callbacks: Add documentation

include/dynarmic/callbacks.h

@@ -16,25 +16,36 @@ class Jit;
 
 /// These function pointers may be inserted into compiled code.
 struct UserCallbacks {
+    // Reads through these callbacks may not be aligned.
+    // Memory must be interpreted as if ENDIANSTATE == 0, endianness will be corrected by the JIT.
     std::uint8_t (*MemoryRead8)(std::uint32_t vaddr);
     std::uint16_t (*MemoryRead16)(std::uint32_t vaddr);
     std::uint32_t (*MemoryRead32)(std::uint32_t vaddr);
     std::uint64_t (*MemoryRead64)(std::uint32_t vaddr);
 
+    // Writes through these callbacks may not be aligned.
+    // Memory must be interpreted as if ENDIANSTATE == 0, endianness will be corrected by the JIT.
     void (*MemoryWrite8)(std::uint32_t vaddr, std::uint8_t value);
     void (*MemoryWrite16)(std::uint32_t vaddr, std::uint16_t value);
     void (*MemoryWrite32)(std::uint32_t vaddr, std::uint32_t value);
     void (*MemoryWrite64)(std::uint32_t vaddr, std::uint64_t value);
 
+    // If this callback returns true, the JIT will assume MemoryRead* callbacks will always
+    // return the same value at any point in time for this vaddr. The JIT may use this information
+    // in optimizations.
+    // An conservative implementation that always returns false is safe.
     bool (*IsReadOnlyMemory)(std::uint32_t vaddr);
 
     /// The intrepreter must execute only one instruction at PC.
     void (*InterpreterFallback)(std::uint32_t pc, Jit* jit, void* user_arg);
     void* user_arg = nullptr;
 
+    // This callback is called whenever a SVC instruction is executed.
     void (*CallSVC)(std::uint32_t swi);
 
     // Page Table
+    // The page table is used for faster memory access. If an entry in the table is nullptr,
+    // the JIT will fallback to calling the MemoryRead*/MemoryWrite* callbacks.
     static constexpr std::size_t PAGE_BITS = 12;
     static constexpr std::size_t NUM_PAGE_TABLE_ENTRIES = 1 << (32 - PAGE_BITS);
     std::array<std::uint8_t*, NUM_PAGE_TABLE_ENTRIES>* page_table = nullptr;