Allow allocating arbitrarily aligned physical memory regions

This will be needed for USB (XHCI controller driver).

Rsconscript

@@ -198,6 +198,7 @@ task "run", desc: "Run HOS in QEMU" do
   sh %W[
     qemu-system-x86_64
     -machine q35
+    -m 128M
     -cpu max
     -smp cpus=4
     -serial file:qemu/serial.out

src/hulk/hippo.d

@@ -89,6 +89,12 @@ struct Hippo
      */
     public static void free_region(T)(T start, ulong size)
     {
+        if (size == PAGE_SIZE)
+        {
+            free_page(start);
+            return;
+        }
+
         if (regions is null)
         {
             /* The free regions list is empty. Append this new region. */
@@ -124,6 +130,72 @@ struct Hippo
         m_n_free_pages += size >> 12;
     }
 
+    /**
+     * Allocate a region with the given size and alignment.
+     *
+     * @param size Region size.
+     * @param alignment Region alignment.
+     */
+    public static void * allocate_aligned_region(ulong size, ulong alignment)
+    {
+        Region ** regionp = &regions;
+        for (;;)
+        {
+            Region * region = *regionp;
+            if (region is null)
+            {
+                break;
+            }
+
+            /* The size of the region must be large enough */
+            if (region.size >= size)
+            {
+                /* The alignment contstraint must be satisfied. */
+                if ((cast(ulong)region & (alignment - 1)) == 0)
+                {
+                    /* The region start address happens to already be aligned. */
+                    if (region.size > size)
+                    {
+                        /* Region found is larger that needed. */
+                        Region * new_region = cast(Region *)(cast(ulong)region + size);
+                        new_region.size = region.size - size;
+                        new_region.next = region.next;
+                        *regionp = new_region;
+                        return region;
+                    }
+                    else
+                    {
+                        /* Region happens to be the size requested. */
+                        *regionp = region.next;
+                        return region;
+                    }
+                }
+                else
+                {
+                    /* This region is large enough but not aligned. See if an
+                     * aligned subregion can be found at the end. */
+                    ulong start = (cast(ulong)region + region.size - size) & ~(alignment - 1);
+                    if (start > cast(ulong)region)
+                    {
+                        /* The aligned subregion does fit in this region. */
+                        region.size = (start - cast(ulong)region);
+                        ulong region_end = cast(ulong)region + region.size;
+                        if (region_end > (start + size))
+                        {
+                            /* There is another region following the aligned
+                             * subregion that we must reclaim. */
+                            free_region(start + size, region_end - (start + size));
+                        }
+                        return cast(void *)start;
+                    }
+                }
+            }
+
+            regionp = &region.next;
+        }
+        return null;
+    }
+
     /**
      * Get the number of free pages.
      *