1 files changed, 20 insertions, 6 deletions

diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index 95a64d1dccc..810c1da2ee4 100644
--- a/arch/x86/kvm/paging_tmpl.h
+++ b/arch/x86/kvm/paging_tmpl.h
@@ -151,7 +151,7 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker,
         pt_element_t pte;
         pt_element_t __user *uninitialized_var(ptep_user);
         gfn_t table_gfn;
-        unsigned index, pt_access, pte_access;
+        unsigned index, pt_access, pte_access, accessed_dirty, shift;
         gpa_t pte_gpa;
         int offset;
         const int write_fault = access & PFERR_WRITE_MASK;
@@ -180,6 +180,7 @@ retry_walk:
         ASSERT((!is_long_mode(vcpu) && is_pae(vcpu)) ||
                (mmu->get_cr3(vcpu) & CR3_NONPAE_RESERVED_BITS) == 0);
 
+        accessed_dirty = PT_ACCESSED_MASK;
         pt_access = pte_access = ACC_ALL;
         ++walker->level;
 
@@ -224,6 +225,7 @@ retry_walk:
                         goto error;
                 }
 
+                accessed_dirty &= pte;
                 pte_access = pt_access & gpte_access(vcpu, pte);
 
                 walker->ptes[walker->level - 1] = pte;
@@ -251,11 +253,23 @@ retry_walk:
         if (!write_fault)
                 protect_clean_gpte(&pte_access, pte);
 
-        ret = FNAME(update_accessed_dirty_bits)(vcpu, mmu, walker, write_fault);
-        if (unlikely(ret < 0))
-                goto error;
-        else if (ret)
-                goto retry_walk;
+        /*
+         * On a write fault, fold the dirty bit into accessed_dirty by shifting it one
+         * place right.
+         *
+         * On a read fault, do nothing.
+         */
+        shift = write_fault >> ilog2(PFERR_WRITE_MASK);
+        shift *= PT_DIRTY_SHIFT - PT_ACCESSED_SHIFT;
+        accessed_dirty &= pte >> shift;
+
+        if (unlikely(!accessed_dirty)) {
+                ret = FNAME(update_accessed_dirty_bits)(vcpu, mmu, walker, write_fault);
+                if (unlikely(ret < 0))
+                        goto error;
+                else if (ret)
+                        goto retry_walk;
+        }
 
         walker->pt_access = pt_access;
         walker->pte_access = pte_access;