,
all AVX512 code is disabled. Checking the version string of the binutils
or assembler is not a good method to detect the issue, as back ported
fixes would not be reflected.

Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>
Acked-by: William Tu <u9012063@gmail.com>
Signed-off-by: Ian Stokes <ian.stokes@intel.com>

1 files changed, 30 insertions, 0 deletions

diff --git a/m4/openvswitch.m4 b/m4/openvswitch.m4
index add3aabcc..7c9a507e5 100644
--- a/m4/openvswitch.m4
+++ b/m4/openvswitch.m4
@@ -404,6 +404,36 @@ AC_DEFUN([OVS_CHECK_SPHINX],
    AC_ARG_VAR([SPHINXBUILD])
    AM_CONDITIONAL([HAVE_SPHINX], [test "$SPHINXBUILD" != none])])
 
+dnl Checks for binutils/assembler known issue with AVX512.
+dnl Due to backports, we probe assembling a reproducer instead of checking
+dnl binutils version string. More details, including ASM dumps and debug here:
+dnl   GCC: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90028
+dnl The checking of binutils funcationality instead of LD version is similar
+dnl to as how DPDK proposes to solve this issue:
+dnl   http://patches.dpdk.org/patch/71723/
+AC_DEFUN([OVS_CHECK_BINUTILS_AVX512],
+  [AC_CACHE_CHECK(
+    [binutils avx512 assembler checks passing],
+    [ovs_cv_binutils_avx512_good],
+    [dnl Assemble a short snippet to test for issue in "build-aux" dir:
+     mkdir -p build-aux
+     OBJFILE=build-aux/binutils_avx512_check.o
+     GATHER_PARAMS='0x8(,%ymm1,1),%ymm0{%k2}'
+     echo "vpgatherqq $GATHER_PARAMS" | as --64 -o $OBJFILE -
+     if ($CC -dumpmachine | grep x86_64) >/dev/null 2>&1; then
+       if (objdump -d  --no-show-raw-insn $OBJFILE | grep -q $GATHER_PARAMS) >/dev/null 2>&1; then
+         ovs_cv_binutils_avx512_good=yes
+         CFLAGS="$CFLAGS -DHAVE_LD_AVX512_GOOD"
+       else
+         ovs_cv_binutils_avx512_good=no
+       fi
+     else
+       ovs_cv_binutils_avx512_good=no
+     fi])
+     rm $OBJFILE
+   AM_CONDITIONAL([HAVE_LD_AVX512_GOOD],
+                  [test "$ovs_cv_binutils_avx512_good" = yes])])
+
 dnl Checks for dot.
 AC_DEFUN([OVS_CHECK_DOT],
   [AC_CACHE_CHECK(