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Re: [Xen-devel] RFC: [0/2] Remove netloop by lazy copying in netback

To: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] RFC: [0/2] Remove netloop by lazy copying in netback
From: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Date: Sun, 25 Mar 2007 21:41:28 +1000
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On Fri, Mar 23, 2007 at 10:42:17PM +1100, Herbert Xu wrote:
> On Fri, Mar 23, 2007 at 10:32:37AM +0000, Keir Fraser wrote:
> > 
> > It still sounds like it would work. The fragment's 'struct page *' will map
> > to a particular kernel virtual addres. That kernel virtual address can be
> > transformed by arithmetic back to the 'struct page *'. The fact that the pte
> > that maps that kernel virtual address actually points over at some other
> > poor unsuspecting pfn (which already has a struct page *, thank you very
> > much) doesn't actually matter, does it? Does anyone ever go look at the pte
> > contents and try to work out the 'struct page *' from that? I doubt it -- or
> > our netback driver would not work right now on x86 (as the mach-to-phys
> > entry is garbage from the p.o.v. of dom0, so any attempt to translate the
> > pte contents into something meaningful in pseudophys space would fail).
> You're right, it's not as bad as I thought.

Actually looking into it further for ia64 there is still a problem.
We use large pages for the identity mapping on ia64.  In order to
modify the PTE we'd have to break the large pages.

Am I missing something obvious?

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