With this change, a FlatView can be used even after a concurrent
update has replaced it. Because we do not have RCU, we use a
mutex to protect the small critical sections that read/write the
as->current_map pointer. Accesses to the FlatView can be done
outside the mutex.
If a MemoryRegion will be used after the FlatView is unref-ed (or after
a MemoryListener callback is returned), a reference has to be added to
that MemoryRegion. For example, memory_region_find adds a reference to
the MemoryRegion that it returns.
Signed-off-by: Paolo Bonzini <pbonz...@redhat.com>
---
memory.c | 75 +++++++++++++++++++++++++++++++++++++++++++++++++++++++------
1 files changed, 67 insertions(+), 8 deletions(-)
diff --git a/memory.c b/memory.c
index 52e1fca..105e211 100644
--- a/memory.c
+++ b/memory.c
@@ -26,12 +26,26 @@ static unsigned memory_region_transaction_depth;
static bool memory_region_update_pending;
static bool global_dirty_log = false;
+/* flat_view_mutex is taken around reading as->current_map; the critical
+ * section is extremely short, so I'm using a single mutex for every AS.
+ * We could also RCU for the read-side.
+ *
+ * The BQL is taken around transaction commits, hence both locks are taken
+ * while writing to as->current_map (with the BQL taken outside).
+ */
+static QemuMutex flat_view_mutex;
+
static QTAILQ_HEAD(memory_listeners, MemoryListener) memory_listeners
= QTAILQ_HEAD_INITIALIZER(memory_listeners);
static QTAILQ_HEAD(, AddressSpace) address_spaces
= QTAILQ_HEAD_INITIALIZER(address_spaces);
+static void memory_init(void)
+{
+ qemu_mutex_init(&flat_view_mutex);
+}
+
typedef struct AddrRange AddrRange;
/*
@@ -222,6 +236,7 @@ struct FlatRange {
* order.
*/
struct FlatView {
+ unsigned ref;
FlatRange *ranges;
unsigned nr;
unsigned nr_allocated;
@@ -243,6 +258,7 @@ static bool flatrange_equal(FlatRange *a, FlatRange *b)
static void flatview_init(FlatView *view)
{
+ view->ref = 1;
view->ranges = NULL;
view->nr = 0;
view->nr_allocated = 0;
@@ -276,6 +292,18 @@ static void flatview_destroy(FlatView *view)
g_free(view);
}
+static void flatview_ref(FlatView *view)
+{
+ __sync_fetch_and_add(&view->ref, 1);
+}
+
+static void flatview_unref(FlatView *view)
+{
+ if (__sync_fetch_and_sub(&view->ref, 1) == 1) {
+ flatview_destroy(view);
+ }
+}
+
static bool can_merge(FlatRange *r1, FlatRange *r2)
{
return int128_eq(addrrange_end(r1->addr), r2->addr.start)
@@ -728,16 +756,38 @@ static void
address_space_update_topology_pass(AddressSpace *as,
}
+static FlatView *address_space_get_flatview(AddressSpace *as)
+{
+ FlatView *view;
+
+ qemu_mutex_lock(&flat_view_mutex);
+ view = as->current_map;
+ flatview_ref(view);
+ qemu_mutex_unlock(&flat_view_mutex);
+ return view;
+}
+
static void address_space_update_topology(AddressSpace *as)
{
- FlatView *old_view = as->current_map;
+ FlatView *old_view = address_space_get_flatview(as);
FlatView *new_view = generate_memory_topology(as->root);
address_space_update_topology_pass(as, old_view, new_view, false);
address_space_update_topology_pass(as, old_view, new_view, true);
+ qemu_mutex_lock(&flat_view_mutex);
+ flatview_unref(as->current_map);
as->current_map = new_view;
- flatview_destroy(old_view);
+ qemu_mutex_unlock(&flat_view_mutex);
+
+ /* Note that all the old MemoryRegions are still alive up to this
+ * point. This relieves most MemoryListeners from the need to
+ * ref/unref the MemoryRegions they get---unless they use them
+ * outside the iothread mutex, in which case precise reference
+ * counting is necessary.
+ */
+ flatview_unref(old_view);
+
address_space_update_ioeventfds(as);
}
@@ -1173,12 +1223,13 @@ void memory_region_sync_dirty_bitmap(MemoryRegion *mr)
FlatRange *fr;
QTAILQ_FOREACH(as, &address_spaces, address_spaces_link) {
- FlatView *view = as->current_map;
+ FlatView *view = address_space_get_flatview(as);
FOR_EACH_FLAT_RANGE(fr, view) {
if (fr->mr == mr) {
MEMORY_LISTENER_UPDATE_REGION(fr, as, Forward, log_sync);
}
}
+ flatview_unref(view);
}
}
@@ -1230,7 +1281,7 @@ static void
memory_region_update_coalesced_range_as(MemoryRegion *mr, AddressSpa
AddrRange tmp;
MemoryRegionSection section;
- view = as->current_map;
+ view = address_space_get_flatview(as);
FOR_EACH_FLAT_RANGE(fr, view) {
if (fr->mr == mr) {
section = (MemoryRegionSection) {
@@ -1256,6 +1307,7 @@ static void
memory_region_update_coalesced_range_as(MemoryRegion *mr, AddressSpa
}
}
}
+ flatview_unref(view);
}
static void memory_region_update_coalesced_range(MemoryRegion *mr)
@@ -1543,7 +1595,7 @@ MemoryRegionSection memory_region_find(MemoryRegion *mr,
as = memory_region_to_address_space(root);
range = addrrange_make(int128_make64(addr), int128_make64(size));
- view = as->current_map;
+ view = address_space_get_flatview(as);
fr = flatview_lookup(view, range);
if (!fr) {
return ret;
@@ -1564,6 +1616,7 @@ MemoryRegionSection memory_region_find(MemoryRegion *mr,
ret.readonly = fr->readonly;
memory_region_ref(ret.mr);
+ flatview_unref(view);
return ret;
}
@@ -1572,10 +1625,11 @@ void address_space_sync_dirty_bitmap(AddressSpace *as)
FlatView *view;
FlatRange *fr;
- view = as->current_map;
+ view = address_space_get_flatview(as);
FOR_EACH_FLAT_RANGE(fr, view) {
MEMORY_LISTENER_UPDATE_REGION(fr, as, Forward, log_sync);
}
+ flatview_unref(view);
}
void memory_global_dirty_log_start(void)
@@ -1607,7 +1661,7 @@ static void listener_add_address_space(MemoryListener
*listener,
}
}
- view = as->current_map;
+ view = address_space_get_flatview(as);
FOR_EACH_FLAT_RANGE(fr, view) {
MemoryRegionSection section = {
.mr = fr->mr,
@@ -1621,6 +1675,7 @@ static void listener_add_address_space(MemoryListener
*listener,
listener->region_add(listener, §ion);
}
}
+ flatview_unref(view);
}
void memory_listener_register(MemoryListener *listener, AddressSpace *filter)
@@ -1654,6 +1709,10 @@ void memory_listener_unregister(MemoryListener *listener)
void address_space_init(AddressSpace *as, MemoryRegion *root, const char *name)
{
+ if (QTAILQ_EMPTY(&address_spaces)) {
+ memory_init();
+ }
+
memory_region_transaction_begin();
as->root = root;
as->current_map = g_new(FlatView, 1);
@@ -1675,7 +1734,7 @@ void address_space_destroy(AddressSpace *as)
memory_region_transaction_commit();
QTAILQ_REMOVE(&address_spaces, as, address_spaces_link);
address_space_destroy_dispatch(as);
- flatview_destroy(as->current_map);
+ flatview_unref(as->current_map);
g_free(as->name);
g_free(as->current_map);
g_free(as->ioeventfds);
--
1.7.1
