KVM: MMU: Avoid access/dirty update loop if all is well

Keep track of accessed/dirty bits; if they are all set, do not
enter the accessed/dirty update loop.

Reviewed-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index 95a64d1dccc795c4f6d2ef5e62d127d70c809eed..810c1da2ee44f26d9dd9939e53ef0e76aad20b12 100644 (file)
--- 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;