MAX_PHYSADDR_BITS v/s MAX_PHYSMEM_BITS

Dec. 20, 2011

      Hi all,

   Consider the following code snippet (for x86_64 arch) from
arch/x86/include/asm/sparsemem.h:

# define SECTION_SIZE_BITS	27 /* matt - 128 is convenient right now */
# define MAX_PHYSADDR_BITS	44
# define MAX_PHYSMEM_BITS	46

Also, consider the following comments (obtained from the same file),

MAX_PHYSADDR_BITS		2^n: max size of physical address space
MAX_PHYSMEM_BITS		2^n: how much memory we can have in
				     that space

According to my understanding MAX_PHYSMEM_BITS has to have a value
less than or equal to MAX_PHYSADDR_BITS. But this is not the case
for x86_64. Can someone explain what these macros really mean?

Thanks,
chandan r