Re: [Xen-devel] Page table and memory management

[Christian Limpach <christian.limpach@xxxxxxxxx>]

On Thu, 3 Mar 2005 17:00:17 -0500, Richard <judicator3@xxxxxxxxx> wrote:
> > Xen will automatically infer the type when you attach an L1 to an
> > existing L2. Xen will infer the L2 type when the L2 gets used as
> > current pagetable base.
> So I do not see where XEN is automatically inferring the L1 type when
> I am inserting for the 1st time the L1 page frame into the L2.

The L1 type gets set in get_page_type, when the page's type count is
zero and the page was of a different type until then (code path with
the "On type change" comment).  For this to work, the type count needs
to be zero which it isn't when there is already a writeable mapping to
the page.

You should use a debug=y build of xen, it will tell you why and when
your attempt to use a new L1 page fails.

> In
> order to use a page frame as an L1 page table, I have to find a way to
> tell XEN to update the corresponding typeinfo field in order to pass
> the verification in get_page_type().

No, you have to make sure the page is not otherwise used, that there
are at most read-only mappings to it.

    christian


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