x86/p2m-pt: fix p2m_flags_to_access()

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: d1bb6c97c3 ("IOMMU: also pass p2m_access_t to p2m_get_iommu_flags())
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: e70a9a043a
master date: 2021-09-07 14:24:49 +0200

xen/arch/x86/mm/p2m-pt.c

@@ -483,7 +483,7 @@ int p2m_pt_handle_deferred_changes(uint64_t gpa)
 /* 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. */