[PATCH 2/2] x86/shadow: streamline shadow_get_page_from_l1e()

[Jan Beulich <jbeulich@suse.com>]

Trying get_page_from_l1e() up to three times isn't helpful; in debug
builds it may lead to log messages mking things look as if there was a
problem somewhere. And there's no need to have more than one try: The
function can only possibly succeed for one domain passed as 3rd
argument (unless the page is an MMIO one to which both have access,
but MMIO pages should be "got" by specifying the requesting domain
anyway). Re-arrange things so just the one call gets made which has a
chance of succeeding.

The code could in principle be arranged such that there's only a single
call to get_page_from_l1e(), but the conditional would become pretty
complex then and hence hard to follow / understand / adjust.

The redundant (with shadow_mode_refcounts()) shadow_mode_translate()
gets dropped.

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

--- a/xen/arch/x86/mm/shadow/set.c
+++ b/xen/arch/x86/mm/shadow/set.c
@@ -112,40 +112,36 @@ shadow_get_page_from_l1e(shadow_l1e_t sl
         return 0;
     }
 
-    res = get_page_from_l1e(sl1e, d, d);
+    if ( owner == dom_io )
+        owner = NULL;
 
     /*
      * If a privileged domain is attempting to install a map of a page it does
      * not own, we let it succeed anyway.
      */
-    if ( unlikely(res < 0) &&
-         !shadow_mode_translate(d) &&
-         owner && (d != owner) )
+    if ( owner && (d != owner) &&
+         !(res = xsm_priv_mapping(XSM_TARGET, d, owner)) )
     {
-        res = xsm_priv_mapping(XSM_TARGET, d, owner);
-        if ( !res )
-        {
-            res = get_page_from_l1e(sl1e, d, owner);
-            SHADOW_PRINTK("privileged %pd installs map of mfn %"PRI_mfn" owned by %pd: %s\n",
-                           d, mfn_x(mfn), owner,
-                           res >= 0 ? "success" : "failed");
-        }
+        res = get_page_from_l1e(sl1e, d, owner);
+        SHADOW_PRINTK("privileged %pd installs map of %pd's mfn %"PRI_mfn": %s\n",
+                      d, owner, mfn_x(mfn),
+                      res >= 0 ? "success" : "failed");
     }
-
     /* Okay, it might still be a grant mapping PTE.  Try it. */
-    if ( unlikely(res < 0) &&
-         (type == p2m_grant_map_rw ||
-          (type == p2m_grant_map_ro &&
-           !(shadow_l1e_get_flags(sl1e) & _PAGE_RW))) )
+    else if ( owner &&
+              (type == p2m_grant_map_rw ||
+               (type == p2m_grant_map_ro &&
+                !(shadow_l1e_get_flags(sl1e) & _PAGE_RW))) )
     {
         /*
          * It's a grant mapping.  The grant table implementation will
          * already have checked that we're supposed to have access, so
          * we can just grab a reference directly.
          */
-        if ( owner )
-            res = get_page_from_l1e(sl1e, d, owner);
+        res = get_page_from_l1e(sl1e, d, owner);
     }
+    else
+        res = get_page_from_l1e(sl1e, d, d);
 
     if ( unlikely(res < 0) )
     {