[PATCH] x86/p2m-pt: fix p2m_flags_to_access()

[Jan Beulich <jbeulich@xxxxxxxx>]

The initial if() was inverted, invalidating all output from this
function. Which in turn means the mirroring of P2M mappings into the
IOMMU didn't always work as intended: Mappings may have got updated when
there was no need to. There would not have been too few (un)mappings;
what saves us is that alongside the flags comparison MFNs also get
compared, with non-present entries always having an MFN of 0 or
INVALID_MFN while present entries always have MFNs different from these
two (0 in the table also meant to cover INVALID_MFN):

OLD                                     NEW
P W     access  MFN                     P W     access  MFN
0 0     r       0                       0 0     n       0
0 1     rw      0                       0 1     n       0
1 0     n       non-0                   1 0     r       non-0
1 1     n       non-0                   1 1     rw      non-0

present <-> non-present transitions are fine because the MFNs differ.
present -> present transitions as well as non-present -> non-present
ones are potentially causing too many map/unmap operations, but never
too few, because in that case old (bogus) and new access differ.

Fixes: d1bb6c97c31e ("IOMMU: also pass p2m_access_t to p2m_get_iommu_flags())
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/mm/p2m-pt.c
+++ b/xen/arch/x86/mm/p2m-pt.c
@@ -548,7 +548,7 @@ int p2m_pt_handle_deferred_changes(uint6
 /* Reconstruct a fake p2m_access_t from stored PTE flags. */
 static p2m_access_t p2m_flags_to_access(unsigned int flags)
 {
-    if ( flags & _PAGE_PRESENT )
+    if ( !(flags & _PAGE_PRESENT) )
         return p2m_access_n;
 
     /* No need to look at _PAGE_NX for now. */