validate_guest_pt_write(), by calling sh_validate_guest_entry(), already
guarantees the needed update of log-dirty information. Move the
operation into the sole code path needing it (when SHOPT_SKIP_VERIFY is
enabled), making clear that only one such call is needed.

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

--- a/xen/arch/x86/mm/shadow/hvm.c
+++ b/xen/arch/x86/mm/shadow/hvm.c
@@ -656,6 +656,7 @@ static void sh_emulate_unmap_dest(struct
     {
         /* Writes with this alignment constraint can't possibly cross pages. */
         ASSERT(!mfn_valid(sh_ctxt->mfn[1]));
+        paging_mark_dirty(v->domain, sh_ctxt->mfn[0]);
     }
     else
 #endif /* SHADOW_OPTIMIZATIONS & SHOPT_SKIP_VERIFY */
@@ -673,12 +674,10 @@ static void sh_emulate_unmap_dest(struct
             validate_guest_pt_write(v, sh_ctxt->mfn[1], addr + b1, b2);
     }
 
-    paging_mark_dirty(v->domain, sh_ctxt->mfn[0]);
     put_page(mfn_to_page(sh_ctxt->mfn[0]));
 
     if ( unlikely(mfn_valid(sh_ctxt->mfn[1])) )
     {
-        paging_mark_dirty(v->domain, sh_ctxt->mfn[1]);
         put_page(mfn_to_page(sh_ctxt->mfn[1]));
         vunmap((void *)((unsigned long)addr & PAGE_MASK));
     }