Shadow warning with HVM guests

Shadow warning with HVM guests

[Andrew Cooper]

Hello,

With Xen-4.2 and unstable, running HVM guests in shadow mode, there is a
reliable error on HVM create:

sh error: sh_remove_all_mappings(): can't find all mappings of mfn
4f5b4: c=c000000000000002 t=7400000000000001

which looks more scary than it actually is.  (The mfn is variable due to
the domheap allocator, but always refers to gmfn ffffe)

The root cause of this is xc_dom_gnttab_hvm_seed() which performs an
add_to_physmap and remove_from_physmap of the new domains grant table
(0th frame) into the new domains memory, so xc_dom_gnttab_seed() can
create a foreign mapping.

The offending call to sh_remove_all_mappings() is on the
remove_from_physmap hypercall path.

The complaint is that after removing all mappings in the shadow
pagetables, there is are still mappings the mfn in question.  In this
case, that is correct, as the grant table is xalloc()'d memory from the
xenheap.

>From my interpretation, sh_remove_all_mappings() has done its job by the
time it complains.  There are no more mappings in the shadow pagetables,
but it does not logically follow that this means that there are no
mappings at all.

Would it be acceptable to make xenheap mappings immune to this error
message, or perhaps to remove the error entirely as we already
special-case HVM domains with toolstack foreign mappings?

~Andrew