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 <pbonz...@redhat.com>
---
configure | 42 +++++++++++++++++++++++++++++++++++-
include/qemu/sys_membarrier.h | 10 +++++++++
util/Makefile.objs | 1 +
util/sys_membarrier.c | 50 +++++++++++++++++++++++++++++++++++++++++++
4 files changed, 102 insertions(+), 1 deletion(-)
create mode 100644 util/sys_membarrier.c
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" >> $config_host_mak
fi
diff --git a/include/qemu/sys_membarrier.h b/include/qemu/sys_membarrier.h
index 9ce7f5210b..316e3dc4a2 100644
--- a/include/qemu/sys_membarrier.h
+++ b/include/qemu/sys_membarrier.h
@@ -9,9 +9,19 @@
#ifndef QEMU_SYS_MEMBARRIER_H
#define QEMU_SYS_MEMBARRIER_H 1
+#ifdef CONFIG_MEMBARRIER
+/* Only block reordering at the compiler level in the performance-critical
+ * side. The slow side forces processor-level ordering on all other cores
+ * through a system call.
+ */
+extern void smp_mb_global_init(void);
+extern void smp_mb_global(void);
+#define smp_mb_placeholder() barrier()
+#else
/* Keep it simple, execute a real memory barrier on both sides. */
static inline void smp_mb_global_init(void) {}
#define smp_mb_global() smp_mb()
#define smp_mb_placeholder() smp_mb()
+#endif
#endif
diff --git a/util/Makefile.objs b/util/Makefile.objs
index ae90b9963d..728c3541db 100644
--- a/util/Makefile.objs
+++ b/util/Makefile.objs
@@ -33,6 +33,7 @@ util-obj-y += throttle.o
util-obj-y += getauxval.o
util-obj-y += readline.o
util-obj-y += rcu.o
+util-obj-$(CONFIG_MEMBARRIER) += sys_membarrier.o
util-obj-y += qemu-coroutine.o qemu-coroutine-lock.o qemu-coroutine-io.o
util-obj-y += qemu-coroutine-sleep.o
util-obj-y += coroutine-$(CONFIG_COROUTINE_BACKEND).o
diff --git a/util/sys_membarrier.c b/util/sys_membarrier.c
new file mode 100644
index 0000000000..8dcb53e63e
--- /dev/null
+++ b/util/sys_membarrier.c
@@ -0,0 +1,50 @@
+/*
+ * Process-global memory barriers
+ *
+ * Copyright (c) 2018 Red Hat, Inc.
+ *
+ * Author: Paolo Bonzini <pbonz...@redhat.com>
+ */
+
+#include <qemu/osdep.h>
+#include <qemu/sys_membarrier.h>
+#include <qemu/error-report.h>
+
+#ifdef CONFIG_LINUX
+#include <linux/membarrier.h>
+#include <sys/syscall.h>
+
+static int
+membarrier(int cmd, int flags)
+{
+ return syscall(__NR_membarrier, cmd, flags);
+}
+#endif
+
+void smp_mb_global(void)
+{
+#if defined CONFIG_WIN32
+ FlushProcessWriteBuffers();
+#elif defined CONFIG_LINUX
+ membarrier(MEMBARRIER_CMD_SHARED, 0);
+#else
+#error --enable-membarrier is not supported on this operating system.
+#endif
+}
+
+void smp_mb_global_init(void)
+{
+#ifdef CONFIG_LINUX
+ int ret = membarrier(MEMBARRIER_CMD_QUERY, 0);
+ if (ret < 0) {
+ error_report("This QEMU binary requires the membarrier system call.");
+ error_report("Please upgrade your system to a newer version of Linux");
+ exit(1);
+ }
+ if (!(ret & MEMBARRIER_CMD_SHARED)) {
+ error_report("This QEMU binary requires MEMBARRIER_CMD_SHARED
support.");
+ error_report("Please upgrade your system to a newer version of Linux");
+ exit(1);
+ }
+#endif
+}
--
2.14.3
