[PATCH v2 2/4] x86/shadow: slightly consolidate sh_unshadow_for_p2m_change() (part II)

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

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

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v2: Split from previous bigger patch. Add grant related assertion for
    L2.

--- a/xen/arch/x86/mm/shadow/hvm.c
+++ b/xen/arch/x86/mm/shadow/hvm.c
@@ -798,7 +798,7 @@ static void cf_check sh_unshadow_for_p2m
     struct domain *d, unsigned long gfn, l1_pgentry_t old, l1_pgentry_t new,
     unsigned int level)
 {
-    mfn_t omfn = l1e_get_mfn(old);
+    mfn_t omfn = l1e_get_mfn(old), nmfn;
     unsigned int oflags = l1e_get_flags(old);
     p2m_type_t p2mt = p2m_flags_to_type(oflags);
     bool flush = false;
@@ -810,19 +810,30 @@ static void cf_check sh_unshadow_for_p2m
     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)) ||
+         !mfn_valid(omfn) )
+        return;
+
+    nmfn = l1e_get_flags(new) & _PAGE_PRESENT ? l1e_get_mfn(new) : INVALID_MFN;
+
     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 ( !mfn_eq(nmfn, omfn) )
         {
             sh_remove_all_shadows_and_parents(d, omfn);
             if ( sh_remove_all_mappings(d, omfn, _gfn(gfn)) )
@@ -836,13 +847,13 @@ static void cf_check sh_unshadow_for_p2m
      * 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);
             l1_pgentry_t *npte = NULL;
 
             /* If we're replacing a superpage with a normal L1 page, map it */