On 25/4/08 01:18, "Jeremy Fitzhardinge" <jer...@goop.org> wrote:

If you have _PAGE_IO then only unpinning of ptes containing grant-table mappings must not be deferred. 'Ordinary' foreign mappings, of the sort that dom0 can create because it is privileged, do not have this constraint. This is because you can tell they are foreign, and hence avoid page refcounting, simply because the pte contains _PAGE_IO. Without _PAGE_IO we relied on checking whether a machine->pseudophys->machine double lookup took us from the machine address in the pte back to the same machine address. If not, we knew the page was not ours. This doesn't work reliably if the pte page is not pinned because in that case the mapped page can be freed from the foreign domain and be reallocated into the local domain: in which case the M->P->M check could succeed! The _PAGE_IO check is more robust.

-- Keir