core: hle: kernel: k_page_bitmap: Refresh.

src/core/hle/kernel/k_page_bitmap.h

@@ -16,107 +16,126 @@
 namespace Kernel {
 
 class KPageBitmap {
-private:
+public:
     class RandomBitGenerator {
-    private:
-        Common::TinyMT rng{};
-        u32 entropy{};
-        u32 bits_available{};
-
-    private:
-        void RefreshEntropy() {
-            entropy = rng.GenerateRandomU32();
-            bits_available = static_cast<u32>(Common::BitSize<decltype(entropy)>());
-        }
-
-        bool GenerateRandomBit() {
-            if (bits_available == 0) {
-                this->RefreshEntropy();
-            }
-
-            const bool rnd_bit = (entropy & 1) != 0;
-            entropy >>= 1;
-            --bits_available;
-            return rnd_bit;
-        }
-
     public:
         RandomBitGenerator() {
-            rng.Initialize(static_cast<u32>(KSystemControl::GenerateRandomU64()));
+            m_rng.Initialize(static_cast<u32>(KSystemControl::GenerateRandomU64()));
         }
 
-        std::size_t SelectRandomBit(u64 bitmap) {
+        u64 SelectRandomBit(u64 bitmap) {
             u64 selected = 0;
 
-            u64 cur_num_bits = Common::BitSize<decltype(bitmap)>() / 2;
-            u64 cur_mask = (1ULL << cur_num_bits) - 1;
+            for (size_t cur_num_bits = Common::BitSize<decltype(bitmap)>() / 2; cur_num_bits != 0;
+                 cur_num_bits /= 2) {
+                const u64 high = (bitmap >> cur_num_bits);
+                const u64 low = (bitmap & (~(UINT64_C(0xFFFFFFFFFFFFFFFF) << cur_num_bits)));
 
-            while (cur_num_bits) {
-                const u64 low = (bitmap >> 0) & cur_mask;
-                const u64 high = (bitmap >> cur_num_bits) & cur_mask;
-
-                bool choose_low;
-                if (high == 0) {
-                    // If only low val is set, choose low.
-                    choose_low = true;
-                } else if (low == 0) {
-                    // If only high val is set, choose high.
-                    choose_low = false;
-                } else {
-                    // If both are set, choose random.
-                    choose_low = this->GenerateRandomBit();
-                }
-
-                // If we chose low, proceed with low.
-                if (choose_low) {
-                    bitmap = low;
-                    selected += 0;
-                } else {
+                // Choose high if we have high and (don't have low or select high randomly).
+                if (high && (low == 0 || this->GenerateRandomBit())) {
                     bitmap = high;
                     selected += cur_num_bits;
+                } else {
+                    bitmap = low;
+                    selected += 0;
                 }
-
-                // Proceed.
-                cur_num_bits /= 2;
-                cur_mask >>= cur_num_bits;
             }
 
             return selected;
         }
+
+        u64 GenerateRandom(u64 max) {
+            // Determine the number of bits we need.
+            const u64 bits_needed = 1 + (Common::BitSize<decltype(max)>() - std::countl_zero(max));
+
+            // Generate a random value of the desired bitwidth.
+            const u64 rnd = this->GenerateRandomBits(static_cast<u32>(bits_needed));
+
+            // Adjust the value to be in range.
+            return rnd - ((rnd / max) * max);
+        }
+
+    private:
+        void RefreshEntropy() {
+            m_entropy = m_rng.GenerateRandomU32();
+            m_bits_available = static_cast<u32>(Common::BitSize<decltype(m_entropy)>());
+        }
+
+        bool GenerateRandomBit() {
+            if (m_bits_available == 0) {
+                this->RefreshEntropy();
+            }
+
+            const bool rnd_bit = (m_entropy & 1) != 0;
+            m_entropy >>= 1;
+            --m_bits_available;
+            return rnd_bit;
+        }
+
+        u64 GenerateRandomBits(u32 num_bits) {
+            u64 result = 0;
+
+            // Iteratively add random bits to our result.
+            while (num_bits > 0) {
+                // Ensure we have random bits to take from.
+                if (m_bits_available == 0) {
+                    this->RefreshEntropy();
+                }
+
+                // Determine how many bits to take this round.
+                const auto cur_bits = std::min(num_bits, m_bits_available);
+
+                // Generate mask for our current bits.
+                const u64 mask = (static_cast<u64>(1) << cur_bits) - 1;
+
+                // Add bits to output from our entropy.
+                result <<= cur_bits;
+                result |= (m_entropy & mask);
+
+                // Remove bits from our entropy.
+                m_entropy >>= cur_bits;
+                m_bits_available -= cur_bits;
+
+                // Advance.
+                num_bits -= cur_bits;
+            }
+
+            return result;
+        }
+
+    private:
+        Common::TinyMT m_rng;
+        u32 m_entropy{};
+        u32 m_bits_available{};
     };
 
 public:
-    static constexpr std::size_t MaxDepth = 4;
-
-private:
-    std::array<u64*, MaxDepth> bit_storages{};
-    RandomBitGenerator rng{};
-    std::size_t num_bits{};
-    std::size_t used_depths{};
+    static constexpr size_t MaxDepth = 4;
 
 public:
     KPageBitmap() = default;
 
-    constexpr std::size_t GetNumBits() const {
-        return num_bits;
+    constexpr size_t GetNumBits() const {
+        return m_num_bits;
     }
     constexpr s32 GetHighestDepthIndex() const {
-        return static_cast<s32>(used_depths) - 1;
+        return static_cast<s32>(m_used_depths) - 1;
     }
 
-    u64* Initialize(u64* storage, std::size_t size) {
+    u64* Initialize(u64* storage, size_t size) {
         // Initially, everything is un-set.
-        num_bits = 0;
+        m_num_bits = 0;
 
         // Calculate the needed bitmap depth.
-        used_depths = static_cast<std::size_t>(GetRequiredDepth(size));
-        ASSERT(used_depths <= MaxDepth);
+        m_used_depths = static_cast<size_t>(GetRequiredDepth(size));
+        ASSERT(m_used_depths <= MaxDepth);
 
         // Set the bitmap pointers.
         for (s32 depth = this->GetHighestDepthIndex(); depth >= 0; depth--) {
-            bit_storages[depth] = storage;
+            m_bit_storages[depth] = storage;
             size = Common::AlignUp(size, Common::BitSize<u64>()) / Common::BitSize<u64>();
             storage += size;
+            m_end_storages[depth] = storage;
         }
 
         return storage;
@@ -128,19 +147,19 @@ public:
 
         if (random) {
             do {
-                const u64 v = bit_storages[depth][offset];
+                const u64 v = m_bit_storages[depth][offset];
                 if (v == 0) {
                     // If depth is bigger than zero, then a previous level indicated a block was
                     // free.
                     ASSERT(depth == 0);
                     return -1;
                 }
-                offset = offset * Common::BitSize<u64>() + rng.SelectRandomBit(v);
+                offset = offset * Common::BitSize<u64>() + m_rng.SelectRandomBit(v);
                 ++depth;
-            } while (depth < static_cast<s32>(used_depths));
+            } while (depth < static_cast<s32>(m_used_depths));
         } else {
             do {
-                const u64 v = bit_storages[depth][offset];
+                const u64 v = m_bit_storages[depth][offset];
                 if (v == 0) {
                     // If depth is bigger than zero, then a previous level indicated a block was
                     // free.
@@ -149,28 +168,69 @@ public:
                 }
                 offset = offset * Common::BitSize<u64>() + std::countr_zero(v);
                 ++depth;
-            } while (depth < static_cast<s32>(used_depths));
+            } while (depth < static_cast<s32>(m_used_depths));
         }
 
         return static_cast<s64>(offset);
     }
 
-    void SetBit(std::size_t offset) {
+    s64 FindFreeRange(size_t count) {
+        // Check that it is possible to find a range.
+        const u64* const storage_start = m_bit_storages[m_used_depths - 1];
+        const u64* const storage_end = m_end_storages[m_used_depths - 1];
+
+        // If we don't have a storage to iterate (or want more blocks than fit in a single storage),
+        // we can't find a free range.
+        if (!(storage_start < storage_end && count <= Common::BitSize<u64>())) {
+            return -1;
+        }
+
+        // Walk the storages to select a random free range.
+        const size_t options_per_storage = std::max<size_t>(Common::BitSize<u64>() / count, 1);
+        const size_t num_entries = std::max<size_t>(storage_end - storage_start, 1);
+
+        const u64 free_mask = (static_cast<u64>(1) << count) - 1;
+
+        size_t num_valid_options = 0;
+        s64 chosen_offset = -1;
+        for (size_t storage_index = 0; storage_index < num_entries; ++storage_index) {
+            u64 storage = storage_start[storage_index];
+            for (size_t option = 0; option < options_per_storage; ++option) {
+                if ((storage & free_mask) == free_mask) {
+                    // We've found a new valid option.
+                    ++num_valid_options;
+
+                    // Select the Kth valid option with probability 1/K. This leads to an overall
+                    // uniform distribution.
+                    if (num_valid_options == 1 || m_rng.GenerateRandom(num_valid_options) == 0) {
+                        // This is our first option, so select it.
+                        chosen_offset = storage_index * Common::BitSize<u64>() + option * count;
+                    }
+                }
+                storage >>= count;
+            }
+        }
+
+        // Return the random offset we chose.*/
+        return chosen_offset;
+    }
+
+    void SetBit(size_t offset) {
         this->SetBit(this->GetHighestDepthIndex(), offset);
-        num_bits++;
+        m_num_bits++;
     }
 
-    void ClearBit(std::size_t offset) {
+    void ClearBit(size_t offset) {
         this->ClearBit(this->GetHighestDepthIndex(), offset);
-        num_bits--;
+        m_num_bits--;
     }
 
-    bool ClearRange(std::size_t offset, std::size_t count) {
+    bool ClearRange(size_t offset, size_t count) {
         s32 depth = this->GetHighestDepthIndex();
-        u64* bits = bit_storages[depth];
-        std::size_t bit_ind = offset / Common::BitSize<u64>();
-        if (count < Common::BitSize<u64>()) {
-            const std::size_t shift = offset % Common::BitSize<u64>();
+        u64* bits = m_bit_storages[depth];
+        size_t bit_ind = offset / Common::BitSize<u64>();
+        if (count < Common::BitSize<u64>()) [[likely]] {
+            const size_t shift = offset % Common::BitSize<u64>();
             ASSERT(shift + count <= Common::BitSize<u64>());
             // Check that all the bits are set.
             const u64 mask = ((u64(1) << count) - 1) << shift;
@@ -189,8 +249,8 @@ public:
             ASSERT(offset % Common::BitSize<u64>() == 0);
             ASSERT(count % Common::BitSize<u64>() == 0);
             // Check that all the bits are set.
-            std::size_t remaining = count;
-            std::size_t i = 0;
+            size_t remaining = count;
+            size_t i = 0;
             do {
                 if (bits[bit_ind + i++] != ~u64(0)) {
                     return false;
@@ -209,18 +269,18 @@ public:
             } while (remaining > 0);
         }
 
-        num_bits -= count;
+        m_num_bits -= count;
         return true;
     }
 
 private:
-    void SetBit(s32 depth, std::size_t offset) {
+    void SetBit(s32 depth, size_t offset) {
         while (depth >= 0) {
-            std::size_t ind = offset / Common::BitSize<u64>();
-            std::size_t which = offset % Common::BitSize<u64>();
+            size_t ind = offset / Common::BitSize<u64>();
+            size_t which = offset % Common::BitSize<u64>();
             const u64 mask = u64(1) << which;
 
-            u64* bit = std::addressof(bit_storages[depth][ind]);
+            u64* bit = std::addressof(m_bit_storages[depth][ind]);
             u64 v = *bit;
             ASSERT((v & mask) == 0);
             *bit = v | mask;
@@ -232,13 +292,13 @@ private:
         }
     }
 
-    void ClearBit(s32 depth, std::size_t offset) {
+    void ClearBit(s32 depth, size_t offset) {
         while (depth >= 0) {
-            std::size_t ind = offset / Common::BitSize<u64>();
-            std::size_t which = offset % Common::BitSize<u64>();
+            size_t ind = offset / Common::BitSize<u64>();
+            size_t which = offset % Common::BitSize<u64>();
             const u64 mask = u64(1) << which;
 
-            u64* bit = std::addressof(bit_storages[depth][ind]);
+            u64* bit = std::addressof(m_bit_storages[depth][ind]);
             u64 v = *bit;
             ASSERT((v & mask) != 0);
             v &= ~mask;
@@ -252,7 +312,7 @@ private:
     }
 
 private:
-    static constexpr s32 GetRequiredDepth(std::size_t region_size) {
+    static constexpr s32 GetRequiredDepth(size_t region_size) {
         s32 depth = 0;
         while (true) {
             region_size /= Common::BitSize<u64>();
@@ -264,8 +324,8 @@ private:
     }
 
 public:
-    static constexpr std::size_t CalculateManagementOverheadSize(std::size_t region_size) {
-        std::size_t overhead_bits = 0;
+    static constexpr size_t CalculateManagementOverheadSize(size_t region_size) {
+        size_t overhead_bits = 0;
         for (s32 depth = GetRequiredDepth(region_size) - 1; depth >= 0; depth--) {
             region_size =
                 Common::AlignUp(region_size, Common::BitSize<u64>()) / Common::BitSize<u64>();
@@ -273,6 +333,13 @@ public:
         }
         return overhead_bits * sizeof(u64);
     }
+
+private:
+    std::array<u64*, MaxDepth> m_bit_storages{};
+    std::array<u64*, MaxDepth> m_end_storages{};
+    RandomBitGenerator m_rng;
+    size_t m_num_bits{};
+    size_t m_used_depths{};
 };
 
 } // namespace Kernel