From f7bff04b7bd06db496a3a78917be38abcc9d52fc Mon Sep 17 00:00:00 2001
From: Fam Zheng <famz@redhat.com>
Date: Wed, 21 Feb 2018 21:10:12 -0500
Subject: [PATCH] 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

Backports commit 729633c2bc30496073431584eb6e304776b4ebd4 from qemu
---
 qemu/exec.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/qemu/exec.c b/qemu/exec.c
index 8a0e26e0..5959e532 100644
--- a/qemu/exec.c
+++ b/qemu/exec.c
@@ -103,6 +103,7 @@ typedef struct PhysPageMap {
 } PhysPageMap;
 
 struct AddressSpaceDispatch {
+    MemoryRegionSection *mru_section;
     /* This is a multi-level map on the physical address space.
      * The bottom level has pointers to MemoryRegionSections.
      */
@@ -300,17 +301,28 @@ bool memory_region_is_unassigned(struct uc_struct* uc, MemoryRegion *mr)
 }
 
 static MemoryRegionSection *address_space_lookup_region(AddressSpaceDispatch *d,
-        hwaddr addr,
-        bool resolve_subpage)
+                                                        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;
 }