identity mapped paging (Vaibhav Jain)

-----Original Message-----
From: Vaibhav Jain [mailto:vjoss197 at gmail.com]
Sent: Tuesday, April 17, 2012 4:07 PM
To: Pranay Kumar Srivastava
Cc: kernelnewbies at kernelnewbies.org
Subject: Re: identity mapped paging (Vaibhav Jain)


On Fri, Apr 13, 2012 at 2:15 AM, Vaibhav Jain [off-list ref]
wrote:

quoted

I am not clear about the use of identity mapped paging while paging

is

quoted

being enabled by the operating system. Also I don't understand at

what

quoted

point are the
identity mappings no longer useful.According to this article
http://geezer.osdevbrasil.net/osd/mem/index.htm#identity - "The page
table
entries used to identity-map kernel memory can be deleted once paging
and
virtual addresses are enabled." Can somebody please explain?

Identity mapping is when VA(Virt Address)=PA(Physical address).

So basically when you set up your page tables you need to make sure
they map identically. This is very easily done if you consider each 4KB
block as a page beginning from location 0 upto whatever you've found to
be the highest memory available either thru BIOS or GRUB.

Remember that while setting up your PTEs and PDE every address is a
physical one. So if you thought that your kernel would be linked
initially to a higher VA since you would remap it to a lower memory
physically then that would be WRONG!. Without PTEs and PDEs installed
don't do that!.

Why would you want it? Well for a simple reason, when your kernel
starts to boot there's no translator,(No PTEs/PDEs and the Paging
Enabled bit of processor is also cleared AFAIK just after the BIOS is
done), yet since you've not enabled your processor for that but you'll
be doing that in a moment.

So let's say you made your kernel to be linked to higher VA like 3Gigs.
Now the addresses would be generated beginning 3Gigs however you still
don't have the Page tables installed since your kernel just started. So
in that case the address is the physical address. And if you've not
loaded your kernel beginning 3Gigs then it would definitely come
crashing down.

To avoid the crash in case you made your kernel to link to higher half
of the memory, you can use GDT trick since segmentation is always on
and you can make the overflow of the address addition to translate to a
lower physical memory even if paging is not enabled yet. Thus it is
possible to load the kernel at lower memory addresses while the linkage
would be for higher VMA. And once your PTEs/PGD are enabled then you
can use those instead of the GDT trick.

Here's a link to that http://wiki.osdev.org/Higher_Half_With_GDT

quoted

Thanks
Vaibhav Jain

Hi,

Thanks for replying but I am still confused. I continued reading about
this thing and what
I have understood is the following :
After the kernel executes the instruction to enable paging the
instruction pointer will contain the
address of the next instruction which will now be treated as a virtual
address. So for the next instruction to be executed
the page table should map this address to itself.
Please correct me if I am wrong.
I am confused by the point about linking  the kernel to higher address.
Could you please put that in a step by step manner
to make it clear what  happens before paging is enabled and what
happens after that.
Also, please explain at what point during the execution of kernel code
are the identity-mapped addresses no longer useful ?




Thanks
Vaibhav
Hi,

I am somewhat understanding your point. But I have some other queries
now in my mind.

If the kernel is linked to 3Gigs is there a way other than the GDT
trick.?

Make your load address = VA when you link so you won't have to worry about doing the GDT trick.

In fact I am wondering that if the kernel is linked to 3Gigs and Grub
loads it at 1MB physical, how will even the first instruction of kernel
execute ?  I mean if all the address generated by kernel are above 3
Gigs and paging is not enabled how will it start
running ?

That's what the GDT trick is for. If you read the intel/amd processor manuals the segmentation is always on. So when the address get generated your segment's base address is still added to the generated address before it is put on wire. You can add a constant offset (in your GDT's base address part) to the generated address to get the address beginning from the load address of your kernel.

I would suggest you make the higher half kernel later and try to first create some code that can fragment your available memory into pages and store this information so you'll know what all pages are there. Next would be to do identity mapping, since your kernel VMA=LMA in your linker script this would be easier to do.

When you get that paging enabled you can move on to higher half kernel. I would suggest you to work on page replacement algos and virtual memory management code side by side for better integration with paging in later stages.

Maybe you can post your code if you are allowed to then I can have a look at it.

Thanks
Vaibhav Jain

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