[PATCH] x86/PAE partially linear page table support

["Jan Beulich" <jbeulich@novell.com>]

While full linear page table support makes little sense (and would be more
complicated to implement), partial linear page table support is almost identical
to that in non-PAE, and is used (at least) by NetWare.

Of course, the question here is whether this shouldn't be generalized - all
validated page tables (except for top level PAE) could be allowed to be installed
at lower levels using the same scheme.

Signed-off-by: Jan Beulich <jbeulich@novell.com>

Index: 2007-02-07/xen/arch/x86/mm.c
===================================================================
--- 2007-02-07.orig/xen/arch/x86/mm.c	2007-02-07 16:08:05.000000000 +0100
+++ 2007-02-07/xen/arch/x86/mm.c	2007-02-07 16:31:33.000000000 +0100
@@ -547,7 +547,7 @@ get_linear_pagetable(
             return 0;
 
         /*
-         * Make sure that the mapped frame is an already-validated L2 table. 
+         * Make sure that the mapped frame is an already-validated root table.
          * If so, atomically increment the count (checking for overflow).
          */
         page = mfn_to_page(pfn);
@@ -569,6 +569,51 @@ get_linear_pagetable(
 }
 #endif /* !CONFIG_X86_PAE */
 
+#if defined(CONFIG_X86_PAE) || defined(CONFIG_X86_64)
+static int
+get_l2_linear_pagetable(
+    l2_pgentry_t l2e, unsigned long l2e_pfn, struct domain *d)
+{
+    unsigned long pfn;
+
+    if ( (l2e_get_flags(l2e) & _PAGE_RW) )
+    {
+        MEM_LOG("Attempt to create linear p.t. with write perms");
+        return 0;
+    }
+
+    if ( (pfn = l2e_get_pfn(l2e)) != l2e_pfn )
+    {
+        unsigned long x, y;
+        struct page_info *page;
+
+        /* Make sure the mapped frame belongs to the correct domain. */
+        if ( unlikely(!get_page_from_pagenr(pfn, d)) )
+            return 0;
+
+        /*
+         * Make sure that the mapped frame is an already-validated L2 table.
+         * If so, atomically increment the count (checking for overflow).
+         */
+        page = mfn_to_page(pfn);
+        y = page->u.inuse.type_info;
+        do {
+            x = y;
+            if ( unlikely((x & PGT_count_mask) == PGT_count_mask) ||
+                 unlikely((x & (PGT_type_mask|PGT_validated)) !=
+                          (PGT_l2_page_table|PGT_validated)) )
+            {
+                put_page(page);
+                return 0;
+            }
+        }
+        while ( (y = cmpxchg(&page->u.inuse.type_info, x, x + 1)) != x );
+    }
+
+    return 1;
+}
+#endif /* !CONFIG_X86_PAE */
+
 int
 get_page_from_l1e(
     l1_pgentry_t l1e, struct domain *d)
@@ -645,10 +690,16 @@ get_page_from_l2e(
     }
 
     rc = get_page_and_type_from_pagenr(l2e_get_pfn(l2e), PGT_l1_page_table, d);
-#if CONFIG_PAGING_LEVELS == 2
     if ( unlikely(!rc) )
+    {
+#if CONFIG_PAGING_LEVELS == 2
         rc = get_linear_pagetable(l2e, pfn, d);
+#else
+        if ( (CONFIG_PAGING_LEVELS == 3 || IS_COMPAT(d)) )
+            rc = get_l2_linear_pagetable(l2e, pfn, d);
 #endif
+    }
+
     return rc;
 }