acinclude.m4

# -*- autoconf -*-

# Copyright (c) 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015 Nicira, Inc.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at:
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

dnl OVS_ENABLE_WERROR
AC_DEFUN([OVS_ENABLE_WERROR],
  [AC_ARG_ENABLE(
     [Werror],
     [AC_HELP_STRING([--enable-Werror], [Add -Werror to CFLAGS])],
     [], [enable_Werror=no])
   AC_CONFIG_COMMANDS_PRE(
     [if test "X$enable_Werror" = Xyes; then
        OVS_CFLAGS="$OVS_CFLAGS -Werror"
      fi])])

dnl OVS_CHECK_LINUX
dnl
dnl Configure linux kernel source tree 
AC_DEFUN([OVS_CHECK_LINUX], [
  AC_ARG_WITH([linux],
              [AC_HELP_STRING([--with-linux=/path/to/linux],
                              [Specify the Linux kernel build directory])])
  AC_ARG_WITH([linux-source],
              [AC_HELP_STRING([--with-linux-source=/path/to/linux-source],
                              [Specify the Linux kernel source directory
                               (usually figured out automatically from build
                               directory)])])

  # Deprecated equivalents to --with-linux, --with-linux-source.
  AC_ARG_WITH([l26])
  AC_ARG_WITH([l26-source])

  if test X"$with_linux" != X; then
    KBUILD=$with_linux
  elif test X"$with_l26" != X; then
    KBUILD=$with_l26
    AC_MSG_WARN([--with-l26 is deprecated, please use --with-linux instead])
  else
    KBUILD=
  fi

  if test X"$KBUILD" != X; then
    if test X"$with_linux_source" != X; then
      KSRC=$with_linux_source
    elif test X"$with_l26_source" != X; then
      KSRC=$with_l26_source
      AC_MSG_WARN([--with-l26-source is deprecated, please use --with-linux-source instead])
    else
      KSRC=
    fi
  elif test X"$with_linux_source" != X || test X"$with_l26_source" != X; then
    AC_MSG_ERROR([Linux source directory may not be specified without Linux build directory])
  fi

  if test -n "$KBUILD"; then
    KBUILD=`eval echo "$KBUILD"`
    case $KBUILD in
        /*) ;;
        *) KBUILD=`pwd`/$KBUILD ;;
    esac

    # The build directory is what the user provided.
    # Make sure that it exists.
    AC_MSG_CHECKING([for Linux build directory])
    if test -d "$KBUILD"; then
        AC_MSG_RESULT([$KBUILD])
        AC_SUBST(KBUILD)
    else
        AC_MSG_RESULT([no])
        AC_ERROR([source dir $KBUILD doesn't exist])
    fi

    # Debian breaks kernel headers into "source" header and "build" headers.
    # We want the source headers, but $KBUILD gives us the "build" headers.
    # Use heuristics to find the source headers.
    AC_MSG_CHECKING([for Linux source directory])
    if test -n "$KSRC"; then
      KSRC=`eval echo "$KSRC"`
      case $KSRC in
          /*) ;;
          *) KSRC=`pwd`/$KSRC ;;
      esac
      if test ! -e $KSRC/include/linux/kernel.h; then
        AC_MSG_ERROR([$KSRC is not a kernel source directory])
      fi
    else
      KSRC=$KBUILD
      if test ! -e $KSRC/include/linux/kernel.h; then
        # Debian kernel build Makefiles tend to include a line of the form:
        # MAKEARGS := -C /usr/src/linux-headers-3.2.0-1-common O=/usr/src/linux-headers-3.2.0-1-486
        # First try to extract the source directory from this line.
        KSRC=`sed -n 's/.*-C \([[^ ]]*\).*/\1/p' "$KBUILD"/Makefile`
        if test ! -e "$KSRC"/include/linux/kernel.h; then
          # Didn't work.  Fall back to name-based heuristics that used to work.
          case `echo "$KBUILD" | sed 's,/*$,,'` in # (
            */build)
              KSRC=`echo "$KBUILD" | sed 's,/build/*$,/source,'`
              ;; # (
            *)
              KSRC=`(cd $KBUILD && pwd -P) | sed 's,-[[^-]]*$,-common,'`
              ;;
          esac
        fi
      fi
      if test ! -e "$KSRC"/include/linux/kernel.h; then
        AC_MSG_ERROR([cannot find source directory (please use --with-linux-source)])
      fi
    fi
    AC_MSG_RESULT([$KSRC])

    AC_MSG_CHECKING([for kernel version])
    version=`sed -n 's/^VERSION = //p' "$KSRC/Makefile"`
    patchlevel=`sed -n 's/^PATCHLEVEL = //p' "$KSRC/Makefile"`
    sublevel=`sed -n 's/^SUBLEVEL = //p' "$KSRC/Makefile"`
    if test X"$version" = X || test X"$patchlevel" = X; then
       AC_ERROR([cannot determine kernel version])
    elif test X"$sublevel" = X; then
       kversion=$version.$patchlevel
    else
       kversion=$version.$patchlevel.$sublevel
    fi
    AC_MSG_RESULT([$kversion])

    if test "$version" -ge 4; then
       if test "$version" = 4 && test "$patchlevel" -le 3; then
          : # Linux 4.x
       else
          AC_ERROR([Linux kernel in $KBUILD is version $kversion, but version newer than 4.3.x is not supported (please refer to the FAQ for advice)])
       fi
    elif test "$version" = 3 && test "$patchlevel" -ge 10; then
       : # Linux 3.x
    else
       AC_ERROR([Linux kernel in $KBUILD is version $kversion, but version 3.10 or later is required])
    fi
    if (test ! -e "$KBUILD"/include/linux/version.h && \
        test ! -e "$KBUILD"/include/generated/uapi/linux/version.h)|| \
       (test ! -e "$KBUILD"/include/linux/autoconf.h && \
        test ! -e "$KBUILD"/include/generated/autoconf.h); then
        AC_MSG_ERROR([Linux kernel source in $KBUILD is not configured])
    fi
    OVS_CHECK_LINUX_COMPAT
  fi
  AM_CONDITIONAL(LINUX_ENABLED, test -n "$KBUILD")
])

dnl OVS_CHECK_DPDK
dnl
dnl Configure DPDK source tree
AC_DEFUN([OVS_CHECK_DPDK], [
  AC_ARG_WITH([dpdk],
              [AC_HELP_STRING([--with-dpdk=/path/to/dpdk],
                              [Specify the DPDK build directory])],
              [have_dpdk=true])

  AC_MSG_CHECKING([whether dpdk datapath is enabled])
  if test "$have_dpdk" != true || test "$with_dpdk" = no; then
    AC_MSG_RESULT([no])
    DPDKLIB_FOUND=false
  else
    AC_MSG_RESULT([yes])
    case "$with_dpdk" in
      yes)
        DPDK_AUTO_DISCOVER="true"
        DPDK_INCLUDE="/usr/local/include/dpdk -I/usr/include/dpdk"
        ;;
      *)
        DPDK_AUTO_DISCOVER="false"
        DPDK_INCLUDE="$with_dpdk/include"
        # If 'with_dpdk' is passed install directory, point to headers
        # installed in $DESTDIR/$prefix/include/dpdk
        AC_CHECK_FILE([$DPDK_INCLUDE/rte_config.h], [],
                      [AC_CHECK_FILE([$DPDK_INCLUDE/dpdk/rte_config.h],
                                     [DPDK_INCLUDE=$DPDK_INCLUDE/dpdk], [])])
        DPDK_LIB_DIR="$with_dpdk/lib"
        ;;
    esac

    DPDK_LIB="-ldpdk"
    DPDK_EXTRA_LIB=""

    ovs_save_CFLAGS="$CFLAGS"
    ovs_save_LDFLAGS="$LDFLAGS"
    CFLAGS="$CFLAGS -I$DPDK_INCLUDE"
    if test "$DPDK_AUTO_DISCOVER" = "false"; then
      LDFLAGS="$LDFLAGS -L${DPDK_LIB_DIR}"
    fi

    AC_COMPILE_IFELSE([
      AC_LANG_PROGRAM(
        [
          #include <rte_config.h>
#if !RTE_LIBRTE_VHOST_USER
#error
#endif
        ], [])
      ], [],
      [AC_DEFINE([VHOST_CUSE], [1], [DPDK vhost-cuse support enabled, vhost-user disabled.])
       DPDK_EXTRA_LIB="-lfuse"])

    # On some systems we have to add -ldl to link with dpdk
    #
    # This code, at first, tries to link without -ldl (""),
    # then adds it and tries again.
    # Before each attempt the search cache must be unset,
    # otherwise autoconf will stick with the old result

    DPDKLIB_FOUND=false
    save_LIBS=$LIBS
    for extras in "" "-ldl"; do
        LIBS="$DPDK_LIB $extras $save_LIBS $DPDK_EXTRA_LIB"
        AC_LINK_IFELSE(
           [AC_LANG_PROGRAM([#include <rte_config.h>
                             #include <rte_eal.h>],
                            [int rte_argc; char ** rte_argv;
                             rte_eal_init(rte_argc, rte_argv);])],
           [DPDKLIB_FOUND=true])
        if $DPDKLIB_FOUND; then
            break
        fi
    done

    # If linking unsuccessful
    if test "$DPDKLIB_FOUND" = "false" ; then
      if $DPDK_AUTO_DISCOVER; then
        AC_MSG_ERROR([Could not find DPDK library in default search path, Use --with-dpdk to specify the DPDK library installed in non-standard location])
      else
        AC_MSG_ERROR([Could not find DPDK libraries in $DPDK_LIB_DIR])
      fi
    fi
    CFLAGS="$ovs_save_CFLAGS"
    LDFLAGS="$ovs_save_LDFLAGS"
    if test "$DPDK_AUTO_DISCOVER" = "false"; then
      OVS_LDFLAGS="$OVS_LDFLAGS -L$DPDK_LIB_DIR"
    fi
    OVS_CFLAGS="$OVS_CFLAGS -I$DPDK_INCLUDE"
    OVS_ENABLE_OPTION([-mssse3])

    # DPDK pmd drivers are not linked unless --whole-archive is used.
    #
    # This happens because the rest of the DPDK code doesn't use any symbol in
    # the pmd driver objects, and the drivers register themselves using an
    # __attribute__((constructor)) function.
    #
    # These options are specified inside a single -Wl directive to prevent
    # autotools from reordering them.
    DPDK_vswitchd_LDFLAGS=-Wl,--whole-archive,$DPDK_LIB,--no-whole-archive
    AC_SUBST([DPDK_vswitchd_LDFLAGS])
    AC_DEFINE([DPDK_NETDEV], [1], [System uses the DPDK module.])
  fi

  AM_CONDITIONAL([DPDK_NETDEV], test "$DPDKLIB_FOUND" = true)
])

dnl OVS_GREP_IFELSE(FILE, REGEX, [IF-MATCH], [IF-NO-MATCH])
dnl
dnl Greps FILE for REGEX.  If it matches, runs IF-MATCH, otherwise IF-NO-MATCH.
dnl If IF-MATCH is empty then it defines to OVS_DEFINE(HAVE_<REGEX>), with
dnl <REGEX> translated to uppercase.
AC_DEFUN([OVS_GREP_IFELSE], [
  AC_MSG_CHECKING([whether $2 matches in $1])
  if test -f $1; then
    grep '$2' $1 >/dev/null 2>&1
    status=$?
    case $status in
      0) 
        AC_MSG_RESULT([yes])
        m4_if([$3], [], [OVS_DEFINE([HAVE_]m4_toupper([$2]))], [$3])
        ;;
      1) 
        AC_MSG_RESULT([no])
        $4
        ;;
      *) 
        AC_MSG_ERROR([grep exited with status $status])
        ;;
    esac
  else
    AC_MSG_RESULT([file not found])
    $4
  fi
])

dnl OVS_FIND_FIELD_IFELSE(FILE, STRUCTURE, REGEX, [IF-MATCH], [IF-NO-MATCH])
dnl
dnl Looks for STRUCTURE in FILE. If it is found, greps for REGEX within the
dnl structure definition. If this is successful, runs IF-MATCH, otherwise
dnl IF_NO_MATCH. If IF-MATCH is empty then it defines to
dnl OVS_DEFINE(HAVE_<STRUCTURE>_WITH_<REGEX>), with <STRUCTURE> and <REGEX>
dnl translated to uppercase.
AC_DEFUN([OVS_FIND_FIELD_IFELSE], [
  AC_MSG_CHECKING([whether $2 has member $3 in $1])
  if test -f $1; then
    awk '/$2.{/,/^}/' $1 2>/dev/null | grep '$3' >/dev/null
    status=$?
    case $status in
      0)
        AC_MSG_RESULT([yes])
        m4_if([$4], [], [OVS_DEFINE([HAVE_]m4_toupper([$2])[_WITH_]m4_toupper([$3]))], [$4])
        ;;
      1)
        AC_MSG_RESULT([no])
        $5
        ;;
      *)
        AC_MSG_ERROR([grep exited with status $status])
        ;;
    esac
  else
    AC_MSG_RESULT([file not found])
    $5
  fi
])

dnl OVS_DEFINE(NAME)
dnl
dnl Defines NAME to 1 in kcompat.h.
AC_DEFUN([OVS_DEFINE], [
  echo '#define $1 1' >> datapath/linux/kcompat.h.new
])

dnl OVS_CHECK_LINUX_COMPAT
dnl
dnl Runs various Autoconf checks on the Linux kernel source in
dnl the directory in $KBUILD.
AC_DEFUN([OVS_CHECK_LINUX_COMPAT], [
  rm -f datapath/linux/kcompat.h.new
  mkdir -p datapath/linux
  : > datapath/linux/kcompat.h.new

  echo '#include <linux/version.h>
#ifndef RHEL_RELEASE_CODE
#define RHEL_RELEASE_CODE 0
#define RHEL_RELEASE_VERSION(a, b) 0
#endif' >> datapath/linux/kcompat.h.new

  OVS_GREP_IFELSE([$KSRC/arch/x86/include/asm/checksum_32.h], [src_err,],
                  [OVS_DEFINE([HAVE_CSUM_COPY_DBG])])

  OVS_GREP_IFELSE([$KSRC/include/net/addrconf.h], [ipv6_dst_lookup.*net],
                  [OVS_DEFINE([HAVE_IPV6_DST_LOOKUP_NET])])
  OVS_GREP_IFELSE([$KSRC/include/net/addrconf.h], [ipv6_stub])

  OVS_GREP_IFELSE([$KSRC/include/linux/err.h], [ERR_CAST])
  OVS_GREP_IFELSE([$KSRC/include/linux/err.h], [IS_ERR_OR_NULL])

  OVS_GREP_IFELSE([$KSRC/include/linux/etherdevice.h], [eth_hw_addr_random])
  OVS_GREP_IFELSE([$KSRC/include/linux/etherdevice.h], [ether_addr_copy])

  OVS_GREP_IFELSE([$KSRC/include/uapi/linux/if_link.h], [IFLA_GENEVE_TOS])
  OVS_GREP_IFELSE([$KSRC/include/uapi/linux/if_link.h], [rtnl_link_stats64])
  OVS_GREP_IFELSE([$KSRC/include/linux/if_link.h], [rtnl_link_stats64])
  OVS_GREP_IFELSE([$KSRC/include/linux/if_vlan.h], [vlan_set_encap_proto])
  OVS_GREP_IFELSE([$KSRC/include/linux/if_vlan.h], [vlan_hwaccel_push_inside])

  OVS_GREP_IFELSE([$KSRC/include/linux/in.h], [ipv4_is_multicast])
  OVS_GREP_IFELSE([$KSRC/include/linux/in.h], [proto_ports_offset])
  OVS_GREP_IFELSE([$KSRC/include/net/ip.h], [__ip_select_ident.*dst_entry],
                  [OVS_DEFINE([HAVE_IP_SELECT_IDENT_USING_DST_ENTRY])])
  OVS_GREP_IFELSE([$KSRC/include/net/ip.h], [__ip_select_ident.*net],
                  [OVS_DEFINE([HAVE_IP_SELECT_IDENT_USING_NET])])

  OVS_GREP_IFELSE([$KSRC/include/net/ip.h], [inet_get_local_port_range.*net],
                  [OVS_DEFINE([HAVE_INET_GET_LOCAL_PORT_RANGE_USING_NET])])
  OVS_GREP_IFELSE([$KSRC/include/net/ip.h], [ip_defrag.*net],
                  [OVS_DEFINE([HAVE_IP_DEFRAG_TAKES_NET])])
  OVS_GREP_IFELSE([$KSRC/include/net/ip.h], [ip_do_fragment])
  OVS_GREP_IFELSE([$KSRC/include/net/ip.h], [ip_skb_dst_mtu])

  OVS_GREP_IFELSE([$KSRC/include/net/ip.h], [IPSKB_FRAG_PMTU],
                  [OVS_DEFINE([HAVE_CORRECT_MRU_HANDLING])])
  OVS_GREP_IFELSE([$KSRC/include/net/ip_tunnels.h], [__ip_tunnel_change_mtu])
  OVS_GREP_IFELSE([$KSRC/include/net/inet_frag.h], [hashfn.*const],
                  [OVS_DEFINE([HAVE_INET_FRAGS_CONST])])
  OVS_GREP_IFELSE([$KSRC/include/net/inet_frag.h], [last_in],
                  [OVS_DEFINE([HAVE_INET_FRAGS_LAST_IN])])
  OVS_GREP_IFELSE([$KSRC/include/net/inet_frag.h], [inet_frag_evicting])
  OVS_FIND_FIELD_IFELSE([$KSRC/include/net/inet_frag.h], [inet_frags],
                        [frags_work])
  OVS_FIND_FIELD_IFELSE([$KSRC/include/net/inet_frag.h], [inet_frags],
                        [rwlock])
  OVS_FIND_FIELD_IFELSE([$KSRC/include/net/inet_frag.h], [inet_frag_queue],
                        [list_evictor])
  OVS_GREP_IFELSE([$KSRC/include/net/inetpeer.h], [vif],
                  [OVS_DEFINE([HAVE_INETPEER_VIF_SUPPORT])])

  OVS_GREP_IFELSE([$KSRC/include/net/dst_metadata.h], [metadata_dst])

  OVS_GREP_IFELSE([$KSRC/include/linux/net.h], [sock_create_kern.*net],
                  [OVS_DEFINE([HAVE_SOCK_CREATE_KERN_NET])])
  OVS_GREP_IFELSE([$KSRC/include/linux/netdevice.h], [dev_disable_lro])
  OVS_GREP_IFELSE([$KSRC/include/linux/netdevice.h], [dev_get_stats])
  OVS_GREP_IFELSE([$KSRC/include/linux/netdevice.h], [dev_get_by_index_rcu])
  OVS_GREP_IFELSE([$KSRC/include/linux/netdevice.h], [dev_recursion_level])
  OVS_GREP_IFELSE([$KSRC/include/linux/netdevice.h], [__skb_gso_segment])
  OVS_GREP_IFELSE([$KSRC/include/linux/netdevice.h], [can_checksum_protocol])
  OVS_GREP_IFELSE([$KSRC/include/linux/netdevice.h], [ndo_get_iflink])
  OVS_GREP_IFELSE([$KSRC/include/linux/netdevice.h], [netdev_features_t])
  dnl Ubuntu kernel 3.13 has defined this struct but not used for netdev->tstats.
  dnl So check type of tstats.
  OVS_GREP_IFELSE([$KSRC/include/linux/netdevice.h], [pcpu_sw_netstats.*tstats],
                  [OVS_DEFINE([HAVE_PCPU_SW_NETSTATS])])
  OVS_GREP_IFELSE([$KSRC/include/linux/netdevice.h], [netif_needs_gso.*net_device],
                  [OVS_DEFINE([HAVE_NETIF_NEEDS_GSO_NETDEV])])
  OVS_GREP_IFELSE([$KSRC/include/linux/netdevice.h], [udp_offload])
  OVS_GREP_IFELSE([$KSRC/include/linux/netdevice.h], [udp_offload.*uoff],
                  [OVS_DEFINE([HAVE_UDP_OFFLOAD_ARG_UOFF])])
  OVS_GREP_IFELSE([$KSRC/include/linux/netdevice.h], [gro_remcsum])

  OVS_GREP_IFELSE([$KSRC/include/linux/netfilter.h], [nf_hook_state])
  OVS_GREP_IFELSE([$KSRC/include/linux/netfilter.h], [nf_register_net_hook])
  OVS_GREP_IFELSE([$KSRC/include/linux/netfilter.h], [nf_hookfn.*nf_hook_ops],
                  [OVS_DEFINE([HAVE_NF_HOOKFN_ARG_OPS])])
  OVS_FIND_FIELD_IFELSE([$KSRC/include/linux/netfilter_ipv6.h], [nf_ipv6_ops],
                        [fragment.*sock], [OVS_DEFINE([HAVE_NF_IPV6_OPS_FRAGMENT])])

  OVS_GREP_IFELSE([$KSRC/include/net/netfilter/nf_conntrack.h],
                  [tmpl_alloc.*conntrack_zone],
                  [OVS_DEFINE([HAVE_NF_CT_TMPL_ALLOC_TAKES_STRUCT_ZONE])])
  OVS_GREP_IFELSE([$KSRC/include/net/netfilter/nf_conntrack.h],
                  [l3num.*struct.net],
                  [OVS_DEFINE([HAVE_NF_CT_GET_TUPLEPR_TAKES_STRUCT_NET])])
  OVS_GREP_IFELSE([$KSRC/include/net/netfilter/nf_conntrack_zones.h],
                  [nf_ct_zone_init])
  OVS_GREP_IFELSE([$KSRC/include/net/netfilter/nf_conntrack_labels.h],
                  [nf_connlabels_get])
  OVS_GREP_IFELSE([$KSRC/include/net/netfilter/ipv6/nf_defrag_ipv6.h],
                  [nf_ct_frag6_consume_orig])
  OVS_GREP_IFELSE([$KSRC/include/net/netfilter/ipv6/nf_defrag_ipv6.h],
                  [nf_ct_frag6_output])

  OVS_GREP_IFELSE([$KSRC/include/linux/random.h], [prandom_u32])
  OVS_GREP_IFELSE([$KSRC/include/linux/random.h], [prandom_u32_max])

  OVS_GREP_IFELSE([$KSRC/include/net/rtnetlink.h], [get_link_net])
  OVS_GREP_IFELSE([$KSRC/include/net/rtnetlink.h], [name_assign_type])
  OVS_GREP_IFELSE([$KSRC/include/net/rtnetlink.h], [rtnl_create_link.*src_net],
                  [OVS_DEFINE([HAVE_RTNL_CREATE_LINK_SRC_NET])])
  OVS_GREP_IFELSE([$KSRC/include/net/net_namespace.h], [possible_net_t])

  OVS_GREP_IFELSE([$KSRC/include/linux/rcupdate.h], [rcu_read_lock_held], [],
                  [OVS_GREP_IFELSE([$KSRC/include/linux/rtnetlink.h],
                                   [rcu_read_lock_held])])
  OVS_GREP_IFELSE([$KSRC/include/linux/rtnetlink.h], [lockdep_rtnl_is_held])
  
  # Check for the proto_data_valid member in struct sk_buff.  The [^@]
  # is necessary because some versions of this header remove the
  # member but retain the kerneldoc comment that describes it (which
  # starts with @).  The brackets must be doubled because of m4
  # quoting rules.
  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h], [[[^@]]proto_data_valid],
                  [OVS_DEFINE([HAVE_PROTO_DATA_VALID])])
  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h], [skb_checksum_start_offset])
  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h], [inner_protocol])
  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h], [inner_mac_header])
  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h], [inner_network_header])
  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h], [kfree_skb_list])
  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h], [skb_scrub_packet.*xnet],
		  [OVS_DEFINE([HAVE_SKB_SCRUB_PACKET_XNET])])
  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h], [rxhash])
  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h], [u16.*rxhash],
                  [OVS_DEFINE([HAVE_U16_RXHASH])])
  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h], [skb_dst(],
                  [OVS_DEFINE([HAVE_SKB_DST_ACCESSOR_FUNCS])])
  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h], 
                  [skb_copy_from_linear_data_offset])
  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h],
                  [skb_reset_tail_pointer])
  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h], [skb_cow_head])
  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h], [skb_warn_if_lro],
                  [OVS_DEFINE([HAVE_SKB_WARN_LRO])])
  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h], [consume_skb])
  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h], [skb_frag_page])
  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h], [skb_has_frag_list])
  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h], [__skb_fill_page_desc])
  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h], [skb_reset_mac_len])
  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h], [skb_unclone])
  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h], [skb_orphan_frags])
  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h], [skb_get_hash(],
                  [OVS_DEFINE([HAVE_SKB_GET_HASH])])
  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h], [skb_clear_hash])
  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h], [int.skb_zerocopy(],
                  [OVS_DEFINE([HAVE_SKB_ZEROCOPY])])
  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h], [u8.*l4_rxhash],
                  [OVS_DEFINE([HAVE_L4_RXHASH])])
  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h], [skb_ensure_writable])
  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h], [skb_vlan_pop])
  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h], [skb_vlan_push])

  OVS_GREP_IFELSE([$KSRC/include/linux/types.h], [bool],
                  [OVS_DEFINE([HAVE_BOOL_TYPE])])
  OVS_GREP_IFELSE([$KSRC/include/linux/types.h], [__wsum],
                  [OVS_DEFINE([HAVE_CSUM_TYPES])])
  OVS_GREP_IFELSE([$KSRC/include/uapi/linux/types.h], [__wsum],
                  [OVS_DEFINE([HAVE_CSUM_TYPES])])

  OVS_GREP_IFELSE([$KSRC/include/net/checksum.h], [csum_replace4])
  OVS_GREP_IFELSE([$KSRC/include/net/checksum.h], [csum_unfold])

  OVS_GREP_IFELSE([$KSRC/include/net/dst.h], [dst_discard_sk])
  OVS_GREP_IFELSE([$KSRC/include/net/dst.h], [__skb_dst_copy])

  OVS_GREP_IFELSE([$KSRC/include/net/genetlink.h], [genl_has_listeners])
  OVS_GREP_IFELSE([$KSRC/include/net/genetlink.h], [mcgrp_offset])
  OVS_GREP_IFELSE([$KSRC/include/net/genetlink.h], [parallel_ops])
  OVS_GREP_IFELSE([$KSRC/include/net/genetlink.h], [genlmsg_new_unicast])
  OVS_GREP_IFELSE([$KSRC/include/net/genetlink.h], [netlink_has_listeners(net->genl_sock],
                  [OVS_DEFINE([HAVE_GENL_HAS_LISTENERS_TAKES_NET])])
  OVS_GREP_IFELSE([$KSRC/include/net/genetlink.h], [genlmsg_parse])
  OVS_GREP_IFELSE([$KSRC/include/net/genetlink.h], [genl_notify.*family],
                  [OVS_DEFINE([HAVE_GENL_NOTIFY_TAKES_FAMILY])])

  OVS_FIND_FIELD_IFELSE([$KSRC/include/net/genetlink.h],
                        [genl_multicast_group], [id])
  OVS_GREP_IFELSE([$KSRC/include/net/geneve.h], [geneve_hdr])

  OVS_GREP_IFELSE([$KSRC/include/net/gre.h], [gre_cisco_register])
  OVS_GREP_IFELSE([$KSRC/include/net/gre.h], [gre_handle_offloads])
  OVS_GREP_IFELSE([$KSRC/include/net/ipv6.h], [IP6_FH_F_SKIP_RH])
  OVS_GREP_IFELSE([$KSRC/include/net/ipv6.h], [ip6_local_out_sk])
  OVS_GREP_IFELSE([$KSRC/include/net/ipv6.h], [__ipv6_addr_jhash])
  OVS_GREP_IFELSE([$KSRC/include/net/ip6_fib.h], [rt6i.*u.dst],
                  [OVS_DEFINE([HAVE_RT6INFO_DST_UNION])])
  OVS_GREP_IFELSE([$KSRC/include/net/ip6_route.h], [ip6_frag.*sock],
                  [OVS_DEFINE([HAVE_IP_FRAGMENT_TAKES_SOCK])])

  OVS_GREP_IFELSE([$KSRC/include/net/netlink.h], [nla_get_be16])
  OVS_GREP_IFELSE([$KSRC/include/net/netlink.h], [nla_put_be16])
  OVS_GREP_IFELSE([$KSRC/include/net/netlink.h], [nla_put_be32])
  OVS_GREP_IFELSE([$KSRC/include/net/netlink.h], [nla_put_be64])
  OVS_GREP_IFELSE([$KSRC/include/net/netlink.h], [nla_put_in_addr])
  OVS_GREP_IFELSE([$KSRC/include/net/netlink.h], [nla_find_nested])
  OVS_GREP_IFELSE([$KSRC/include/net/netlink.h], [nla_is_last])
  OVS_GREP_IFELSE([$KSRC/include/linux/netlink.h], [void.*netlink_set_err],
                  [OVS_DEFINE([HAVE_VOID_NETLINK_SET_ERR])])

  OVS_GREP_IFELSE([$KSRC/include/net/sctp/checksum.h], [sctp_compute_cksum])

  OVS_GREP_IFELSE([$KSRC/include/linux/if_vlan.h], [ADD_ALL_VLANS_CMD],
                  [OVS_DEFINE([HAVE_VLAN_BUG_WORKAROUND])])
  OVS_GREP_IFELSE([$KSRC/include/linux/if_vlan.h], [vlan_insert_tag_set_proto])
  OVS_GREP_IFELSE([$KSRC/include/linux/if_vlan.h], [__vlan_insert_tag])
  OVS_GREP_IFELSE([$KSRC/include/linux/if_vlan.h], [vlan_get_protocol])

  OVS_GREP_IFELSE([$KSRC/include/linux/u64_stats_sync.h], [u64_stats_fetch_begin_irq])

  OVS_GREP_IFELSE([$KSRC/include/net/vxlan.h], [struct vxlan_metadata],
                  [OVS_DEFINE([HAVE_VXLAN_METADATA])])
  OVS_GREP_IFELSE([$KSRC/include/net/vxlan.h], [VXLAN_HF_RCO])
  OVS_GREP_IFELSE([$KSRC/include/net/udp.h], [udp_flow_src_port],
                  [OVS_GREP_IFELSE([$KSRC/include/net/udp.h], [inet_get_local_port_range(net],
                                   [OVS_DEFINE([HAVE_UDP_FLOW_SRC_PORT])])])
  OVS_GREP_IFELSE([$KSRC/include/net/udp.h], [udp_v4_check])
  OVS_GREP_IFELSE([$KSRC/include/net/udp.h], [udp_set_csum])
  OVS_GREP_IFELSE([$KSRC/include/net/udp_tunnel.h], [udp_tunnel_gro_complete])
  OVS_GREP_IFELSE([$KSRC/include/net/udp_tunnel.h], [ipv6_v6only],
                  [OVS_DEFINE([HAVE_UDP_TUNNEL_IPV6])])

  OVS_GREP_IFELSE([$KSRC/include/linux/skbuff.h], [ignore_df],
                  [OVS_DEFINE([HAVE_IGNORE_DF_RENAME])])
  OVS_GREP_IFELSE([$KSRC/include/uapi/linux/netdevice.h], [NET_NAME_UNKNOWN],
                  [OVS_DEFINE([HAVE_NET_NAME_UNKNOWN])])

  OVS_GREP_IFELSE([$KSRC/include/linux/utsrelease.h], [el6],
                  [OVS_DEFINE([HAVE_RHEL6_PER_CPU])])

  if cmp -s datapath/linux/kcompat.h.new \
            datapath/linux/kcompat.h >/dev/null 2>&1; then
    rm datapath/linux/kcompat.h.new
  else
    mv datapath/linux/kcompat.h.new datapath/linux/kcompat.h
  fi
])

dnl Checks for net/if_packet.h.
AC_DEFUN([OVS_CHECK_IF_PACKET],
  [AC_CHECK_HEADER([net/if_packet.h],
                   [HAVE_IF_PACKET=yes],
                   [HAVE_IF_PACKET=no])
   AM_CONDITIONAL([HAVE_IF_PACKET], [test "$HAVE_IF_PACKET" = yes])
   if test "$HAVE_IF_PACKET" = yes; then
      AC_DEFINE([HAVE_IF_PACKET], [1],
                [Define to 1 if net/if_packet.h is available.])
   fi])

dnl Checks for net/if_dl.h.
dnl
dnl (We use this as a proxy for checking whether we're building on FreeBSD
dnl or NetBSD.)
AC_DEFUN([OVS_CHECK_IF_DL],
  [AC_CHECK_HEADER([net/if_dl.h],
                   [HAVE_IF_DL=yes],
                   [HAVE_IF_DL=no])
   AM_CONDITIONAL([HAVE_IF_DL], [test "$HAVE_IF_DL" = yes])
   if test "$HAVE_IF_DL" = yes; then
      AC_DEFINE([HAVE_IF_DL], [1],
                [Define to 1 if net/if_dl.h is available.])

      # On these platforms we use libpcap to access network devices.
      AC_SEARCH_LIBS([pcap_open_live], [pcap])
   fi])

dnl Checks for buggy strtok_r.
dnl
dnl Some versions of glibc 2.7 has a bug in strtok_r when compiling
dnl with optimization that can cause segfaults:
dnl
dnl http://sources.redhat.com/bugzilla/show_bug.cgi?id=5614.
AC_DEFUN([OVS_CHECK_STRTOK_R],
  [AC_CACHE_CHECK(
     [whether strtok_r macro segfaults on some inputs],
     [ovs_cv_strtok_r_bug],
     [AC_RUN_IFELSE(
        [AC_LANG_PROGRAM([#include <stdio.h>
                          #include <string.h>
                         ],
                         [[#if __GLIBC__ == 2 && __GLIBC_MINOR__ < 8
                           /* Assume bug is present, because relatively minor
                              changes in compiler settings (e.g. optimization
                              level) can make it crop up. */
                           return 1;
                           #else
                           char string[] = ":::";
                           char *save_ptr = (char *) 0xc0ffee;
                           char *token1, *token2;
                           token1 = strtok_r(string, ":", &save_ptr);
                           token2 = strtok_r(NULL, ":", &save_ptr);
                           freopen ("/dev/null", "w", stdout);
                           printf ("%s %s\n", token1, token2);
                           return 0;
                           #endif
                          ]])],
        [ovs_cv_strtok_r_bug=no],
        [ovs_cv_strtok_r_bug=yes],
        [ovs_cv_strtok_r_bug=yes])])
   if test $ovs_cv_strtok_r_bug = yes; then
     AC_DEFINE([HAVE_STRTOK_R_BUG], [1],
               [Define if strtok_r macro segfaults on some inputs])
   fi
])

dnl ----------------------------------------------------------------------
dnl These macros are from GNU PSPP, with the following original license:
dnl Copyright (C) 2005, 2006, 2007 Free Software Foundation, Inc.
dnl This file is free software; the Free Software Foundation
dnl gives unlimited permission to copy and/or distribute it,
dnl with or without modifications, as long as this notice is preserved.

AC_DEFUN([_OVS_CHECK_CC_OPTION], [dnl
  m4_define([ovs_cv_name], [ovs_cv_[]m4_translit([$1], [-=], [__])])dnl
  AC_CACHE_CHECK([whether $CC accepts $1], [ovs_cv_name], 
    [ovs_save_CFLAGS="$CFLAGS"
     dnl Include -Werror in the compiler options, because without -Werror
     dnl clang's GCC-compatible compiler driver does not return a failure
     dnl exit status even though it complains about options it does not
     dnl understand.
     dnl
     dnl Also, check stderr as gcc exits with status 0 for options
     dnl rejected at getopt level.
     dnl    % touch /tmp/a.c
     dnl    % gcc -g -c -Werror -Qunused-arguments /tmp/a.c; echo $?
     dnl    gcc: unrecognized option '-Qunused-arguments'
     dnl    0
     dnl    %
     CFLAGS="$CFLAGS $WERROR $1"
     AC_COMPILE_IFELSE([AC_LANG_PROGRAM(,)], [if test -s conftest.err && grep "unrecognized option" conftest.err; then ovs_cv_name[]=no; else ovs_cv_name[]=yes; fi], [ovs_cv_name[]=no])
     CFLAGS="$ovs_save_CFLAGS"])
  if test $ovs_cv_name = yes; then
    m4_if([$2], [], [:], [$2])
  else
    m4_if([$3], [], [:], [$3])
  fi
])

dnl OVS_CHECK_WERROR
dnl
dnl Check whether the C compiler accepts -Werror.
dnl Sets $WERROR to "-Werror", if so, and otherwise to the empty string.
AC_DEFUN([OVS_CHECK_WERROR],
  [WERROR=
   _OVS_CHECK_CC_OPTION([-Werror], [WERROR=-Werror])])

dnl OVS_CHECK_CC_OPTION([OPTION], [ACTION-IF-ACCEPTED], [ACTION-IF-REJECTED])
dnl Check whether the given C compiler OPTION is accepted.
dnl If so, execute ACTION-IF-ACCEPTED, otherwise ACTION-IF-REJECTED.
AC_DEFUN([OVS_CHECK_CC_OPTION],
  [AC_REQUIRE([OVS_CHECK_WERROR])
   _OVS_CHECK_CC_OPTION([$1], [$2], [$3])])

dnl OVS_ENABLE_OPTION([OPTION])
dnl Check whether the given C compiler OPTION is accepted.
dnl If so, add it to WARNING_FLAGS.
dnl Example: OVS_ENABLE_OPTION([-Wdeclaration-after-statement])
AC_DEFUN([OVS_ENABLE_OPTION], 
  [OVS_CHECK_CC_OPTION([$1], [WARNING_FLAGS="$WARNING_FLAGS $1"])
   AC_SUBST([WARNING_FLAGS])])

dnl OVS_CONDITIONAL_CC_OPTION([OPTION], [CONDITIONAL])
dnl Check whether the given C compiler OPTION is accepted.
dnl If so, enable the given Automake CONDITIONAL.

dnl Example: OVS_CONDITIONAL_CC_OPTION([-Wno-unused], [HAVE_WNO_UNUSED])
AC_DEFUN([OVS_CONDITIONAL_CC_OPTION],
  [OVS_CHECK_CC_OPTION(
    [$1], [ovs_have_cc_option=yes], [ovs_have_cc_option=no])
   AM_CONDITIONAL([$2], [test $ovs_have_cc_option = yes])])
dnl ----------------------------------------------------------------------

dnl Check for too-old XenServer.
AC_DEFUN([OVS_CHECK_XENSERVER_VERSION],
  [AC_CACHE_CHECK([XenServer release], [ovs_cv_xsversion],
    [if test -e /etc/redhat-release; then
       ovs_cv_xsversion=`sed -n 's/^XenServer DDK release \([[^-]]*\)-.*/\1/p' /etc/redhat-release`
     fi
     if test -z "$ovs_cv_xsversion"; then
       ovs_cv_xsversion=none
     fi])
  case $ovs_cv_xsversion in
    none)
      ;;

    [[1-9]][[0-9]]* |                    dnl XenServer 10 or later
    [[6-9]]* |                           dnl XenServer 6 or later
    5.[[7-9]]* |                         dnl XenServer 5.7 or later
    5.6.[[1-9]][[0-9]][[0-9]][[0-9]]* |  dnl XenServer 5.6.1000 or later
    5.6.[[2-9]][[0-9]][[0-9]]* |         dnl XenServer 5.6.200 or later
    5.6.1[[0-9]][[0-9]])                 dnl Xenserver 5.6.100 or later
      ;;

    *)
      AC_MSG_ERROR([This appears to be XenServer $ovs_cv_xsversion, but only XenServer 5.6.100 or later is supported.  (If you are really using a supported version of XenServer, you may override this error message by specifying 'ovs_cv_xsversion=5.6.100' on the "configure" command line.)])
      ;;
  esac])

dnl OVS_MAKE_HAS_IF([if-true], [if-false])
dnl
dnl Checks whether make has the GNU make $(if condition,then,else) extension.
dnl Runs 'if-true' if so, 'if-false' otherwise.
AC_DEFUN([OVS_CHECK_MAKE_IF],
  [AC_CACHE_CHECK(
     [whether ${MAKE-make} has GNU make \$(if) extension],
     [ovs_cv_gnu_make_if],
     [cat <<'EOF' > conftest.mk
conftest.out:
	echo $(if x,y,z) > conftest.out
.PHONY: all
EOF
      rm -f conftest.out
      AS_ECHO(["$as_me:$LINENO: invoking ${MAKE-make} -f conftest.mk all:"]) >&AS_MESSAGE_LOG_FD 2>&1
      ${MAKE-make} -f conftest.mk conftest.out >&AS_MESSAGE_LOG_FD 2>&1
      AS_ECHO(["$as_me:$LINENO: conftest.out contains:"]) >&AS_MESSAGE_LOG_FD 2>&1
      cat conftest.out >&AS_MESSAGE_LOG_FD 2>&1
      result=`cat conftest.out`
      rm -f conftest.mk conftest.out
      if test "X$result" = "Xy"; then
        ovs_cv_gnu_make_if=yes
      else
        ovs_cv_gnu_make_if=no
      fi])])

dnl OVS_CHECK_GNU_MAKE
dnl
dnl Checks whether make is GNU make (because Linux kernel Makefiles
dnl only work with GNU make).
AC_DEFUN([OVS_CHECK_GNU_MAKE],
  [AC_CACHE_CHECK(
     [whether ${MAKE-make} is GNU make],
     [ovs_cv_gnu_make],
     [rm -f conftest.out
      AS_ECHO(["$as_me:$LINENO: invoking ${MAKE-make} --version:"]) >&AS_MESSAGE_LOG_FD 2>&1
      ${MAKE-make} --version >conftest.out 2>&1
      cat conftest.out >&AS_MESSAGE_LOG_FD 2>&1
      result=`cat conftest.out`
      rm -f conftest.mk conftest.out

      case $result in # (
        GNU*) ovs_cv_gnu_make=yes ;; # (
        *) ovs_cv_gnu_make=no ;;
      esac])
   AM_CONDITIONAL([GNU_MAKE], [test $ovs_cv_gnu_make = yes])])

dnl OVS_CHECK_SPARSE_TARGET
dnl
dnl The "cgcc" script from "sparse" isn't very good at detecting the
dnl target for which the code is being built.  This helps it out.
AC_DEFUN([OVS_CHECK_SPARSE_TARGET],
  [AC_CACHE_CHECK(
    [target hint for cgcc],
    [ac_cv_sparse_target],
    [AS_CASE([`$CC -dumpmachine 2>/dev/null`],
       [i?86-* | athlon-*], [ac_cv_sparse_target=x86],
       [x86_64-*], [ac_cv_sparse_target=x86_64],
       [ac_cv_sparse_target=other])])
   AS_CASE([$ac_cv_sparse_target],
     [x86], [SPARSEFLAGS= CGCCFLAGS=-target=i86],
     [x86_64], [SPARSEFLAGS=-m64 CGCCFLAGS=-target=x86_64],
     [SPARSEFLAGS= CGCCFLAGS=])
   AC_SUBST([SPARSEFLAGS])
   AC_SUBST([CGCCFLAGS])])

dnl OVS_SPARSE_EXTRA_INCLUDES
dnl
dnl The cgcc script from "sparse" does not search gcc's default
dnl search path. Get the default search path from GCC and pass
dnl them to sparse.
AC_DEFUN([OVS_SPARSE_EXTRA_INCLUDES],
    AC_SUBST([SPARSE_EXTRA_INCLUDES],
           [`$CC -v -E - </dev/null 2>&1 >/dev/null | sed -n -e '/^#include.*search.*starts.*here:/,/^End.*of.*search.*list\./s/^ \(.*\)/-I \1/p' |grep -v /usr/lib | grep -x -v '\-I /usr/include' | tr \\\n ' ' `] ))

dnl OVS_ENABLE_SPARSE
AC_DEFUN([OVS_ENABLE_SPARSE],
  [AC_REQUIRE([OVS_CHECK_SPARSE_TARGET])
   AC_REQUIRE([OVS_CHECK_MAKE_IF])
   AC_REQUIRE([OVS_SPARSE_EXTRA_INCLUDES])
   : ${SPARSE=sparse}
   AC_SUBST([SPARSE])
   AC_CONFIG_COMMANDS_PRE(
     [if test $ovs_cv_gnu_make_if = yes; then
        CC='$(if $(C),env REAL_CC="'"$CC"'" CHECK="$(SPARSE) -I $(top_srcdir)/include/sparse $(SPARSEFLAGS) $(SPARSE_EXTRA_INCLUDES) " cgcc $(CGCCFLAGS),'"$CC"')'
      fi])])

dnl OVS_PTHREAD_SET_NAME
dnl
dnl This checks for three known variants of pthreads functions for setting
dnl the name of the current thread:
dnl
dnl   glibc: int pthread_setname_np(pthread_t, const char *name);
dnl   NetBSD: int pthread_setname_np(pthread_t, const char *format, void *arg);
dnl   FreeBSD: int pthread_set_name_np(pthread_t, const char *name);
dnl
dnl For glibc and FreeBSD, the arguments are just a thread and its name.  For
dnl NetBSD, 'format' is a printf() format string and 'arg' is an argument to
dnl provide to it.
dnl
dnl This macro defines:
dnl
dnl    glibc: HAVE_GLIBC_PTHREAD_SETNAME_NP
dnl    NetBSD: HAVE_NETBSD_PTHREAD_SETNAME_NP
dnl    FreeBSD: HAVE_PTHREAD_SET_NAME_NP
AC_DEFUN([OVS_CHECK_PTHREAD_SET_NAME],
  [AC_CHECK_FUNCS([pthread_set_name_np])
   if test $ac_cv_func_pthread_set_name_np != yes; then
     AC_CACHE_CHECK(
       [for pthread_setname_np() variant],
       [ovs_cv_pthread_setname_np],
       [AC_LINK_IFELSE(
         [AC_LANG_PROGRAM([#include <pthread.h>
  ], [pthread_setname_np(pthread_self(), "name");])],
         [ovs_cv_pthread_setname_np=glibc],
         [AC_LINK_IFELSE(
           [AC_LANG_PROGRAM([#include <pthread.h>
], [pthread_setname_np(pthread_self(), "%s", "name");])],
           [ovs_cv_pthread_setname_np=netbsd],
           [ovs_cv_pthread_setname_np=none])])])
     case $ovs_cv_pthread_setname_np in # (
       glibc)
          AC_DEFINE(
            [HAVE_GLIBC_PTHREAD_SETNAME_NP], [1],
            [Define to 1 if pthread_setname_np() is available and takes 2 parameters (like glibc).])
          ;; # (
       netbsd)
          AC_DEFINE(
            [HAVE_NETBSD_PTHREAD_SETNAME_NP], [1],
            [Define to 1 if pthread_setname_np() is available and takes 3 parameters (like NetBSD).])
          ;;
     esac
   fi])

dnl OVS_CHECK_LINUX_HOST.
dnl
dnl Checks whether we're building for a Linux host, based on the presence of
dnl the __linux__ preprocessor symbol, and sets up an Automake conditional
dnl LINUX based on the result.
AC_DEFUN([OVS_CHECK_LINUX_HOST],
  [AC_CACHE_CHECK(
     [whether __linux__ is defined],
     [ovs_cv_linux],
     [AC_COMPILE_IFELSE(
        [AC_LANG_PROGRAM([enum { LINUX = __linux__};], [])],
        [ovs_cv_linux=true],
        [ovs_cv_linux=false])])
   AM_CONDITIONAL([LINUX], [$ovs_cv_linux])])