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

To: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] RFC: [0/2] Remove netloop by lazy copying in netback
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Tue, 27 Mar 2007 02:02:39 +0100
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On 27/3/07 01:52, "Herbert Xu" <herbert@xxxxxxxxxxxxxxxxxxx> wrote:

> It sounds good but looking further there is still an issue.
> The problem is that the gpfn itself is not sufficient if we want to
> release the grant table entry.  We need to have the grant handle too
> in order to free the entry completely.  If we did provide the handle
> then this will just be the same operation that I was trying to add
> except that now we're adding it under XENMEM_add_to_physmap :)

Well, it could be a bit more work to change, but what does a p2m entry look
like for a grant mapping? The p2m table should be a very convenient place to
have typed entries, with one of those types being grant-mapping, and
including the maptrack handle. The MFN can be derived from the maptrack
handle where that is required. This would then conveniently be enough
information to perform implicit unmaps (the lack of support of which is an
annoying limitation for non-auto-translate guests which it would be really
nice to avoid for auto-translate guests, particularly since it ought to be
not really all that hard afaics).

This is pretty much the avenue (hopefully not a dead end!) that I pushed
Isaku Yamahata down when he queried physmap semantics a week or so ago.
Sadly we've had no need for grant mappings in x86 auto-translate guests yet,
so there's no example x86 implementation to lead the way. :-)

 -- Keir

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