x86/shadow: slightly consolidate sh_unshadow_for_p2m_change() (part II)

Pull common checks out of the switch(). This includes extending a
_PAGE_PRESENT check to L1 as well, which presumably was deemed redundant
with p2m_is_valid() || p2m_is_grant(), but I think we are better off
being explicit in all cases. Note that for L2 (or higher) the grant
check isn't strictly necessary, as grants are only ever single pages.
Leave a respective assertion.

With _PAGE_PRESENT checked uniformly, the suspicious mfn_valid(omfn)
checks can be dropped rather than moved/folded - if anything we'd need
to compare against INVALID_MFN, but that won't come out of l1e_get_mfn().

For L1 replace the moved out condition with a PTE comparison: There's
no need for any update or flushing when the two match.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Tim Deegan <tim@xen.org>

xen/arch/x86/mm/shadow/hvm.c

@@ -810,19 +810,27 @@ static void cf_check sh_unshadow_for_p2m_change(
     if ( unlikely(!d->arch.paging.shadow.total_pages) )
         return;
 
+    /* Only previously present / valid entries need processing. */
+    if ( !(oflags & _PAGE_PRESENT) ||
+         (!p2m_is_valid(p2mt) && !p2m_is_grant(p2mt)) )
+        return;
+
     switch ( level )
     {
     default:
         /*
          * The following assertion is to make sure we don't step on 1GB host
-         * page support of HVM guest.
+         * page support of HVM guest. Plus we rely on ->set_entry() to never
+         * be called with orders above PAGE_ORDER_2M, not even to install
+         * non-present entries (which in principle ought to be fine even
+         * without respective large page support).
          */
-        ASSERT(!((oflags & _PAGE_PRESENT) && (oflags & _PAGE_PSE)));
+        ASSERT(!(oflags & _PAGE_PSE));
         break;
 
     /* If we're removing an MFN from the p2m, remove it from the shadows too */
     case 1:
-        if ( (p2m_is_valid(p2mt) || p2m_is_grant(p2mt)) && mfn_valid(omfn) )
+        if ( l1e_get_intpte(old) != l1e_get_intpte(new) )
         {
             sh_remove_all_shadows_and_parents(d, omfn);
             if ( sh_remove_all_mappings(d, omfn, _gfn(gfn)) )
@@ -836,10 +844,11 @@ static void cf_check sh_unshadow_for_p2m_change(
      * scheme, that's OK, but otherwise they must be unshadowed.
      */
     case 2:
-        if ( !(oflags & _PAGE_PRESENT) || !(oflags & _PAGE_PSE) )
+        if ( !(oflags & _PAGE_PSE) )
             break;
 
-        if ( p2m_is_valid(p2mt) && mfn_valid(omfn) )
+        ASSERT(!p2m_is_grant(p2mt));
+
         {
             unsigned int i;
             mfn_t nmfn = l1e_get_mfn(new);