Re: using segmentation in the kernel

[Denis Vlasenko <vda@ilport.com.ua>]

On Wednesday 12 October 2005 16:03, linux-os (Dick Johnson) wrote:
> On the ix86 you have a problem. Let's say that you write some
> code from scratch, that runs the CPU in 32-bit linear address-mode
> without paging. Then you want to activate paging. To activate
> paging, you MUST have provided some code and some data-space for
> your descriptors, where there is a 1:1 mapping between virtual
> and bus addresses. If you don't do this, at the instant you
> change to paging mode, you crash. The CPU fetches garbage.
> 
> This is why the first few megabytes of Linux are unity-mapped.
> You will always need to run the kernel out of an area where
> a portion of that "segment" is unity-mapped. That segment
> is where the descriptors for addressing, paging, and interrupts
> must reside.
>
> If you truly wanted to run the kernel from 3-4 GB as you state,
> you must have RAM there, i.e., some physical stuff so that
> a 1:1 mapping could be implemented. The 3-4 GB region is
> where a lot of PCI addressing occurs on 32-bit machines and,
> in fact, there are some "do-not-touch" addresses in that
> region as well.

This is untrue. After paging is enabled, you can jump
to non-unity mapped location and remove small unity-mapped region.

> Remember that the kernel runs in virtual address mode, but
> the descriptors that specify that mode need to be in physical
> memory, addressed at the same offset. You can experiment

Some of us are smart enough to add an offset when doing virt<->phys
conversion, if needed.
--
vda
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