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

To: Isaku Yamahata <yamahata@xxxxxxxxxxxxx>, 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 08:36:39 +0100
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On 27/3/07 04:34, "Isaku Yamahata" <yamahata@xxxxxxxxxxxxx> wrote:

> The ia64 p2m entry is 64bit and some bits are already used
> so that there is no room for storing grant table related handle.
> I don't think storing the maptrack handle instead of MFN is good idea
> because it would make the p2m more complicated.
> Is it possible to look up grant table reference by MFN effectively?
> Probably MFN-keyed hash table whose entry is struct active_grant_table.
> I'm not sure whether it is effective compared to guest providing
> grant refernece, though.

This is quite a shame, since the swap-grant-mapping-for-memory hypercall
simply is not very nice. What if we wanted to swap a grant mapping for
another grant mapping in future? Or some other kind of mapping? We end up
needing a new, complicated, grant-table hypercall every time. Or in six
months time we decide this wasn't such a good idea, implement another
scheme, but have this special-purpose grant-table hypercall hanging around
unused forever more.

Could we shatter the ia64 guest large mappings easily? They have no benefit
on Xen anyway apart from some inconsequential memory saving.

 -- Keir

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