[Xen-devel] [PATCH] Fix guest_physmap_add_entry checks

["George Dunlap" <George.Dunlap@xxxxxxxxxxxxx>]

guest_physmap_add_entry() checks to see if the given mfn and gpfn
range in the p2m and m2p tables is already mapped before overwriting
the maps, and attempts to do something reasonable so that we don't
have any "dangling" pointers.

Unfortunately, these checks got broken when the page_order argument
was added.  Each individual p2m and m2p entry needs to be checked, not
just the first page in a page order.

This patch implements that.

One decision that deserves some double-checking: when removing old p2m
mappings to the given MFN range, it now calls p2m_remove_page() with a
page_order of 0 (since we don't know how big the original order was
supposed to be) for each individual mfn.  It didn't look like this
would cause a problem anywhere.

Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>

diff -r b53b02976633 xen/arch/x86/mm/p2m.c
--- a/xen/arch/x86/mm/p2m.c     Wed Sep 24 12:47:33 2008 +0100
+++ b/xen/arch/x86/mm/p2m.c     Wed Sep 24 16:22:37 2008 +0100
@@ -953,38 +953,47 @@ guest_physmap_add_entry(struct domain *d

     P2M_DEBUG("adding gfn=%#lx mfn=%#lx\n", gfn, mfn);

-    omfn = gfn_to_mfn(d, gfn, &ot);
-    if ( p2m_is_ram(ot) )
+    /* First, remove m->p mappings for existing p->m mappings */
+    for ( i = 0; i < (1UL << page_order); i++ )
     {
-        ASSERT(mfn_valid(omfn));
-        for ( i = 0; i < (1UL << page_order); i++ )
-            set_gpfn_from_mfn(mfn_x(omfn)+i, INVALID_M2P_ENTRY);
-    }
-
-    ogfn = mfn_to_gfn(d, _mfn(mfn));
-    if (
-#ifdef __x86_64__
-        (ogfn != 0x5555555555555555L)
-#else
-        (ogfn != 0x55555555L)
-#endif
-        && (ogfn != INVALID_M2P_ENTRY)
-        && (ogfn != gfn) )
-    {
-        /* This machine frame is already mapped at another physical address */
-        P2M_DEBUG("aliased! mfn=%#lx, old gfn=%#lx, new gfn=%#lx\n",
-                  mfn, ogfn, gfn);
-        omfn = gfn_to_mfn(d, ogfn, &ot);
+        omfn = gfn_to_mfn(d, gfn, &ot);
         if ( p2m_is_ram(ot) )
         {
             ASSERT(mfn_valid(omfn));
-            P2M_DEBUG("old gfn=%#lx -> mfn %#lx\n",
-                      ogfn , mfn_x(omfn));
-            if ( mfn_x(omfn) == mfn )
-                p2m_remove_page(d, ogfn, mfn, page_order);
+            set_gpfn_from_mfn(mfn_x(omfn)+i, INVALID_M2P_ENTRY);
         }
     }

+    /* Then, look for m->p mappings for this range and deal with them */
+    for ( i = 0; i < (1UL << page_order); i++ )
+    {
+        ogfn = mfn_to_gfn(d, _mfn(mfn));
+        if (
+#ifdef __x86_64__
+            (ogfn != 0x5555555555555555L)
+#else
+            (ogfn != 0x55555555L)
+#endif
+            && (ogfn != INVALID_M2P_ENTRY)
+            && (ogfn != gfn) )
+        {
+            /* This machine frame is already mapped at another physical
+             * address */
+            P2M_DEBUG("aliased! mfn=%#lx, old gfn=%#lx, new gfn=%#lx\n",
+                      mfn, ogfn, gfn);
+            omfn = gfn_to_mfn(d, ogfn, &ot);
+            if ( p2m_is_ram(ot) )
+            {
+                ASSERT(mfn_valid(omfn));
+                P2M_DEBUG("old gfn=%#lx -> mfn %#lx\n",
+                          ogfn , mfn_x(omfn));
+                if ( mfn_x(omfn) == mfn )
+                    p2m_remove_page(d, ogfn, mfn, 0);
+            }
+        }
+    }
+
+    /* Now, actually do the two-way mapping */
     if ( mfn_valid(_mfn(mfn)) )
     {
         if ( !set_p2m_entry(d, gfn, _mfn(mfn), page_order, t) )

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