x86/biosmem: improve clarity

2 files changed, 29 insertions, 1 deletions

diff --git a/arch/x86/machine/biosmem.c b/arch/x86/machine/biosmem.c
index 245181fc..f385067b 100644
--- a/arch/x86/machine/biosmem.c
+++ b/arch/x86/machine/biosmem.c
@@ -516,6 +516,8 @@ biosmem_map_show(void)
         printk("biosmem: %018llx:%018llx, %s\n", entry->base_addr,
                entry->base_addr + entry->length,
                biosmem_type_desc(entry->type));
+
+    printk("biosmem: heap: %x-%x\n", biosmem_heap_start, biosmem_heap_end);
 }
 
 static int __init
@@ -649,12 +651,27 @@ biosmem_find_reserved_area_update(phys_addr_t min, phys_addr_t *start,
     }
 }
 
+/*
+ * Return the boundaries of a reserved area.
+ *
+ * The area returned is the lowest in memory, between min (included) and
+ * max (excluded).
+ */
 static phys_addr_t __init
 biosmem_find_reserved_area(phys_addr_t min, phys_addr_t max,
                            phys_addr_t *endp)
 {
     phys_addr_t start, end = end;
 
+    /*
+     * Obviously, the kernel is reserved. Since all allocations until now
+     * were made from the heap, and since the unused pages of the heap
+     * have already been loaded as available in the VM system, consider
+     * the whole heap reserved as well.
+     *
+     * Everything else is freed to the VM system.
+     */
+
     start = max;
     biosmem_find_reserved_area_update(min, &start, &end, (unsigned long)&_boot,
                                       BOOT_VTOP((unsigned long)&_end));
@@ -673,6 +690,13 @@ biosmem_free_usable_range(phys_addr_t start, phys_addr_t end)
 {
     struct vm_page *page;
 
+    if (start == end)
+        return;
+
+    printk("biosmem: release to vm_page: %llx-%llx (%lluk)\n",
+           (unsigned long long)start, (unsigned long long)end,
+           (unsigned long long)((end - start) >> 10));
+
     while (start < end) {
         page = vm_page_lookup(start);
         assert(page != NULL);
diff --git a/arch/x86/machine/biosmem.h b/arch/x86/machine/biosmem.h
index c712c266..fc65c761 100644
--- a/arch/x86/machine/biosmem.h
+++ b/arch/x86/machine/biosmem.h
@@ -51,7 +51,11 @@ void biosmem_bootstrap(struct multiboot_raw_info *mbi);
 /*
  * Allocate contiguous physical pages during bootstrap.
  *
- * This function is called before paging is enabled.
+ * This function is called before paging is enabled. It should only be used
+ * to allocate initial page table pages. Those pages are later loaded into
+ * the VM system (as reserved pages) which means they can be freed like other
+ * regular pages. Users should fix up the type of those pages once the VM
+ * system is initialized.
  */
 void * biosmem_bootalloc(unsigned int nr_pages);