Virtual Memory map

[arshad hussain]

On Wed, Dec 13, 2017 at 12:44 AM, Damian Tometzki
<damian.tometzki at icloud.com> wrote:
> Hello all,
>
> i have a short question:
>
> How are the bits (in brackets) calculated in the virtual memory map ?
>
> for example the first line (=47 bits)
>
> Virtual memory map with 4 level page tables:
>
> 0000000000000000 - 00007fffffffffff (=47 bits) user space, different per mm
> hole caused by [47:63] sign extension
> ffff800000000000 - ffff87ffffffffff (=43 bits) guard hole, reserved for hypervisor
>

Presently for x86-64 kernel uses 4-level paging. Out of 64bit only 48 bit
is enabled (Which boils down to Terabyte of address space). For case
where getpagesize() is 4K the breakup is :-

CR3 -> Global(9 bits) -> Upper(9 bits) -> Middle(9 bits) -> Page
table(9 bits) -> [12 bits for page or 4096 bytes] = (48 bits)

Thanks