x86/mm: Refactor map_pages_to_xen to have only a single exit path

We will soon need to perform clean-ups before returning.

No functional change.

This is part of XSA-345.

Reported-by: Hongyan Xia <hongyxia@amazon.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Hongyan Xia <hongyxia@amazon.com>
Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/mm.c

@@ -275,7 +275,7 @@ static l4_pgentry_t __read_mostly split_l4e;
  * Originally cloned from share_xen_page_with_guest(), just to avoid setting
  * PGC_xen_heap on non-heap (typically) MMIO pages. Other pieces got dropped
  * simply because they're not needed in this context.
- */ 
+ */
 static void __init assign_io_page(struct page_info *page)
 {
     set_gpfn_from_mfn(mfn_x(page_to_mfn(page)), INVALID_M2P_ENTRY);
@@ -5079,6 +5079,7 @@ int map_pages_to_xen(
     l2_pgentry_t *pl2e, ol2e;
     l1_pgentry_t *pl1e, ol1e;
     unsigned int  i;
+    int rc = -ENOMEM;
 
 #define flush_flags(oldf) do {                 \
     unsigned int o_ = (oldf);                  \
@@ -5099,7 +5100,8 @@ int map_pages_to_xen(
         l3_pgentry_t ol3e, *pl3e = virt_to_xen_l3e(virt);
 
         if ( !pl3e )
-            return -ENOMEM;
+            goto out;
+
         ol3e = *pl3e;
 
         if ( cpu_has_page1gb &&
@@ -5189,7 +5191,7 @@ int map_pages_to_xen(
 
             l2t = alloc_xen_pagetable();
             if ( l2t == NULL )
-                return -ENOMEM;
+                goto out;
 
             for ( i = 0; i < L2_PAGETABLE_ENTRIES; i++ )
                 l2e_write(l2t + i,
@@ -5218,7 +5220,7 @@ int map_pages_to_xen(
 
         pl2e = virt_to_xen_l2e(virt);
         if ( !pl2e )
-            return -ENOMEM;
+            goto out;
 
         if ( ((((virt >> PAGE_SHIFT) | mfn_x(mfn)) &
                ((1u << PAGETABLE_ORDER) - 1)) == 0) &&
@@ -5262,7 +5264,7 @@ int map_pages_to_xen(
             {
                 pl1e = virt_to_xen_l1e(virt);
                 if ( pl1e == NULL )
-                    return -ENOMEM;
+                    goto out;
             }
             else if ( l2e_get_flags(*pl2e) & _PAGE_PSE )
             {
@@ -5290,7 +5292,7 @@ int map_pages_to_xen(
 
                 l1t = alloc_xen_pagetable();
                 if ( l1t == NULL )
-                    return -ENOMEM;
+                    goto out;
 
                 for ( i = 0; i < L1_PAGETABLE_ENTRIES; i++ )
                     l1e_write(&l1t[i],
@@ -5436,7 +5438,10 @@ int map_pages_to_xen(
 
 #undef flush_flags
 
-    return 0;
+    rc = 0;
+
+ out:
+    return rc;
 }
 
 int populate_pt_range(unsigned long virt, unsigned long nr_mfns)