Subject: x86/shadow: move OOS flag bit positions
From: Jan Beulich <[email-masked]>

In preparation of reducing struct page_info's shadow_flags field to 16
bits, lower the bit positions used for SHF_out_of_sync and
SHF_oos_may_write.

Instead of also adjusting the open coded use in _get_page_type(),
introduce shadow_prepare_page_type_change() to contain knowledge of the
bit positions to shadow code.

This is part of XSA-280.

Signed-off-by: Jan Beulich <[email-masked]>
Reviewed-by: Tim Deegan <[email-masked]>
---
v2: Rename function and pass full type.

--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -2712,17 +2712,8 @@ static int _get_page_type(struct page_in
         {
             struct domain *d = page_get_owner(page);
 
-            /*
-             * Normally we should never let a page go from type count 0
-             * to type count 1 when it is shadowed. One exception:
-             * out-of-sync shadowed pages are allowed to become
-             * writeable.
-             */
-            if ( d && shadow_mode_enabled(d)
-                 && (page->count_info & PGC_page_table)
-                 && !((page->shadow_flags & (1u<<29))
-                      && type == PGT_writable_page) )
-               shadow_remove_all_shadows(d, page_to_mfn(page));
+            if ( d && shadow_mode_enabled(d) )
+               shadow_prepare_page_type_change(d, page, type);
 
             ASSERT(!(x & PGT_pae_xen_l2));
             if ( (x & PGT_type_mask) != type )
--- a/xen/arch/x86/mm/shadow/common.c
+++ b/xen/arch/x86/mm/shadow/common.c
@@ -749,6 +749,9 @@ int sh_unsync(struct vcpu *v, mfn_t gmfn
          || !v->domain->arch.paging.shadow.oos_active )
         return 0;
 
+    BUILD_BUG_ON(!(typeof(pg->shadow_flags))SHF_out_of_sync);
+    BUILD_BUG_ON(!(typeof(pg->shadow_flags))SHF_oos_may_write);
+
     pg->shadow_flags |= SHF_out_of_sync|SHF_oos_may_write;
     oos_hash_add(v, gmfn);
     perfc_incr(shadow_unsync);
@@ -2413,6 +2416,26 @@ void sh_remove_shadows(struct domain *d,
     paging_unlock(d);
 }
 
+void shadow_prepare_page_type_change(struct domain *d, struct page_info *page,
+                                     unsigned long new_type)
+{
+    if ( !(page->count_info & PGC_page_table) )
+        return;
+
+#if (SHADOW_OPTIMIZATIONS & SHOPT_OUT_OF_SYNC)
+    /*
+     * Normally we should never let a page go from type count 0 to type
+     * count 1 when it is shadowed. One exception: out-of-sync shadowed
+     * pages are allowed to become writeable.
+     */
+    if ( (page->shadow_flags & SHF_oos_may_write) &&
+         new_type == PGT_writable_page )
+        return;
+#endif
+
+    shadow_remove_all_shadows(d, page_to_mfn(page));
+}
+
 static void
 sh_remove_all_shadows_and_parents(struct domain *d, mfn_t gmfn)
 /* Even harsher: this is a HVM page that we thing is no longer a pagetable.
--- a/xen/arch/x86/mm/shadow/private.h
+++ b/xen/arch/x86/mm/shadow/private.h
@@ -285,8 +285,8 @@ static inline void sh_terminate_list(str
  * codepath is called during that time and is sensitive to oos issues, it may
  * need to use the second flag.
  */
-#define SHF_out_of_sync (1u<<30)
-#define SHF_oos_may_write (1u<<29)
+#define SHF_out_of_sync (1u << (SH_type_max_shadow + 1))
+#define SHF_oos_may_write (1u << (SH_type_max_shadow + 2))
 
 #endif /* (SHADOW_OPTIMIZATIONS & SHOPT_OUT_OF_SYNC) */
 
--- a/xen/include/asm-x86/shadow.h
+++ b/xen/include/asm-x86/shadow.h
@@ -81,6 +81,10 @@ void shadow_final_teardown(struct domain
 
 void sh_remove_shadows(struct domain *d, mfn_t gmfn, int fast, int all);
 
+/* Adjust shadows ready for a guest page to change its type. */
+void shadow_prepare_page_type_change(struct domain *d, struct page_info *page,
+                                     unsigned long new_type);
+
 /* Discard _all_ mappings from the domain's shadows. */
 void shadow_blow_tables_per_domain(struct domain *d);
 
@@ -105,6 +109,10 @@ int shadow_set_allocation(struct domain
 static inline void sh_remove_shadows(struct domain *d, mfn_t gmfn,
                                      int fast, int all) {}
 
+static inline void shadow_prepare_page_type_change(struct domain *d,
+                                                   struct page_info *page,
+                                                   unsigned long new_type) {}
+
 static inline void shadow_blow_tables_per_domain(struct domain *d) {}
 
 static inline int shadow_domctl(struct domain *d,