[kernel] Add add_existing() to page_tree

This ended up being unused, but still probably useful: Refactor out the
"find" logic of page_tree::find_or_add (note that this is different than
the "find" logic of page_tree::find, as it potentially modifies the tree
to add a location to accommodate the page being searched for) into a new
page_tree::get_entry method. That was then used to add an add_existing
method for inserting pages into the page_tree.

src/kernel/page_tree.cpp

@@ -58,30 +58,8 @@ page_tree::contains(uint64_t offset, uint8_t &index) const
     return (offset & level_mask(m_level)) == m_base;
 }
 
-bool
+uintptr_t &
-page_tree::find(const page_tree *root, uint64_t offset, uintptr_t &page)
+page_tree::get_entry(page_tree * &root, uint64_t offset)
-{
-    page_tree const *node = root;
-    while (node) {
-        uint8_t level = node->m_level;
-        uint8_t index = 0;
-        if (!node->contains(offset, index))
-            return false;
-
-        if (!level) {
-            uintptr_t entry = node->m_entries[index].entry;
-            page = entry & ~0xfffull;
-            return (entry & 1); // bit 0 marks 'present'
-        }
-
-        node = node->m_entries[index].child;
-    }
-
-    return false;
-}
-
-bool
-page_tree::find_or_add(page_tree * &root, uint64_t offset, uintptr_t &page)
 {
     page_tree *level0 = nullptr;
 
@@ -125,7 +103,7 @@ page_tree::find_or_add(page_tree * &root, uint64_t offset, uintptr_t &page)
             node = *parent;
         }
 
-        kassert( node || parent, "Both node and parent were null in find_or_add");
+        kassert( node || parent, "Both node and parent were null in page_tree::get_entry");
 
         if (!node) {
             // We found a parent with an empty spot where this node should
@@ -135,9 +113,39 @@ page_tree::find_or_add(page_tree * &root, uint64_t offset, uintptr_t &page)
         }
     }
 
-    kassert(level0, "Got through find_or_add without a level0");
+    kassert(level0, "Got through page_tree::get_entry without a level0");
+
     uint8_t index = index_for(offset, 0);
-    uint64_t &ent = level0->m_entries[index].entry;
+    return level0->m_entries[index].entry;
+}
+
+bool
+page_tree::find(const page_tree *root, uint64_t offset, uintptr_t &page)
+{
+    page_tree const *node = root;
+    while (node) {
+        uint8_t level = node->m_level;
+        uint8_t index = 0;
+        if (!node->contains(offset, index))
+            return false;
+
+        if (!level) {
+            uintptr_t entry = node->m_entries[index].entry;
+            page = entry & ~0xfffull;
+            return (entry & 1); // bit 0 marks 'present'
+        }
+
+        node = node->m_entries[index].child;
+    }
+
+    return false;
+}
+
+bool
+page_tree::find_or_add(page_tree * &root, uint64_t offset, uintptr_t &page)
+{
+    uint64_t &ent = get_entry(root, offset);
+
     if (!(ent & 1)) {
         // No entry for this page exists, so make one
         if (!frame_allocator::get().allocate(1, &ent))
@@ -148,3 +156,12 @@ page_tree::find_or_add(page_tree * &root, uint64_t offset, uintptr_t &page)
     page = ent & ~0xfffull;
     return true;
 }
+
+void
+page_tree::add_existing(page_tree * &root, uint64_t offset, uintptr_t page)
+{
+    uint64_t &ent = get_entry(root, offset);
+    kassert(!(ent & 1), "Replacing existing mapping in page_tree::add_existing");
+
+    ent = page | 1;
+}

src/kernel/page_tree.h

@@ -23,6 +23,12 @@ public:
     /// \returns     True if a page was found
     static bool find_or_add(page_tree * &root, uint64_t offset, uintptr_t &page);
 
+    /// Add an existing mapping not allocated via find_or_add.
+    /// \arg root    [inout] The root node of the tree. This pointer may be updated.
+    /// \arg offset  Offset into the VMA, in bytes
+    /// \arg page    The mapped page physical address
+    static void add_existing(page_tree * &root, uint64_t offset, uintptr_t page);
+
     ~page_tree();
 
 private:
@@ -34,6 +40,9 @@ private:
     /// \returns     True if the address is contained
     bool contains(uintptr_t offset, uint8_t &index) const;
 
+    /// Get a (writable) reference to a page in the tree
+    static uintptr_t & get_entry(page_tree * &root, uint64_t offset);
+
     /// Stores the page offset of the start of this node's pages virtual addresses
     uint64_t m_base;