[PATCH RFC v2 3/3] x86/altp2m: p2m_altp2m_propagate_change() should honor present page order

[Jan Beulich <jbeulich@xxxxxxxx>]

For higher order mappings the comparison against p2m->min_remapped_gfn
needs to take the upper bound of the covered GFN range into account, not
just the base GFN.

Otherwise, i.e. when dropping a mapping and not overlapping the remapped
range, XXX.

Note that there's no need to call get_gfn_type_access() ahead of the
check against the remapped range boundaries: None of its outputs are
needed earlier. Local variables get moved into the more narrow scope to
demonstrate this.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
I may be entirely wrong and hence that part of the change may also be
wrong, but I'm having trouble seeing why the original
"!mfn_eq(m, INVALID_MFN)" wasn't "mfn_eq(m, INVALID_MFN)". Isn't the
goal there to pre-fill entries that were previously invalid, instead of
undoing prior intentional divergence from the host P2M? (I have
intentionally not reflected this aspect in the description yet; I can't
really write a description of this without understanding what's going on
in case the original code was correct.)

When cur_order is below the passed in page_order, the p2m_set_entry() is
of course not covering the full range. This could be put in a loop, but
locking looks to be a little problematic: If a set of lower order pages
gets re-combined to a large one while already having progressed into the
range, we'd need to carefully back off. Alternatively the full incoming
GFN range could be held locked for the entire loop; this would likely
mean relying on gfn_lock() actually resolving to p2m_lock(). But perhaps
that's not a big problem, considering that core functions like
p2m_get_gfn_type_access() or __put_gfn() assume so, too (because of
not taking the order into account at all)?

--- a/xen/arch/x86/mm/p2m.c
+++ b/xen/arch/x86/mm/p2m.c
@@ -2532,9 +2532,6 @@ int p2m_altp2m_propagate_change(struct d
                                 p2m_type_t p2mt, p2m_access_t p2ma)
 {
     struct p2m_domain *p2m;
-    p2m_access_t a;
-    p2m_type_t t;
-    mfn_t m;
     unsigned int i;
     unsigned int reset_count = 0;
     unsigned int last_reset_idx = ~0;
@@ -2551,12 +2548,30 @@ int p2m_altp2m_propagate_change(struct d
             continue;
 
         p2m = d->arch.altp2m_p2m[i];
-        m = get_gfn_type_access(p2m, gfn_x(gfn), &t, &a, 0, NULL);
 
+        if ( !mfn_eq(mfn, INVALID_MFN) )
+        {
+            p2m_access_t a;
+            p2m_type_t t;
+            unsigned int cur_order;
+
+            if ( mfn_eq(get_gfn_type_access(p2m, gfn_x(gfn), &t, &a, 0,
+                                            &cur_order),
+                        INVALID_MFN) )
+            {
+                int rc = p2m_set_entry(p2m, gfn, mfn, min(cur_order, 
page_order),
+                                       p2mt, p2ma);
+
+                /* Best effort: Don't bail on error. */
+                if ( !ret )
+                    ret = rc;
+            }
+
+            __put_gfn(p2m, gfn_x(gfn));
+        }
         /* Check for a dropped page that may impact this altp2m */
-        if ( mfn_eq(mfn, INVALID_MFN) &&
-             gfn_x(gfn) >= p2m->min_remapped_gfn &&
-             gfn_x(gfn) <= p2m->max_remapped_gfn )
+        else if ( gfn_x(gfn) + (1UL << page_order) > p2m->min_remapped_gfn &&
+                  gfn_x(gfn) <= p2m->max_remapped_gfn )
         {
             if ( !reset_count++ )
             {
@@ -2566,8 +2581,6 @@ int p2m_altp2m_propagate_change(struct d
             else
             {
                 /* At least 2 altp2m's impacted, so reset everything */
-                __put_gfn(p2m, gfn_x(gfn));
-
                 for ( i = 0; i < MAX_ALTP2M; i++ )
                 {
                     if ( i == last_reset_idx ||
@@ -2581,16 +2594,6 @@ int p2m_altp2m_propagate_change(struct d
                 break;
             }
         }
-        else if ( !mfn_eq(m, INVALID_MFN) )
-        {
-            int rc = p2m_set_entry(p2m, gfn, mfn, page_order, p2mt, p2ma);
-
-            /* Best effort: Don't bail on error. */
-            if ( !ret )
-                ret = rc;
-        }
-
-        __put_gfn(p2m, gfn_x(gfn));
     }
 
     altp2m_list_unlock(d);