From b3587102d160fb74a12935a79f06ee8a12712f12 Mon Sep 17 00:00:00 2001
From: ReinUsesLisp <reinuseslisp@airmail.cc>
Date: Tue, 29 Dec 2020 21:16:57 -0300
Subject: core/memory: Read and write page table atomically

Squash attributes into the pointer's integer, making them an uintptr_t
pair containing 2 bits at the bottom and then the pointer. These bits
are currently unused thanks to alignment requirements.

Configure Dynarmic to mask out these bits on pointer reads.

While we are at it, remove some unused attributes carried over from
Citra.

Read/Write and other hot functions use a two step unpacking process that
is less readable to stop MSVC from emitting an extra AND instruction in
the hot path:

 mov         rdi,rcx
 shr         rdx,0Ch
 mov         r8,qword ptr [rax+8]
 mov         rax,qword ptr [r8+rdx*8]
 mov         rdx,rax
-and         al,3
 and         rdx,0FFFFFFFFFFFFFFFCh
 je          Core::Memory::Memory::Impl::Read<unsigned char>
 mov         rax,qword ptr [vaddr]
 movzx       eax,byte ptr [rdx+rax]
---
 src/common/page_table.h | 68 +++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 57 insertions(+), 11 deletions(-)

(limited to 'src/common/page_table.h')

diff --git a/src/common/page_table.h b/src/common/page_table.h
index 9754fabf9..8d4ee9249 100644
--- a/src/common/page_table.h
+++ b/src/common/page_table.h
@@ -4,6 +4,7 @@
 
 #pragma once
 
+#include <atomic>
 #include <tuple>
 
 #include "common/common_types.h"
@@ -20,10 +21,6 @@ enum class PageType : u8 {
     /// Page is mapped to regular memory, but also needs to check for rasterizer cache flushing and
     /// invalidation
     RasterizerCachedMemory,
-    /// Page is mapped to a I/O region. Writing and reading to this page is handled by functions.
-    Special,
-    /// Page is allocated for use.
-    Allocated,
 };
 
 struct SpecialRegion {
@@ -48,6 +45,59 @@ struct SpecialRegion {
  * mimics the way a real CPU page table works.
  */
 struct PageTable {
+    /// Number of bits reserved for attribute tagging.
+    /// This can be at most the guaranteed alignment of the pointers in the page table.
+    static constexpr int ATTRIBUTE_BITS = 2;
+
+    /**
+     * Pair of host pointer and page type attribute.
+     * This uses the lower bits of a given pointer to store the attribute tag.
+     * Writing and reading the pointer attribute pair is guaranteed to be atomic for the same method
+     * call. In other words, they are guaranteed to be synchronized at all times.
+     */
+    class PageInfo {
+    public:
+        /// Returns the page pointer
+        [[nodiscard]] u8* Pointer() const noexcept {
+            return ExtractPointer(raw.load(std::memory_order_relaxed));
+        }
+
+        /// Returns the page type attribute
+        [[nodiscard]] PageType Type() const noexcept {
+            return ExtractType(raw.load(std::memory_order_relaxed));
+        }
+
+        /// Returns the page pointer and attribute pair, extracted from the same atomic read
+        [[nodiscard]] std::pair<u8*, PageType> PointerType() const noexcept {
+            const uintptr_t non_atomic_raw = raw.load(std::memory_order_relaxed);
+            return {ExtractPointer(non_atomic_raw), ExtractType(non_atomic_raw)};
+        }
+
+        /// Returns the raw representation of the page information.
+        /// Use ExtractPointer and ExtractType to unpack the value.
+        [[nodiscard]] uintptr_t Raw() const noexcept {
+            return raw.load(std::memory_order_relaxed);
+        }
+
+        /// Write a page pointer and type pair atomically
+        void Store(u8* pointer, PageType type) noexcept {
+            raw.store(reinterpret_cast<uintptr_t>(pointer) | static_cast<uintptr_t>(type));
+        }
+
+        /// Unpack a pointer from a page info raw representation
+        [[nodiscard]] static u8* ExtractPointer(uintptr_t raw) noexcept {
+            return reinterpret_cast<u8*>(raw & (~uintptr_t{0} << ATTRIBUTE_BITS));
+        }
+
+        /// Unpack a page type from a page info raw representation
+        [[nodiscard]] static PageType ExtractType(uintptr_t raw) noexcept {
+            return static_cast<PageType>(raw & ((uintptr_t{1} << ATTRIBUTE_BITS) - 1));
+        }
+
+    private:
+        std::atomic<uintptr_t> raw;
+    };
+
     PageTable();
     ~PageTable() noexcept;
 
@@ -63,20 +113,16 @@ struct PageTable {
      *
      * @param address_space_width_in_bits The address size width in bits.
      * @param page_size_in_bits           The page size in bits.
-     * @param has_attribute               Whether or not this page has any backing attributes.
      */
-    void Resize(std::size_t address_space_width_in_bits, std::size_t page_size_in_bits,
-                bool has_attribute);
+    void Resize(size_t address_space_width_in_bits, size_t page_size_in_bits);
 
     /**
      * Vector of memory pointers backing each page. An entry can only be non-null if the
-     * corresponding entry in the `attributes` vector is of type `Memory`.
+     * corresponding attribute element is of type `Memory`.
      */
-    VirtualBuffer<u8*> pointers;
+    VirtualBuffer<PageInfo> pointers;
 
     VirtualBuffer<u64> backing_addr;
-
-    VirtualBuffer<PageType> attributes;
 };
 
 } // namespace Common
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