From c4609c92eea30558473f02082733c7e59c2d2013 Mon Sep 17 00:00:00 2001
From: Markus Wick <markus@selfnet.de>
Date: Sat, 5 Jun 2021 11:47:08 +0200
Subject: [PATCH] common/host_memory: Optimize for huge tables.

In theory, if we have 2 MB continously mapped, this should save one layer of TLB.
Let's make it at least more likely by aligning the memory.
---
 src/common/host_memory.cpp | 29 +++++++++++++++++++----------
 src/common/host_memory.h   |  6 +++++-
 2 files changed, 24 insertions(+), 11 deletions(-)

diff --git a/src/common/host_memory.cpp b/src/common/host_memory.cpp
index eb50fbd9f8..8a328f916d 100644
--- a/src/common/host_memory.cpp
+++ b/src/common/host_memory.cpp
@@ -27,6 +27,7 @@
 
 #include <mutex>
 
+#include "common/alignment.h"
 #include "common/assert.h"
 #include "common/host_memory.h"
 #include "common/logging/log.h"
@@ -35,6 +36,7 @@
 namespace Common {
 
 constexpr size_t PageAlignment = 0x1000;
+constexpr size_t HugePageSize = 0x200000;
 
 #ifdef _WIN32
 
@@ -385,9 +387,16 @@ private:
 
 #endif
 
-HostMemory::HostMemory(size_t backing_size, size_t virtual_size)
-    : impl{std::make_unique<HostMemory::Impl>(backing_size, virtual_size)},
-      backing_base{impl->backing_base}, virtual_base{impl->virtual_base} {}
+HostMemory::HostMemory(size_t backing_size_, size_t virtual_size_)
+    : backing_size(backing_size_),
+      virtual_size(virtual_size_), impl{std::make_unique<HostMemory::Impl>(
+                                       AlignUp(backing_size, PageAlignment),
+                                       AlignUp(virtual_size, PageAlignment) + 3 * HugePageSize)},
+      backing_base{impl->backing_base}, virtual_base{impl->virtual_base} {
+    virtual_base += 2 * HugePageSize - 1;
+    virtual_base -= reinterpret_cast<size_t>(virtual_base) & (HugePageSize - 1);
+    virtual_base_offset = virtual_base - impl->virtual_base;
+}
 
 HostMemory::~HostMemory() = default;
 
@@ -399,32 +408,32 @@ void HostMemory::Map(size_t virtual_offset, size_t host_offset, size_t length) {
     ASSERT(virtual_offset % PageAlignment == 0);
     ASSERT(host_offset % PageAlignment == 0);
     ASSERT(length % PageAlignment == 0);
-    ASSERT(virtual_offset + length <= impl->virtual_size);
-    ASSERT(host_offset + length <= impl->backing_size);
+    ASSERT(virtual_offset + length <= virtual_size);
+    ASSERT(host_offset + length <= backing_size);
     if (length == 0) {
         return;
     }
-    impl->Map(virtual_offset, host_offset, length);
+    impl->Map(virtual_offset + virtual_base_offset, host_offset, length);
 }
 
 void HostMemory::Unmap(size_t virtual_offset, size_t length) {
     ASSERT(virtual_offset % PageAlignment == 0);
     ASSERT(length % PageAlignment == 0);
-    ASSERT(virtual_offset + length <= impl->virtual_size);
+    ASSERT(virtual_offset + length <= virtual_size);
     if (length == 0) {
         return;
     }
-    impl->Unmap(virtual_offset, length);
+    impl->Unmap(virtual_offset + virtual_base_offset, length);
 }
 
 void HostMemory::Protect(size_t virtual_offset, size_t length, bool read, bool write) {
     ASSERT(virtual_offset % PageAlignment == 0);
     ASSERT(length % PageAlignment == 0);
-    ASSERT(virtual_offset + length <= impl->virtual_size);
+    ASSERT(virtual_offset + length <= virtual_size);
     if (length == 0) {
         return;
     }
-    impl->Protect(virtual_offset, length, read, write);
+    impl->Protect(virtual_offset + virtual_base_offset, length, read, write);
 }
 
 } // namespace Common
diff --git a/src/common/host_memory.h b/src/common/host_memory.h
index 98005df7a2..eaa7d18ab5 100644
--- a/src/common/host_memory.h
+++ b/src/common/host_memory.h
@@ -15,7 +15,7 @@ namespace Common {
  */
 class HostMemory {
 public:
-    explicit HostMemory(size_t backing_size, size_t virtual_size);
+    explicit HostMemory(size_t backing_size_, size_t virtual_size_);
     ~HostMemory();
 
     /**
@@ -52,11 +52,15 @@ public:
     }
 
 private:
+    size_t backing_size{};
+    size_t virtual_size{};
+
     // Low level handler for the platform dependent memory routines
     class Impl;
     std::unique_ptr<Impl> impl;
     u8* backing_base{};
     u8* virtual_base{};
+    size_t virtual_base_offset{};
 };
 
 } // namespace Common