KVM: x86: flush TLB when D bit is manually changed.

When software changes D bit (either from 1 to 0, or 0 to 1), the
corresponding TLB entity in the hardware won't be updated immediately. We
should flush it to guarantee the consistence of D bit between TLB and
MMU page table in memory.  This is especially important when clearing
the D bit, since it may cause false negatives in reporting dirtiness.

Sanity test was done on my machine with Intel processor.
Signed-off-by: Kai Huang <kai.huang@linux.intel.com>
[Check A bit too. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu.c

@@ -532,6 +532,11 @@ static bool spte_is_bit_cleared(u64 old_spte, u64 new_spte, u64 bit_mask)
 	return (old_spte & bit_mask) && !(new_spte & bit_mask);
 }
 
+static bool spte_is_bit_changed(u64 old_spte, u64 new_spte, u64 bit_mask)
+{
+	return (old_spte & bit_mask) != (new_spte & bit_mask);
+}
+
 /* Rules for using mmu_spte_set:
  * Set the sptep from nonpresent to present.
  * Note: the sptep being assigned *must* be either not present
@@ -582,6 +587,14 @@ static bool mmu_spte_update(u64 *sptep, u64 new_spte)
 	if (!shadow_accessed_mask)
 		return ret;
 
+	/*
+	 * Flush TLB when accessed/dirty bits are changed in the page tables,
+	 * to guarantee consistency between TLB and page tables.
+	 */
+	if (spte_is_bit_changed(old_spte, new_spte,
+                                shadow_accessed_mask | shadow_dirty_mask))
+		ret = true;
+
 	if (spte_is_bit_cleared(old_spte, new_spte, shadow_accessed_mask))
 		kvm_set_pfn_accessed(spte_to_pfn(old_spte));
 	if (spte_is_bit_cleared(old_spte, new_spte, shadow_dirty_mask))