Don't unshadow when the shadow l2 is the current used shadow table.

Otherwise, when destroying the domain, if the current sl2 is unpinned
then the shadow page will be freed without accquire the shadow_lock.
This may also improve the performance due to avoiding uncessary
unshadow.

Signed-off-by: Xiaofeng Ling <xiaofeng.ling@intel.com>

xen/arch/x86/shadow.c

@@ -1450,6 +1450,7 @@ static int resync_all(struct domain *d, u32 stype)
     int changed;
     u32 min_max_shadow, min_max_snapshot;
     int min_shadow, max_shadow, min_snapshot, max_snapshot;
+    struct vcpu *v;
 
     ASSERT(shadow_lock_is_acquired(d));
 
@@ -1739,6 +1740,9 @@ static int resync_all(struct domain *d, u32 stype)
 
         if ( unlikely(unshadow) )
         {
+            for_each_vcpu(d, v)
+                if(smfn == pagetable_get_pfn(v->arch.shadow_table))
+                    return need_flush;
             perfc_incrc(unshadow_l2_count);
             shadow_unpin(smfn);
 #if CONFIG_PAGING_LEVELS == 2

xen/arch/x86/shadow32.c

@@ -2326,6 +2326,7 @@ static int resync_all(struct domain *d, u32 stype)
     int changed;
     u32 min_max_shadow, min_max_snapshot;
     int min_shadow, max_shadow, min_snapshot, max_snapshot;
+    struct vcpu *v;
 
     ASSERT(shadow_lock_is_acquired(d));
 
@@ -2527,6 +2528,9 @@ static int resync_all(struct domain *d, u32 stype)
 
         if ( unlikely(unshadow) )
         {
+            for_each_vcpu(d, v)
+                if(smfn == pagetable_get_pfn(v->arch.shadow_table))
+                    return need_flush;
             perfc_incrc(unshadow_l2_count);
             shadow_unpin(smfn);
             if ( unlikely(shadow_mode_external(d)) )