[PATCH] x86/PV: make post-migration page state consistent

[Jan Beulich <jbeulich@xxxxxxxx>]

When a page table page gets de-validated, its type reference count drops
to zero (and PGT_validated gets cleared), but its type remains intact.
XEN_DOMCTL_getpageframeinfo3, therefore, so far reported prior usage for
such pages. An intermediate write to such a page via e.g.
MMU_NORMAL_PT_UPDATE, however, would transition the page's type to
PGT_writable_page, thus altering what XEN_DOMCTL_getpageframeinfo3 would
return. In libxc the decision which pages to normalize / localize
depends solely on the type returned from the domctl. As a result without
further precautions the guest won't be able to tell whether such a page
has had its (apparent) PTE entries transitioned to the new MFNs.

Add a check of PGT_validated, thus consistently avoiding normalization /
localization in the tool stack.

Alongside using XEN_DOMCTL_PFINFO_NOTAB instead of plain zero for the
change at hand, also change the variable's initializer to use this
constant, too. Take the opportunity and also adjust its type.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/domctl.c
+++ b/xen/arch/x86/domctl.c
@@ -215,7 +215,8 @@ long arch_do_domctl(
 
         for ( i = 0; i < num; ++i )
         {
-            unsigned long gfn = 0, type = 0;
+            unsigned long gfn = 0;
+            unsigned int type = XEN_DOMCTL_PFINFO_NOTAB;
             struct page_info *page;
             p2m_type_t t;
 
@@ -255,6 +256,8 @@ long arch_do_domctl(
 
                 if ( page->u.inuse.type_info & PGT_pinned )
                     type |= XEN_DOMCTL_PFINFO_LPINTAB;
+                else if ( !(page->u.inuse.type_info & PGT_validated) )
+                    type = XEN_DOMCTL_PFINFO_NOTAB;
 
                 if ( page->count_info & PGC_broken )
                     type = XEN_DOMCTL_PFINFO_BROKEN;