nEPT: Support shadow paging for guest paging without A/D bits

Some guest paging modes do not support A/D bits. Add support for such
modes in shadow page code. For such modes PT_GUEST_DIRTY_MASK,
PT_GUEST_ACCESSED_MASK, PT_GUEST_DIRTY_SHIFT and PT_GUEST_ACCESSED_SHIFT
should be set to zero.

Reviewed-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 13 insertions, 3 deletions

diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index 83702a68e987..656f7fae312a 100644
--- a/arch/x86/kvm/paging_tmpl.h
+++ b/arch/x86/kvm/paging_tmpl.h
@@ -92,6 +92,10 @@ static inline void FNAME(protect_clean_gpte)(unsigned *access, unsigned gpte)
 {
         unsigned mask;
 
+        /* dirty bit is not supported, so no need to track it */
+        if (!PT_GUEST_DIRTY_MASK)
+                return;
+
         BUILD_BUG_ON(PT_WRITABLE_MASK != ACC_WRITE_MASK);
 
         mask = (unsigned)~ACC_WRITE_MASK;
@@ -147,7 +151,8 @@ static bool FNAME(prefetch_invalid_gpte)(struct kvm_vcpu *vcpu,
         if (!FNAME(is_present_gpte)(gpte))
                 goto no_present;
 
-        if (!(gpte & PT_GUEST_ACCESSED_MASK))
+        /* if accessed bit is not supported prefetch non accessed gpte */
+        if (PT_GUEST_ACCESSED_MASK && !(gpte & PT_GUEST_ACCESSED_MASK))
                 goto no_present;
 
         return false;
@@ -178,6 +183,10 @@ static int FNAME(update_accessed_dirty_bits)(struct kvm_vcpu *vcpu,
         gfn_t table_gfn;
         int ret;
 
+        /* dirty/accessed bits are not supported, so no need to update them */
+        if (!PT_GUEST_DIRTY_MASK)
+                return 0;
+
         for (level = walker->max_level; level >= walker->level; --level) {
                 pte = orig_pte = walker->ptes[level - 1];
                 table_gfn = walker->table_gfn[level - 1];
@@ -316,8 +325,9 @@ retry_walk:
                 FNAME(protect_clean_gpte)(&pte_access, pte);
         else
                 /*
-                 * On a write fault, fold the dirty bit into accessed_dirty by
-                 * shifting it one place right.
+                 * On a write fault, fold the dirty bit into accessed_dirty.
+                 * For modes without A/D bits support accessed_dirty will be
+                 * always clear.
                  */
                 accessed_dirty &= pte >>
                         (PT_GUEST_DIRTY_SHIFT - PT_GUEST_ACCESSED_SHIFT);