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Re: [Xen-devel] Re: [PATCH]Add a flag for shadow pages

To: Tim Deegan <Tim.Deegan@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Re: [PATCH]Add a flag for shadow pages
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Wed, 04 Mar 2009 13:19:17 +0000
Cc: Gianluca Guida <Gianluca.Guida@xxxxxxxxxxxxx>, "Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
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Thread-topic: [Xen-devel] Re: [PATCH]Add a flag for shadow pages
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On 04/03/2009 12:13, "Tim Deegan" <Tim.Deegan@xxxxxxxxxxxxx> wrote:

> shadow_get_page_from_l1e() should never be handling a pointer to a
> shadow -- if it does that then we've let the guest see the shadows and
> all invariants go out the window.
> sh_mfn_is_a_pagetable() looks OK too; it only gets called based on the
> contents of shadow PTEs or the MFNs that guests are writing to, both of
> which should be safe.
> It all feels a bit fragile to me though, compared to the old layout
> where we always knew the owner field would be NULL.

This is the tradeoff for the new get_page() implementation and wider
reference counts. We just need to be careful and use
page_get_owner_and_reference() in the few cases it seems to be needed.

 -- Keir

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