From mboxrd@z Thu Jan  1 00:00:00 1970
From: arshad.super@gmail.com (arshad hussain)
Date: Wed, 13 Dec 2017 10:05:57 +0530
Subject: Virtual Memory map
In-Reply-To: <20171212191435.GB1921@zrhn9910b>
References: <20171212191435.GB1921@zrhn9910b>
Message-ID: <CAKsu7qzTgM8SVJY9Yy6faKp4+x0N2--POzJ-R-=1u9hZj4eHkQ@mail.gmail.com>
To: kernelnewbies@lists.kernelnewbies.org
List-Id: kernelnewbies.lists.kernelnewbies.org

On Wed, Dec 13, 2017 at 12:44 AM, Damian Tometzki
<damian.tometzki@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