memory: Share special empty FlatView

This shares an cached empty FlatView among address spaces. The empty
FV is used every time when a root MR renders into a FV without memory
sections which happens when MR or its children are not enabled or
zero-sized. The empty_view is not NULL to keep the rest of memory
API intact; it also has a dispatch tree for the same reason.

On POWER8 with 255 CPUs, 255 virtio-net, 40 PCI bridges guest this halves
the amount of FlatView's in use (557 -> 260) and dispatch tables
(~800000 -> ~370000). In an unrelated experiment with 112 non-virtio
devices on x86 ("-M pc"), only 4 FlatViews are alive, and about ~2000
are created at startup.

Backports commit 092aa2fc65b7a35121616aad8f39d47b8f921618 from qemu

include/uc_priv.h

@@ -199,6 +199,7 @@ struct uc_struct {
     uintptr_t helper_retaddr;
 
     // qemu/memory.c
+    FlatView *empty_view;
     GHashTable *flat_views;
     bool global_dirty_log;
 

qemu/aarch64.h

@@ -2998,6 +2998,7 @@
 #define unassigned_mem_ops unassigned_mem_ops_aarch64
 #define unassigned_mem_read unassigned_mem_read_aarch64
 #define unassigned_mem_write unassigned_mem_write_aarch64
+#define unicorn_free_empty_flat_view unicorn_free_empty_flat_view_aarch64
 #define update_spsel update_spsel_aarch64
 #define use_idiv_instructions_rt use_idiv_instructions_rt_aarch64
 #define v6_cp_reginfo v6_cp_reginfo_aarch64

qemu/aarch64eb.h

@@ -2998,6 +2998,7 @@
 #define unassigned_mem_ops unassigned_mem_ops_aarch64eb
 #define unassigned_mem_read unassigned_mem_read_aarch64eb
 #define unassigned_mem_write unassigned_mem_write_aarch64eb
+#define unicorn_free_empty_flat_view unicorn_free_empty_flat_view_aarch64eb
 #define update_spsel update_spsel_aarch64eb
 #define use_idiv_instructions_rt use_idiv_instructions_rt_aarch64eb
 #define v6_cp_reginfo v6_cp_reginfo_aarch64eb

qemu/arm.h

@@ -2998,6 +2998,7 @@
 #define unassigned_mem_ops unassigned_mem_ops_arm
 #define unassigned_mem_read unassigned_mem_read_arm
 #define unassigned_mem_write unassigned_mem_write_arm
+#define unicorn_free_empty_flat_view unicorn_free_empty_flat_view_arm
 #define update_spsel update_spsel_arm
 #define use_idiv_instructions_rt use_idiv_instructions_rt_arm
 #define v6_cp_reginfo v6_cp_reginfo_arm

qemu/armeb.h

@@ -2998,6 +2998,7 @@
 #define unassigned_mem_ops unassigned_mem_ops_armeb
 #define unassigned_mem_read unassigned_mem_read_armeb
 #define unassigned_mem_write unassigned_mem_write_armeb
+#define unicorn_free_empty_flat_view unicorn_free_empty_flat_view_armeb
 #define update_spsel update_spsel_armeb
 #define use_idiv_instructions_rt use_idiv_instructions_rt_armeb
 #define v6_cp_reginfo v6_cp_reginfo_armeb

qemu/header_gen.py

@@ -3004,6 +3004,7 @@ symbols = (
     'unassigned_mem_ops',
     'unassigned_mem_read',
     'unassigned_mem_write',
+    'unicorn_free_empty_flat_view',
     'update_spsel',
     'use_idiv_instructions_rt',
     'v6_cp_reginfo',

qemu/include/exec/memory.h

@@ -1379,6 +1379,8 @@ address_space_write_cached(MemoryRegionCache *cache, hwaddr addr,
     address_space_write(cache->as, cache->xlat + addr, MEMTXATTRS_UNSPECIFIED, buf, len);
 }
 
+void unicorn_free_empty_flat_view(struct uc_struct *uc);
+
 #endif
 
 #endif

qemu/m68k.h

@@ -2998,6 +2998,7 @@
 #define unassigned_mem_ops unassigned_mem_ops_m68k
 #define unassigned_mem_read unassigned_mem_read_m68k
 #define unassigned_mem_write unassigned_mem_write_m68k
+#define unicorn_free_empty_flat_view unicorn_free_empty_flat_view_m68k
 #define update_spsel update_spsel_m68k
 #define use_idiv_instructions_rt use_idiv_instructions_rt_m68k
 #define v6_cp_reginfo v6_cp_reginfo_m68k

qemu/memory.c

@@ -346,6 +346,15 @@ static void flatview_unref(FlatView *view)
     }
 }
 
+void unicorn_free_empty_flat_view(struct uc_struct *uc)
+{
+    if (!uc->empty_view) {
+        return;
+    }
+
+    flatview_destroy(uc->empty_view);
+}
+
 FlatView *address_space_to_flatview(AddressSpace *as)
 {
     // Unicorn: atomic_read used instead of atomic_rcu_read
@@ -728,7 +737,7 @@ static MemoryRegion *memory_region_get_flatview_root(MemoryRegion *mr)
 }
 
 /* Render a memory topology into a list of disjoint absolute ranges. */
-static FlatView *generate_memory_topology(MemoryRegion *mr)
+static FlatView *generate_memory_topology(struct uc_struct *uc, MemoryRegion *mr)
 {
     int i;
     FlatView *view;
@@ -741,14 +750,14 @@ static FlatView *generate_memory_topology(MemoryRegion *mr)
     }
     flatview_simplify(view);
 
-    view->dispatch = address_space_dispatch_new(mr->uc, view);
+    view->dispatch = address_space_dispatch_new(uc, view);
     for (i = 0; i < view->nr; i++) {
         MemoryRegionSection mrs =
             section_from_flat_range(&view->ranges[i], view);
         flatview_add_to_dispatch(view, &mrs);
     }
     address_space_dispatch_compact(view->dispatch);
-    g_hash_table_replace(mr->uc->flat_views, mr, view);
+    g_hash_table_replace(uc->flat_views, mr, view);
 
     return view;
 }
@@ -838,6 +847,15 @@ static void flatviews_init(struct uc_struct *uc)
 
     uc->flat_views = g_hash_table_new_full(g_direct_hash, g_direct_equal, NULL,
                                            (GDestroyNotify) flatview_unref);
+
+    if (!uc->empty_view) {
+        uc->empty_view = generate_memory_topology(uc, NULL);
+        /* We keep it alive forever in the global variable.  */
+        flatview_ref(uc->empty_view);
+    } else {
+        g_hash_table_replace(uc->flat_views, NULL, uc->empty_view);
+        flatview_ref(uc->empty_view);
+    }
 }
 
 static void flatviews_reset(struct uc_struct *uc)
@@ -858,7 +876,7 @@ static void flatviews_reset(struct uc_struct *uc)
             continue;
         }
 
-        generate_memory_topology(physmr);
+        generate_memory_topology(uc, physmr);
     }
 }
 
@@ -915,7 +933,7 @@ static void address_space_update_topology(AddressSpace *as)
 
     flatviews_init(uc);
     if (!g_hash_table_lookup(uc->flat_views, physmr)) {
-        generate_memory_topology(physmr);
+        generate_memory_topology(uc, physmr);
     }
     address_space_set_flatview(as);
 }

qemu/mips.h

@@ -2998,6 +2998,7 @@
 #define unassigned_mem_ops unassigned_mem_ops_mips
 #define unassigned_mem_read unassigned_mem_read_mips
 #define unassigned_mem_write unassigned_mem_write_mips
+#define unicorn_free_empty_flat_view unicorn_free_empty_flat_view_mips
 #define update_spsel update_spsel_mips
 #define use_idiv_instructions_rt use_idiv_instructions_rt_mips
 #define v6_cp_reginfo v6_cp_reginfo_mips

qemu/mips64.h

@@ -2998,6 +2998,7 @@
 #define unassigned_mem_ops unassigned_mem_ops_mips64
 #define unassigned_mem_read unassigned_mem_read_mips64
 #define unassigned_mem_write unassigned_mem_write_mips64
+#define unicorn_free_empty_flat_view unicorn_free_empty_flat_view_mips64
 #define update_spsel update_spsel_mips64
 #define use_idiv_instructions_rt use_idiv_instructions_rt_mips64
 #define v6_cp_reginfo v6_cp_reginfo_mips64

qemu/mips64el.h

@@ -2998,6 +2998,7 @@
 #define unassigned_mem_ops unassigned_mem_ops_mips64el
 #define unassigned_mem_read unassigned_mem_read_mips64el
 #define unassigned_mem_write unassigned_mem_write_mips64el
+#define unicorn_free_empty_flat_view unicorn_free_empty_flat_view_mips64el
 #define update_spsel update_spsel_mips64el
 #define use_idiv_instructions_rt use_idiv_instructions_rt_mips64el
 #define v6_cp_reginfo v6_cp_reginfo_mips64el

qemu/mipsel.h

@@ -2998,6 +2998,7 @@
 #define unassigned_mem_ops unassigned_mem_ops_mipsel
 #define unassigned_mem_read unassigned_mem_read_mipsel
 #define unassigned_mem_write unassigned_mem_write_mipsel
+#define unicorn_free_empty_flat_view unicorn_free_empty_flat_view_mipsel
 #define update_spsel update_spsel_mipsel
 #define use_idiv_instructions_rt use_idiv_instructions_rt_mipsel
 #define v6_cp_reginfo v6_cp_reginfo_mipsel

qemu/powerpc.h

@@ -2998,6 +2998,7 @@
 #define unassigned_mem_ops unassigned_mem_ops_powerpc
 #define unassigned_mem_read unassigned_mem_read_powerpc
 #define unassigned_mem_write unassigned_mem_write_powerpc
+#define unicorn_free_empty_flat_view unicorn_free_empty_flat_view_powerpc
 #define update_spsel update_spsel_powerpc
 #define use_idiv_instructions_rt use_idiv_instructions_rt_powerpc
 #define v6_cp_reginfo v6_cp_reginfo_powerpc

qemu/sparc.h

@@ -2998,6 +2998,7 @@
 #define unassigned_mem_ops unassigned_mem_ops_sparc
 #define unassigned_mem_read unassigned_mem_read_sparc
 #define unassigned_mem_write unassigned_mem_write_sparc
+#define unicorn_free_empty_flat_view unicorn_free_empty_flat_view_sparc
 #define update_spsel update_spsel_sparc
 #define use_idiv_instructions_rt use_idiv_instructions_rt_sparc
 #define v6_cp_reginfo v6_cp_reginfo_sparc

qemu/sparc64.h

@@ -2998,6 +2998,7 @@
 #define unassigned_mem_ops unassigned_mem_ops_sparc64
 #define unassigned_mem_read unassigned_mem_read_sparc64
 #define unassigned_mem_write unassigned_mem_write_sparc64
+#define unicorn_free_empty_flat_view unicorn_free_empty_flat_view_sparc64
 #define update_spsel update_spsel_sparc64
 #define use_idiv_instructions_rt use_idiv_instructions_rt_sparc64
 #define v6_cp_reginfo v6_cp_reginfo_sparc64

qemu/unicorn_common.h

@@ -80,6 +80,7 @@ static void release_common(void *t)
 
     // Destory flat view hash table
     g_hash_table_destroy(s->uc->flat_views);
+    unicorn_free_empty_flat_view(s->uc);
 
     // TODO(danghvu): these function is not available outside qemu
     // so we keep them here instead of outside uc_close.

qemu/x86_64.h

@@ -2998,6 +2998,7 @@
 #define unassigned_mem_ops unassigned_mem_ops_x86_64
 #define unassigned_mem_read unassigned_mem_read_x86_64
 #define unassigned_mem_write unassigned_mem_write_x86_64
+#define unicorn_free_empty_flat_view unicorn_free_empty_flat_view_x86_64
 #define update_spsel update_spsel_x86_64
 #define use_idiv_instructions_rt use_idiv_instructions_rt_x86_64
 #define v6_cp_reginfo v6_cp_reginfo_x86_64