page tables location

[PUCCETTI Armand <armand.puccetti@cea.fr>]

A few simple questions:

1. page tables: Could please someone lighten me up on where the page 
tables tree physically resides
on a 64 bits machine?  Are they contiguous and are they constrained some 
address space? which one?
Where (function) are these tables extended, when necessary?

2. alloc_bitmap: the variable alloc_bitmap that contains the flags to 
check if a page is allocated or not is initialised like this:

...
xenheap_phys_start = init_boot_allocator(_pa(&_end));
...

unsigned long init_boot_allocator(unsigned long bitmap_start)
{
  bitmap_start = round_pgup(bitmap_start);

  /* Allocate space for the allocation bitmap. */
  bitmap_size = max_page / 8;
  bitmap_size = round_pgup(bitmap_size);
  alloc_bitmap = (unsigned long *)phys_to_virt(bitmap_start);
  /* All allocated by default. */
  memset(alloc_bitmap, ~0, bitmap_size);
  return bitmap_start + bitmap_size;
}

so, alloc_bitmap is apparently put somewhere in virtual memory (a little 
after 0xFFFF830000100000
according to xen.lds.S). However, it seems that its 36K are physically 
allocated between 0x00000000004b0000  and 0x00000000004b9000. This is a 
reserved mem area, so how is alloc_bitmap placed at this
location?

3. Is there any physical memory map, explaining what objects are located 
in low mem at fixed addresses?
something that refines the one printed at boot time.

thanks
Armand