Re: identity mapped paging (Vaibhav Jain)

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On Fri, Apr 13, 2012 at 02:15:20AM -0700, Vaibhav Jain wrote:
> >
> > > I am not clear about the use of identity mapped paging while paging is
> > > being enabled by the operating system. Also I don't understand at what
> > > 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
> >
> > > 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

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Hi

May be notes in [1] make it a bit clear ;)

[1] http://pankaj-techstuff.blogspot.com/2007/12/initialization-of-arm-mmu-in-linux.html

Best wishes
Vladimir Murzin

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