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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: Tue, 27 Mar 2007 10:52:33 +1000
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On Tue, Mar 27, 2007 at 01:45:54AM +0100, Keir Fraser wrote:
> As an arguably better alternative, perhaps we could simply make
> XENMEM_add_to_physmap an atomic switch operation (from old mapping to new
> mapping)? That is probably not a very hard thing to guarantee.
> So you would XENMEM_add_to_physmap over the top of the existing grant
> mapping, and expect the grant to simply 'fall away' as a result of this
> operation (i.e., implicit removal from physmap triggers release of the
> grant).
> This sounds neat to me.

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 :)

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