[RFC 1/8] memory: Allow eventfd add/del without starting a transaction

[Greg Kurz <groug@kaod.org>]

Each addition or deletion of an eventfd happens in its own MR
transaction. This doesn't scale well with multiqueue devices
that do 1:1 queue:vCPU mapping (e.g. virtio-scsi-pci or
virtio-blk-pci) : these devices typically create at least one
eventfd per queue and memory_region_transaction_commit(),
which is called during commit, also loops on eventfds, resulting
in a quadratic time complexity. This calls for batching : a
device should be able to add or delete its eventfds in a single
transaction.

Prepare ground for this by introducing extended versions of
memory_region_add_eventfd() and memory_region_del_eventfd()
that take an extra bool argument to control if a transaction
should be started or not.

No behavior change at this point.

Signed-off-by: Greg Kurz <groug@kaod.org>
---
 include/exec/memory.h | 48 ++++++++++++++++++++++++++++++++-----------
 softmmu/memory.c      | 42 ++++++++++++++++++++++---------------
 2 files changed, 62 insertions(+), 28 deletions(-)

diff --git a/include/exec/memory.h b/include/exec/memory.h
index 5728a681b27d..98ed552e001c 100644
--- a/include/exec/memory.h
+++ b/include/exec/memory.h
@@ -1848,13 +1848,25 @@ void memory_region_clear_flush_coalesced(MemoryRegion *mr);
  * @match_data: whether to match against @data, instead of just @addr
  * @data: the data to match against the guest write
  * @e: event notifier to be triggered when @addr, @size, and @data all match.
+ * @transaction: whether to start a transaction for the change
  **/
-void memory_region_add_eventfd(MemoryRegion *mr,
-                               hwaddr addr,
-                               unsigned size,
-                               bool match_data,
-                               uint64_t data,
-                               EventNotifier *e);
+void memory_region_add_eventfd_full(MemoryRegion *mr,
+                                    hwaddr addr,
+                                    unsigned size,
+                                    bool match_data,
+                                    uint64_t data,
+                                    EventNotifier *e,
+                                    bool transaction);
+
+static inline void memory_region_add_eventfd(MemoryRegion *mr,
+                                             hwaddr addr,
+                                             unsigned size,
+                                             bool match_data,
+                                             uint64_t data,
+                                             EventNotifier *e)
+{
+    memory_region_add_eventfd_full(mr, addr, size, match_data, data, e, true);
+}
 
 /**
  * memory_region_del_eventfd: Cancel an eventfd.
@@ -1868,13 +1880,25 @@ void memory_region_add_eventfd(MemoryRegion *mr,
  * @match_data: whether to match against @data, instead of just @addr
  * @data: the data to match against the guest write
  * @e: event notifier to be triggered when @addr, @size, and @data all match.
+ * @transaction: whether to start a transaction for the change
  */
-void memory_region_del_eventfd(MemoryRegion *mr,
-                               hwaddr addr,
-                               unsigned size,
-                               bool match_data,
-                               uint64_t data,
-                               EventNotifier *e);
+void memory_region_del_eventfd_full(MemoryRegion *mr,
+                                    hwaddr addr,
+                                    unsigned size,
+                                    bool match_data,
+                                    uint64_t data,
+                                    EventNotifier *e,
+                                    bool transaction);
+
+static inline void memory_region_del_eventfd(MemoryRegion *mr,
+                                             hwaddr addr,
+                                             unsigned size,
+                                             bool match_data,
+                                             uint64_t data,
+                                             EventNotifier *e)
+{
+    memory_region_del_eventfd_full(mr, addr, size, match_data, data, e, true);
+}
 
 /**
  * memory_region_add_subregion: Add a subregion to a container.
diff --git a/softmmu/memory.c b/softmmu/memory.c
index d4493ef9e430..1b1942d521cc 100644
--- a/softmmu/memory.c
+++ b/softmmu/memory.c
@@ -2341,12 +2341,13 @@ void memory_region_clear_flush_coalesced(MemoryRegion *mr)
 
 static bool userspace_eventfd_warning;
 
-void memory_region_add_eventfd(MemoryRegion *mr,
-                               hwaddr addr,
-                               unsigned size,
-                               bool match_data,
-                               uint64_t data,
-                               EventNotifier *e)
+void memory_region_add_eventfd_full(MemoryRegion *mr,
+                                    hwaddr addr,
+                                    unsigned size,
+                                    bool match_data,
+                                    uint64_t data,
+                                    EventNotifier *e,
+                                    bool transaction)
 {
     MemoryRegionIoeventfd mrfd = {
         .addr.start = int128_make64(addr),
@@ -2367,7 +2368,9 @@ void memory_region_add_eventfd(MemoryRegion *mr,
     if (size) {
         adjust_endianness(mr, &mrfd.data, size_memop(size) | MO_TE);
     }
-    memory_region_transaction_begin();
+    if (transaction) {
+        memory_region_transaction_begin();
+    }
     for (i = 0; i < mr->ioeventfd_nb; ++i) {
         if (memory_region_ioeventfd_before(&mrfd, &mr->ioeventfds[i])) {
             break;
@@ -2380,15 +2383,18 @@ void memory_region_add_eventfd(MemoryRegion *mr,
             sizeof(*mr->ioeventfds) * (mr->ioeventfd_nb-1 - i));
     mr->ioeventfds[i] = mrfd;
     ioeventfd_update_pending |= mr->enabled;
-    memory_region_transaction_commit();
+    if (transaction) {
+        memory_region_transaction_commit();
+    }
 }
 
-void memory_region_del_eventfd(MemoryRegion *mr,
-                               hwaddr addr,
-                               unsigned size,
-                               bool match_data,
-                               uint64_t data,
-                               EventNotifier *e)
+void memory_region_del_eventfd_full(MemoryRegion *mr,
+                                    hwaddr addr,
+                                    unsigned size,
+                                    bool match_data,
+                                    uint64_t data,
+                                    EventNotifier *e,
+                                    bool transaction)
 {
     MemoryRegionIoeventfd mrfd = {
         .addr.start = int128_make64(addr),
@@ -2402,7 +2408,9 @@ void memory_region_del_eventfd(MemoryRegion *mr,
     if (size) {
         adjust_endianness(mr, &mrfd.data, size_memop(size) | MO_TE);
     }
-    memory_region_transaction_begin();
+    if (transaction) {
+        memory_region_transaction_begin();
+    }
     for (i = 0; i < mr->ioeventfd_nb; ++i) {
         if (memory_region_ioeventfd_equal(&mrfd, &mr->ioeventfds[i])) {
             break;
@@ -2415,7 +2423,9 @@ void memory_region_del_eventfd(MemoryRegion *mr,
     mr->ioeventfds = g_realloc(mr->ioeventfds,
                                   sizeof(*mr->ioeventfds)*mr->ioeventfd_nb + 1);
     ioeventfd_update_pending |= mr->enabled;
-    memory_region_transaction_commit();
+    if (transaction) {
+        memory_region_transaction_commit();
+    }
 }
 
 static void memory_region_update_container_subregions(MemoryRegion *subregion)
-- 
2.26.3