exec: Introduce AddressSpaceDispatch.mru_section

Under heavy workloads the lookup will likely end up with the same
MemoryRegionSection from last time. Using a pointer to cache the result,
like ram_list.mru_block, significantly reduces cost of
address_space_translate.

During address space topology update, as->dispatch will be reallocated
so the pointer is invalidated automatically.

Perf reports a visible drop on the cpu usage, because phys_page_find is
not called.  Before:

   2.35%  qemu-system-x86_64       [.] phys_page_find
   0.97%  qemu-system-x86_64       [.] address_space_translate_internal
   0.95%  qemu-system-x86_64       [.] address_space_translate
   0.55%  qemu-system-x86_64       [.] address_space_lookup_region

After:

   0.97%  qemu-system-x86_64       [.] address_space_translate_internal
   0.97%  qemu-system-x86_64       [.] address_space_lookup_region
   0.84%  qemu-system-x86_64       [.] address_space_translate

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <1456813104-25902-8-git-send-email-famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

exec.c

@@ -135,6 +135,7 @@ typedef struct PhysPageMap {
 struct AddressSpaceDispatch {
     struct rcu_head rcu;
 
+    MemoryRegionSection *mru_section;
     /* This is a multi-level map on the physical address space.
      * The bottom level has pointers to MemoryRegionSections.
      */
@@ -351,14 +352,25 @@ static MemoryRegionSection *address_space_lookup_region(AddressSpaceDispatch *d,
                                                         hwaddr addr,
                                                         bool resolve_subpage)
 {
-    MemoryRegionSection *section;
+    MemoryRegionSection *section = atomic_read(&d->mru_section);
     subpage_t *subpage;
+    bool update;
 
-    section = phys_page_find(d->phys_map, addr, d->map.nodes, d->map.sections);
+    if (section && section != &d->map.sections[PHYS_SECTION_UNASSIGNED] &&
+        section_covers_addr(section, addr)) {
+        update = false;
+    } else {
+        section = phys_page_find(d->phys_map, addr, d->map.nodes,
+                                 d->map.sections);
+        update = true;
+    }
     if (resolve_subpage && section->mr->subpage) {
         subpage = container_of(section->mr, subpage_t, iomem);
         section = &d->map.sections[subpage->sub_section[SUBPAGE_IDX(addr)]];
     }
+    if (update) {
+        atomic_set(&d->mru_section, section);
+    }
     return section;
 }