[Xen-devel] [PATCH] EPT: do away with hidden GUEST_TABLE_MAP_FAILED == 0 assumptions

[Jan Beulich <jbeulich@xxxxxxxx>]

The code is quite a bit easier to read and to reason about this way,
I think.

In ept_set_entry() additionally change the function's return value in
the MAP_FAILED case to -ENOMEM; -ENOENT would be applicable only when
ept_next_entry() was invoked with "read_only" set to true.

In two cases, where ept_next_level() follows an ept_split_superpage()
invocation, actually tighten the loop exit condition from
"== MAP_FAILED" to "!= NORMAL_PAGE". Continuing these loops for other
than NORMAL_PAGE is invalid, and there are ASSERT()s in place after
these loops.

Also reduce the scope of "ret" variables where possible, in particular
to better distinguish them from "rc" often used in the same function.

Finally drop pointless "else" in a few areas touched anyway.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/mm/p2m-ept.c
+++ b/xen/arch/x86/mm/p2m-ept.c
@@ -292,8 +292,8 @@ static bool_t ept_split_super_page(struc
  * and map the next table, if available.  If the entry is empty
  * and read_only is set, 
  * Return values:
- *  0: Failed to map.  Either read_only was set and the entry was
- *   empty, or allocating a new page failed.
+ *  GUEST_TABLE_MAP_FAILED: Failed to map.  Either read_only was set and the
+ *   entry was empty, or allocating a new page failed.
  *  GUEST_TABLE_NORMAL_PAGE: next level mapped normally
  *  GUEST_TABLE_SUPER_PAGE:
  *   The next entry points to a superpage, and caller indicates
@@ -404,12 +404,13 @@ static int ept_invalidate_emt_range(stru
     ept_entry_t *table;
     unsigned long gfn_remainder = first_gfn;
     unsigned int i, index;
-    int wrc, rc = 0, ret = GUEST_TABLE_MAP_FAILED;
+    int wrc, rc = 0;
 
     table = map_domain_page(pagetable_get_mfn(p2m_get_pagetable(p2m)));
     for ( i = p2m->ept.wl; i > target; --i )
     {
-        ret = ept_next_level(p2m, 1, &table, &gfn_remainder, i);
+        int ret = ept_next_level(p2m, 1, &table, &gfn_remainder, i);
+
         if ( ret == GUEST_TABLE_MAP_FAILED )
             goto out;
         if ( ret != GUEST_TABLE_NORMAL_PAGE )
@@ -434,8 +435,10 @@ static int ept_invalidate_emt_range(stru
         ASSERT(wrc == 0);
 
         for ( ; i > target; --i )
-            if ( !ept_next_level(p2m, 1, &table, &gfn_remainder, i) )
+            if ( ept_next_level(p2m, 1, &table, &gfn_remainder, i) !=
+                 GUEST_TABLE_NORMAL_PAGE )
                 break;
+        /* We just installed the pages we need. */
         ASSERT(i == target);
     }
 
@@ -694,12 +697,12 @@ ept_set_entry(struct p2m_domain *p2m, gf
     for ( i = ept->wl; i > target; i-- )
     {
         ret = ept_next_level(p2m, 0, &table, &gfn_remainder, i);
-        if ( !ret )
+        if ( ret == GUEST_TABLE_MAP_FAILED )
         {
-            rc = -ENOENT;
+            rc = -ENOMEM;
             goto out;
         }
-        else if ( ret != GUEST_TABLE_NORMAL_PAGE )
+        if ( ret != GUEST_TABLE_NORMAL_PAGE )
             break;
     }
 
@@ -756,7 +759,8 @@ ept_set_entry(struct p2m_domain *p2m, gf
 
         /* then move to the level we want to make real changes */
         for ( ; i > target; i-- )
-            if ( !ept_next_level(p2m, 0, &table, &gfn_remainder, i) )
+            if ( ept_next_level(p2m, 0, &table, &gfn_remainder, i) !=
+                 GUEST_TABLE_NORMAL_PAGE )
                 break;
         /* We just installed the pages we need. */
         ASSERT(i == target);
@@ -859,7 +863,6 @@ static mfn_t ept_get_entry(struct p2m_do
     ept_entry_t *ept_entry;
     u32 index;
     int i;
-    int ret = 0;
     bool_t recalc = 0;
     mfn_t mfn = INVALID_MFN;
     struct ept_data *ept = &p2m->ept;
@@ -883,13 +886,15 @@ static mfn_t ept_get_entry(struct p2m_do
 
     for ( i = ept->wl; i > 0; i-- )
     {
+        int ret;
+
     retry:
         if ( table[gfn_remainder >> (i * EPT_TABLE_ORDER)].recalc )
             recalc = 1;
         ret = ept_next_level(p2m, 1, &table, &gfn_remainder, i);
-        if ( !ret )
+        if ( ret == GUEST_TABLE_MAP_FAILED )
             goto out;
-        else if ( ret == GUEST_TABLE_POD_PAGE )
+        if ( ret == GUEST_TABLE_POD_PAGE )
         {
             if ( !(q & P2M_ALLOC) )
             {
@@ -905,10 +910,9 @@ static mfn_t ept_get_entry(struct p2m_do
 
             if ( p2m_pod_demand_populate(p2m, gfn_, i * EPT_TABLE_ORDER) )
                 goto retry;
-            else
-                goto out;
+            goto out;
         }
-        else if ( ret == GUEST_TABLE_SUPER_PAGE )
+        if ( ret == GUEST_TABLE_SUPER_PAGE )
             break;
     }
 
@@ -1289,7 +1293,6 @@ static void ept_dump_p2m_table(unsigned
     ept_entry_t *table, *ept_entry;
     int order;
     int i;
-    int ret = 0;
     unsigned long gfn, gfn_remainder;
     unsigned long record_counter = 0;
     struct p2m_domain *p2m;
@@ -1307,6 +1310,7 @@ static void ept_dump_p2m_table(unsigned
         for ( gfn = 0; gfn <= p2m->max_mapped_pfn; gfn += 1UL << order )
         {
             char c = 0;
+            int ret = GUEST_TABLE_MAP_FAILED;
 
             gfn_remainder = gfn;
             table = map_domain_page(pagetable_get_mfn(p2m_get_pagetable(p2m)));

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