Re: [Xen-devel] [PATCH v2 0/2] map grant refs at pfn = mfn

[Tim Deegan <tim@xxxxxxx>]

At 18:18 +0100 on 23 Jul (1406135907), Stefano Stabellini wrote:
> Hi all,
> this patch series introduces a second p2m mapping of grant reference on
> ARM at guest physical address == machine address of the grant ref.  It
> is safe because dom0 is already mapped 1:1. We export
> XENFEAT_grant_map_identity to signal the guest that this second p2m
> mapping is
> available.
> 
> One reason for wanting the second p2m mapping is to avoid tracking mfn
> to pfn mappings in the guest kernel. Since the same mfn can be granted
> multiple times to the backend, finding the right pfn corresponding to a
> given mfn can be difficult and expensive. Providing a second mapping at
> a known address allow the kernel to access the page without knowing the
> pfn.

Hrmn.  If your guest-kernel code relies on this new flag, you're 
effectively requiring that driver domains and middlebox VMs be
built 1:1 as well.  Unless they can deal with having a heavily
fragmented memory map that's going to be a problem.

Also it prevents dom0 mapping anything else into its p2m.  Unless dom0
knows where all the RAM in the system is (which is really none of its
business) it can't be sure that a GFN is safe to use for the _first_
p2m mapping, or any other p2m tricks it might like to play.

Tim.




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