From 8d50dd703e65846449f17a11d610c5da171a0e0e Mon Sep 17 00:00:00 2001
From: Michael Mueller <michael.mueller@uos.de>
Date: Fri, 7 Mar 2025 17:50:00 +0100
Subject: [PATCH] ealanos: Ported NOVA's bit allocator to Genode's user-space
 and added method to count number of available bits. Also added documentation.

---
 .../ealanos/include/ealanos/util/bit_alloc.h  | 166 ++++++++++++++++++
 1 file changed, 166 insertions(+)
 create mode 100644 repos/ealanos/include/ealanos/util/bit_alloc.h

diff --git a/repos/ealanos/include/ealanos/util/bit_alloc.h b/repos/ealanos/include/ealanos/util/bit_alloc.h
new file mode 100644
index 0000000000..746d662673
--- /dev/null
+++ b/repos/ealanos/include/ealanos/util/bit_alloc.h
@@ -0,0 +1,166 @@
+/*
+ * Bit allocator
+ *
+ * Copyright (C) 2020 Alexander Boettcher, Genode Labs GmbH
+ * Copyright (C) 2025 Michael Müller, Osnabrück University
+ *
+ * This file is part of the NOVA microhypervisor.
+ *
+ * NOVA is free software: you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * NOVA is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License version 2 for more details.
+ */
+
+
+#pragma once
+
+#include <tukija/bits.h>
+#include <tukija/atomic.h>
+#include <tukija/stdint.h>
+
+#define ACCESS_ONCE(x) (*static_cast<volatile typeof(x) *>(&(x)))
+
+template<unsigned INV, unsigned C>
+class Bit_alloc
+{
+    private:
+        Tukija::mword_t bits[C / 8 / sizeof(Tukija::mword_t)];
+        Tukija::mword_t last { 0 };
+
+        enum
+        {
+            BITS_CNT = sizeof(bits[0]) * 8,
+            MAX = C / 8 / sizeof(Tukija::mword_t),
+        };
+
+    public:
+        /**
+         * @brief Return the number of "free", i.e. unset, bits
+         * 
+         * @return Tukija::mword_t 
+         */
+        inline Tukija::mword_t left() const
+        {
+            Tukija::mword_t count = 0;
+            for (Tukija::mword_t i = 0; i < MAX; i++) {
+                count += popcount(~bits[i]);
+            }
+            return count;
+        }
+
+        /**
+         * @brief Return the maximum number of bits available.
+         * 
+         * @return Tukija::mword_t 
+         */
+        inline Tukija::mword_t max() const { return C; }
+
+        inline Bit_alloc(Tukija::mword_t count)
+        {
+            static_assert(MAX * BITS_CNT == C, "bit allocator");
+            static_assert(INV < C, "bit allocator");
+            Atomic::test_set_bit(bits[INV / BITS_CNT], INV % BITS_CNT);
+            reserve(count, C - count);
+        }
+
+        /**
+         * @brief Allocate a bit
+         * 
+         * @return Tukija::mword_t - the index of the allocated bit
+         */
+        inline Tukija::mword_t alloc()
+        {
+            for (Tukija::mword_t i = ACCESS_ONCE(last), j = 0; j < MAX; i++, j++)
+            {
+                i %= MAX;
+
+                if (ACCESS_ONCE(bits[i]) == ~0UL)
+                    continue;
+
+                long b = bit_scan_forward (~bits[i]);
+                if (b < 0 || b >= BITS_CNT || Atomic::test_set_bit (bits[i], b)) {
+                    j--;
+                    i--;
+                    continue;
+                }
+
+                if (bits[i] != ~0UL && last != i)
+                    last = i;
+
+                return i * BITS_CNT + b;
+            }
+
+            return INV;
+        }
+
+        /**
+         * @brief Release, i.e. clear, a bit
+         * 
+         * @param id - Bit to release to the allocator
+         */
+        inline void release(Tukija::mword_t const id)
+        {
+            if (id == INV || id >= C)
+                return;
+
+            Tukija::mword_t i = id / BITS_CNT;
+            Tukija::mword_t b = id % BITS_CNT;
+
+            while (ACCESS_ONCE(bits[i]) & (1ul << b))
+                 Atomic::test_clr_bit (ACCESS_ONCE(bits[i]), b);
+        }
+
+        /**
+         * @brief Reserve a range of bits, thus, making them unavailable
+         * 
+         * @details Reserve a range of bits. This method can be used to
+         *          limit the number of bits than can be allocated independent
+         *          of the template parameter C. This is especially useful
+         *          for cases, where more a consecutive range of bits is needed,
+         *          or the number of bits allocatable shall be reduced to less
+         *          than C. 
+         * 
+         * @param start - First bit from which the reservation shall start
+         * @param count - Number of consecutive bits to reserve
+         */
+        void reserve(Tukija::mword_t const start, Tukija::mword_t const count)
+        {
+            if (start >= C)
+                return;
+
+            Tukija::mword_t i = start / BITS_CNT;
+            Tukija::mword_t b = start % BITS_CNT;
+
+            Tukija::mword_t cnt = count > C ? C : count;
+            if (start + cnt > C)
+                cnt = C - start;
+
+            while (cnt) {
+                Tukija::mword_t const c  = (cnt > BITS_CNT) ? Tukija::mword_t(BITS_CNT) : cnt;
+                Tukija::mword_t const bc = (c > (BITS_CNT - b)) ? Tukija::mword_t(BITS_CNT - b) : c;
+                if (bits[i] != ~0UL) {
+                    if (bc >= BITS_CNT) {
+                        bits[i] = ~0UL;
+                    } else {
+                        bits[i] |= ((1ul << bc) - 1) << b;
+                    }
+                }
+                i++;
+                cnt -= bc;
+                b = 0;
+            }
+        }
+
+        /**
+         * @brief Create a new Bit allocator at address p
+         * 
+         * @param p 
+         * @return void* 
+         */
+        void *operator new(Genode::size_t, void *p) { return p; }
+};