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Re: [Xen-devel] RFC : 4KiB page support - x86_32.S

To: Adam Sulmicki <adam@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] RFC : 4KiB page support - x86_32.S
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Mon, 22 Nov 2004 22:23:56 +0000
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> I see few issues.
> For one thing PAGE_OFFSET points to "4gb - 60mb" and 60mb. This is where
> the current page table stuff live. This is fine with 4mb pages as in this
> configuration page table/directory occupy only 4kb.
> However, once I want to duplicate current setup using 4kb of pages, I need
> 4MB for page tables (+4kb for page directory).
> It seems to me that once I have 4mb of page tables + other stuff from
> x86_32.S I would run over the 4gb mark (although with a bit of empirical
> testing it did not crash right away.. not sure If I'm missing something.).
> I have tried to change PAGE_OFFSET to point to "4gb - 56", but then all
> hell would break lose and it would not even work with 4mb pages on pentium
> 3.
> Thus in my code I just put the 4mb wroth of page tables at 56mb-60mb
> address space, what should fit well with both low and high memory. It
> seems to work fine other than it just my idea to use this particular space
> for the page.

56-60mb is probably okay, but kernel.c will need fixing because I
think it currently copies dom0 image there temporarily during Xen
bootstrap. As you say, checking CPUID is a good idea. mmu_cr4_features
is initialised statically as a C variable.

I'll try to find time to take a closer look and hopefully get 4kb-page
support checked in.


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