Messages in this thread | | | From | Denis Vlasenko <> | Subject | Re: using segmentation in the kernel | Date | Thu, 13 Oct 2005 11:51:16 +0300 |
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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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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