From 59b4874443184305274f671a7bbc94773ef3f9a9 Mon Sep 17 00:00:00 2001
From: Andreas Kling <kling@serenityos.org>
Date: Sun, 15 Nov 2020 17:30:32 +0100
Subject: [PATCH] UserspaceEmulator: Add a page-address-to-MMU-region lookup
 map

To make SoftMMU::find_region() O(1), this patch invests 3MiB into a
lookup table where we track each possible page base address and map
them to the SoftMMU::Region corresponding to that address.

This is another large improvement to general emulation performance. :^)
---
 DevTools/UserspaceEmulator/SoftMMU.cpp | 19 ++++++++++++++-----
 DevTools/UserspaceEmulator/SoftMMU.h   |  2 ++
 2 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/DevTools/UserspaceEmulator/SoftMMU.cpp b/DevTools/UserspaceEmulator/SoftMMU.cpp
index 9d330f2305c..58b7f724756 100644
--- a/DevTools/UserspaceEmulator/SoftMMU.cpp
+++ b/DevTools/UserspaceEmulator/SoftMMU.cpp
@@ -37,24 +37,33 @@ SoftMMU::Region* SoftMMU::find_region(X86::LogicalAddress address)
     if (address.selector() == 0x28)
         return m_tls_region.ptr();
 
-    for (auto& region : m_regions) {
-        if (region.contains(address.offset()))
-            return &region;
-    }
-    return nullptr;
+    size_t page_index = (address.offset() & ~(PAGE_SIZE - 1)) / PAGE_SIZE;
+    return m_page_to_region_map[page_index];
 }
 
 void SoftMMU::add_region(NonnullOwnPtr<Region> region)
 {
     ASSERT(!find_region({ 0x20, region->base() }));
+
     // FIXME: More sanity checks pls
     if (region->is_shared_buffer())
         m_shbuf_regions.set(static_cast<SharedBufferRegion*>(region.ptr())->shbuf_id(), region.ptr());
+
+    size_t first_page_in_region = region->base() / PAGE_SIZE;
+    for (size_t i = 0; i < ceil_div(region->size(), PAGE_SIZE); ++i) {
+        m_page_to_region_map[first_page_in_region + i] = region.ptr();
+    }
+
     m_regions.append(move(region));
 }
 
 void SoftMMU::remove_region(Region& region)
 {
+    size_t first_page_in_region = region.base() / PAGE_SIZE;
+    for (size_t i = 0; i < ceil_div(region.size(), PAGE_SIZE); ++i) {
+        m_page_to_region_map[first_page_in_region + i] = nullptr;
+    }
+
     if (region.is_shared_buffer())
         m_shbuf_regions.remove(static_cast<SharedBufferRegion&>(region).shbuf_id());
     m_regions.remove_first_matching([&](auto& entry) { return entry.ptr() == &region; });
diff --git a/DevTools/UserspaceEmulator/SoftMMU.h b/DevTools/UserspaceEmulator/SoftMMU.h
index 52e9b2a2492..f09c7f97767 100644
--- a/DevTools/UserspaceEmulator/SoftMMU.h
+++ b/DevTools/UserspaceEmulator/SoftMMU.h
@@ -125,6 +125,8 @@ public:
     }
 
 private:
+    Region* m_page_to_region_map[786432];
+
     OwnPtr<Region> m_tls_region;
     NonnullOwnPtrVector<Region> m_regions;
     HashMap<int, Region*> m_shbuf_regions;