[Xen-devel] P2M aliasing check

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

Hello,

While trying to get hotplug scripts to work on FreeBSD, I've realized that 
there's a check in the P2M code that prevents having multiple gfn pointing 
to the same mfn. The check in question is performed at 
guest_physmap_add_entry, and it means that the existing gfn -> mfn is 
removed and the new one is added, leaving the previous gfn without a valid 
mfn.

This is a problem specially for PVH Dom0, since it means local-attach of 
virtual disk devices lead to a crash of the Dom0. This happens because 
Dom0 is sharing a grant with itself, and then the original gfn -> mfn 
assignment gets destroyed in favor of the new one created by mapping the 
grant reference.

I would like to understand the reason why this check is in place, and why 
aliased gfn -> mfn mappings are not permitted. This is a different behavior 
as compared to PV guests, where multiple VA -> mfn mappings are permitted.

Roger.

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