From 20b1bd767ffc52ba5873f070ac6c36e0f31074c1 Mon Sep 17 00:00:00 2001
From: Paolo Bonzini <pbonzini@redhat.com>
Date: Sun, 11 Mar 2018 22:19:47 -0400
Subject: [PATCH] memory: seek FlatView sharing candidates among children
 subregions

A container can be used instead of an alias to allow switching between
multiple subregions. In this case we cannot directly share the
subregions (since they only belong to a single parent), but if the
subregions are aliases we can in turn walk those.

This is not enough to remove all source of quadratic FlatView creation,
but it enables sharing of the PCI bus master FlatViews (and their
AddressSpaceDispatch structures) across all PCI devices. For 112
virtio-net-pci devices, boot time is reduced from 25 to 10 seconds and
memory consumption from 1.4 to 1 G.

Backports commit e673ba9af9bf8fd8e0f44025ac738b8285b3ed27 from qemu
---
 qemu/memory.c | 40 ++++++++++++++++++++++++++++++++++------
 1 file changed, 34 insertions(+), 6 deletions(-)

diff --git a/qemu/memory.c b/qemu/memory.c
index 16891425..3ec68a8e 100644
--- a/qemu/memory.c
+++ b/qemu/memory.c
@@ -688,12 +688,40 @@ static void render_memory_region(FlatView *view,
 
 static MemoryRegion *memory_region_get_flatview_root(MemoryRegion *mr)
 {
-    while (mr->alias && !mr->alias_offset &&
-           int128_ge(mr->size, mr->alias->size)) {
-        /* The alias is included in its entirety.  Use it as
-         * the "real" root, so that we can share more FlatViews.
-         */
-        mr = mr->alias;
+    while (mr->enabled) {
+        if (mr->alias) {
+            if (!mr->alias_offset && int128_ge(mr->size, mr->alias->size)) {
+                /* The alias is included in its entirety.  Use it as
+                 * the "real" root, so that we can share more FlatViews.
+                 */
+                mr = mr->alias;
+                continue;
+            }
+        } else if (!mr->terminates) {
+            unsigned int found = 0;
+            MemoryRegion *child, *next = NULL;
+            QTAILQ_FOREACH(child, &mr->subregions, subregions_link) {
+                if (child->enabled) {
+                    if (++found > 1) {
+                        next = NULL;
+                        break;
+                    }
+                    if (!child->addr && int128_ge(mr->size, child->size)) {
+                        /* A child is included in its entirety.  If it's the only
+                         * enabled one, use it in the hope of finding an alias down the
+                         * way. This will also let us share FlatViews.
+                         */
+                        next = child;
+                    }
+                }
+            }
+            if (next) {
+                mr = next;
+                continue;
+            }
+        }
+
+        break;
     }
 
     return mr;