[PATCH v3] x86/PoD: tie together P2M update and increment of entry count

[Jan Beulich <jbeulich@xxxxxxxx>]

When not holding the PoD lock across the entire region covering P2M
update and stats update, the entry count - if to be incorrect at all -
should indicate too large a value in preference to a too small one, to
avoid functions bailing early when they find the count is zero. However,
instead of moving the increment ahead (and adjust back upon failure),
extend the PoD-locked region.

Fixes: 99af3cd40b6e ("x86/mm: Rework locking in the PoD layer")
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
The locked region could be shrunk again, by having multiple unlock
calls. But I think both ioreq_request_mapcache_invalidate() and
domain_crash() are fair enough to call with the lock still held?
---
v3: Extend locked region instead. Add Fixes: tag.
v2: Add comments.

--- a/xen/arch/x86/mm/p2m-pod.c
+++ b/xen/arch/x86/mm/p2m-pod.c
@@ -1348,16 +1348,22 @@ mark_populate_on_demand(struct domain *d
         }
     }
 
+    /*
+     * P2M update and stats increment need to collectively be under PoD lock,
+     * to prevent code elsewhere observing PoD entry count being zero despite
+     * there actually still being PoD entries (created by the p2m_set_entry()
+     * invocation below).
+     */
+    pod_lock(p2m);
+
     /* Now, actually do the two-way mapping */
     rc = p2m_set_entry(p2m, gfn, INVALID_MFN, order,
                        p2m_populate_on_demand, p2m->default_access);
     if ( rc == 0 )
     {
-        pod_lock(p2m);
         p2m->pod.entry_count += 1UL << order;
         p2m->pod.entry_count -= pod_count;
         BUG_ON(p2m->pod.entry_count < 0);
-        pod_unlock(p2m);
 
         ioreq_request_mapcache_invalidate(d);
     }
@@ -1373,6 +1379,8 @@ mark_populate_on_demand(struct domain *d
         domain_crash(d);
     }
 
+    pod_unlock(p2m);
+
 out:
     gfn_unlock(p2m, gfn, order);