kvm: set page dirty only if page has been writable
authorYu Zhao <yuzhao@google.com>
Wed, 30 Mar 2016 20:38:09 +0000 (13:38 -0700)
committerPaolo Bonzini <pbonzini@redhat.com>
Fri, 1 Apr 2016 10:10:10 +0000 (12:10 +0200)
In absence of shadow dirty mask, there is no need to set page dirty
if page has never been writable. This is a tiny optimization but
good to have for people who care much about dirty page tracking.

Signed-off-by: Yu Zhao <yuzhao@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
arch/x86/kvm/mmu.c

index 70e95d097ef104ac489a41a17dce2296dad5123c..1ff4dbb73fb7a7ee56c2c28e0d37a6a049f3adc9 100644 (file)
@@ -557,8 +557,15 @@ static bool mmu_spte_update(u64 *sptep, u64 new_spte)
              !is_writable_pte(new_spte))
                ret = true;
 
-       if (!shadow_accessed_mask)
+       if (!shadow_accessed_mask) {
+               /*
+                * We don't set page dirty when dropping non-writable spte.
+                * So do it now if the new spte is becoming non-writable.
+                */
+               if (ret)
+                       kvm_set_pfn_dirty(spte_to_pfn(old_spte));
                return ret;
+       }
 
        /*
         * Flush TLB when accessed/dirty bits are changed in the page tables,
@@ -605,7 +612,8 @@ static int mmu_spte_clear_track_bits(u64 *sptep)
 
        if (!shadow_accessed_mask || old_spte & shadow_accessed_mask)
                kvm_set_pfn_accessed(pfn);
-       if (!shadow_dirty_mask || (old_spte & shadow_dirty_mask))
+       if (old_spte & (shadow_dirty_mask ? shadow_dirty_mask :
+                                           PT_WRITABLE_MASK))
                kvm_set_pfn_dirty(pfn);
        return 1;
 }