[PATCH v7 6/6] memory: Introduce address_space_to_flatview_rcu()

[Chuang Xu]

In last patch, we wrap vm_load with begin/commit, here we introduce
address_space_to_flatview_rcu() to avoid unnecessary enforce commit
during vm_load.

Signed-off-by: Chuang Xu <xuchuangxclwt@bytedance.com>
---
 include/exec/memory-internal.h |  2 +-
 include/exec/memory.h          | 20 ++++++++++++++++++++
 softmmu/memory.c               |  2 +-
 3 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/include/exec/memory-internal.h b/include/exec/memory-internal.h
index 100c1237ac..1432240449 100644
--- a/include/exec/memory-internal.h
+++ b/include/exec/memory-internal.h
@@ -30,7 +30,7 @@ static inline AddressSpaceDispatch 
*flatview_to_dispatch(FlatView *fv)
 
 static inline AddressSpaceDispatch *address_space_to_dispatch(AddressSpace *as)
 {
-    return flatview_to_dispatch(address_space_to_flatview(as));
+    return flatview_to_dispatch(address_space_to_flatview_rcu(as));
 }
 
 FlatView *address_space_get_flatview(AddressSpace *as);
diff --git a/include/exec/memory.h b/include/exec/memory.h
index d6fd89db64..235e3017bc 100644
--- a/include/exec/memory.h
+++ b/include/exec/memory.h
@@ -1100,6 +1100,9 @@ bool memory_region_transaction_in_progress(void);
 
 void memory_region_transaction_do_commit(void);
 
+/*
+ * We recommend using this by default.
+ */
 static inline FlatView *address_space_to_flatview(AddressSpace *as)
 {
     if (qemu_mutex_iothread_locked()) {
@@ -1123,6 +1126,23 @@ static inline FlatView 
*address_space_to_flatview(AddressSpace *as)
     return qatomic_rcu_read(&as->current_map);
 }
 
+/*
+ * We recommend using address_space_to_flatview() rather than this one.
+ * Note that if we use this during a memory region transaction, we may
+ * see obsolete flatviews. We must bear with an obsolete map until commit.
+ * And outside a memory region transaction, this is basically the same as
+ * address_space_to_flatview().
+ */
+static inline FlatView *address_space_to_flatview_rcu(AddressSpace *as)
+{
+    /*
+     * Before using any flatview, sanity check BQL or RCU is held.
+     */
+    assert(qemu_mutex_iothread_locked() || rcu_read_is_locked());
+
+    return qatomic_rcu_read(&as->current_map);
+}
+
 /**
  * typedef flatview_cb: callback for flatview_for_each_range()
  *
diff --git a/softmmu/memory.c b/softmmu/memory.c
index 6a8e8b4e71..33d14e967d 100644
--- a/softmmu/memory.c
+++ b/softmmu/memory.c
@@ -815,7 +815,7 @@ FlatView *address_space_get_flatview(AddressSpace *as)
 
     RCU_READ_LOCK_GUARD();
     do {
-        view = address_space_to_flatview(as);
+        view = address_space_to_flatview_rcu(as);
         /* If somebody has replaced as->current_map concurrently,
          * flatview_ref returns false.
          */
-- 
2.20.1