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Re: [Xen-devel] Xeno Linux never pins L1 tables ?

To: "Jeremy Fitzhardinge" <jeremy@xxxxxxxx>

Date: Tue, 16 Oct 2007 16:59:26 -0400

Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>

Delivery-date: Tue, 16 Oct 2007 14:00:07 -0700

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On 10/16/07, Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:

Satya wrote:
> I'm developing my own 32-bit (no PAE) paravirtualized kernel for xen
> with Mini-OS as a starting point. I am currently working on process
> page table support (equivalent of arch/i386/mm/pgtable- xen.c) and
> mostly following Linux for the moment. I noticed that linux-2.6.18-xen
> never pins an L1 table (a pte), yet __pgd_pin() walks the page
> directory and gives up write access on the kernel mappings of pte
> pages and only pins the pgd page. How do set_pte() and set_pte_at()
> macros work if they are writing directly to the page table entires ?
> do we fault in the kernel to handle this ?!

Pinning the top level of a pagetable implicitly pins all the lower
levels, so they are all pinned.

Wonder what happens if an L1 page is created *after* a PGD is pinned ? I think I have to explicitly pin the L1 page then? or unlikely( does Xen pin it when I map into into the pinned PGD using a hypercall ) ? :)

thanks,
satya.

J

http://cs.uic.edu/~spopuri

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