From a40161cbe9ccbcbab798c3e4d257c4bba99d153a Mon Sep 17 00:00:00 2001
From: Paolo Bonzini <pbonzini@redhat.com>
Date: Fri, 16 Feb 2018 10:05:23 +0100
Subject: membarrier: add --enable-membarrier

Actually enable the global memory barriers if supported by the OS.
Because only recent versions of Linux include the support, they
are disabled by default.  Note that it also has to be disabled
for QEMU to run under Wine.

Before this patch, rcutorture reports 85 ns/read for my machine,
after the patch it reports 12.5 ns/read.  On the other hand updates
go from 50 *micro*seconds to 20 *milli*seconds.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 configure | 42 +++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 41 insertions(+), 1 deletion(-)

(limited to 'configure')

diff --git a/configure b/configure
index 26d56eb5bb..db87fc4fed 100755
--- a/configure
+++ b/configure
@@ -342,7 +342,7 @@ attr=""
 libattr=""
 xfs=""
 tcg="yes"
-
+membarrier=""
 vhost_net="no"
 vhost_crypto="no"
 vhost_scsi="no"
@@ -1161,6 +1161,10 @@ for opt do
   ;;
   --enable-attr) attr="yes"
   ;;
+  --disable-membarrier) membarrier="no"
+  ;;
+  --enable-membarrier) membarrier="yes"
+  ;;
   --disable-blobs) blobs="no"
   ;;
   --with-pkgversion=*) pkgversion="$optarg"
@@ -1577,6 +1581,7 @@ disabled with --disable-FEATURE, default is enabled if available:
   xen-pci-passthrough
   brlapi          BrlAPI (Braile)
   curl            curl connectivity
+  membarrier      membarrier system call (for Linux 4.14+ or Windows)
   fdt             fdt device tree
   bluez           bluez stack connectivity
   kvm             KVM acceleration support
@@ -5137,6 +5142,37 @@ if compile_prog "" "" ; then
     have_fsxattr=yes
 fi
 
+##########################################
+# check for usable membarrier system call
+if test "$membarrier" = "yes"; then
+    have_membarrier=no
+    if test "$mingw32" = "yes" ; then
+        have_membarrier=yes
+    elif test "$linux" = "yes" ; then
+        cat > $TMPC << EOF
+    #include <linux/membarrier.h>
+    #include <sys/syscall.h>
+    #include <unistd.h>
+    #include <stdlib.h>
+    int main(void) {
+        syscall(__NR_membarrier, MEMBARRIER_CMD_QUERY, 0);
+        syscall(__NR_membarrier, MEMBARRIER_CMD_SHARED, 0);
+	exit(0);
+    }
+EOF
+        if compile_prog "" "" ; then
+            have_membarrier=yes
+        fi
+    fi
+    if test "$have_membarrier" = "no"; then
+      feature_not_found "membarrier" "membarrier system call not available"
+    fi
+else
+    # Do not enable it by default even for Mingw32, because it doesn't
+    # work on Wine.
+    membarrier=no
+fi
+
 ##########################################
 # check if rtnetlink.h exists and is useful
 have_rtnetlink=no
@@ -5763,6 +5799,7 @@ fi
 echo "malloc trim support $malloc_trim"
 echo "RDMA support      $rdma"
 echo "fdt support       $fdt"
+echo "membarrier        $membarrier"
 echo "preadv support    $preadv"
 echo "fdatasync         $fdatasync"
 echo "madvise           $madvise"
@@ -6245,6 +6282,9 @@ fi
 if test "$fdt" = "yes" ; then
   echo "CONFIG_FDT=y" >> $config_host_mak
 fi
+if test "$membarrier" = "yes" ; then
+  echo "CONFIG_MEMBARRIER=y" >> $config_host_mak
+fi
 if test "$signalfd" = "yes" ; then
   echo "CONFIG_SIGNALFD=y" >> $config_host_mak
 fi
-- 
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