KVM: MMU: Make flooding detection work when guest page faults are bypassed

When we allow guest page faults to reach the guests directly, we lose
the fault tracking which allows us to detect demand paging.  So we provide
an alternate mechnism by clearing the accessed bit when we set a pte, and
checking it later to see if the guest actually used it.

Signed-off-by: Avi Kivity <avi@qumranet.com>

1 files changed, 8 insertions, 1 deletions

diff --git a/drivers/kvm/paging_tmpl.h b/drivers/kvm/paging_tmpl.h
index 99ac9b15f773..be0f85231da9 100644
--- a/drivers/kvm/paging_tmpl.h
+++ b/drivers/kvm/paging_tmpl.h
@@ -238,7 +238,12 @@ static void FNAME(set_pte_common)(struct kvm_vcpu *vcpu,
                 FNAME(mark_pagetable_dirty)(vcpu->kvm, walker);
         }
 
-        spte = PT_PRESENT_MASK | PT_ACCESSED_MASK | PT_DIRTY_MASK;
+        /*
+         * We don't set the accessed bit, since we sometimes want to see
+         * whether the guest actually used the pte (in order to detect
+         * demand paging).
+         */
+        spte = PT_PRESENT_MASK | PT_DIRTY_MASK;
         spte |= gpte & PT64_NX_MASK;
         if (!dirty)
                 access_bits &= ~PT_WRITABLE_MASK;
@@ -291,6 +296,8 @@ unshadowed:
         page_header_update_slot(vcpu->kvm, shadow_pte, gaddr);
         if (!was_rmapped)
                 rmap_add(vcpu, shadow_pte);
+        if (!ptwrite || !*ptwrite)
+                vcpu->last_pte_updated = shadow_pte;
 }
 
 static void FNAME(set_pte)(struct kvm_vcpu *vcpu, pt_element_t gpte,