Virtual Memory map

[Damian Tometzki]

Hello,

On more question. The second example:

>> ffff800000000000 - ffff87ffffffffff (=43 bits) guard hole, reserved for hypervisor

What are the relation between the Bits (=43bits) and the Adresse Range ?

Is it possible to calculate the 43 Bits from the range ?  If yes, how ? 


Best regards
Damian 




> Am 13.12.2017 um 05:35 schrieb arshad hussain <arshad.super at gmail.com>:
> 
> 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
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