KVM: x86: Further clarify the logic and comments for toggling log dirty

Add a sanity check in kvm_mmu_slot_apply_flags to assert that the
LOG_DIRTY_PAGES flag is indeed being toggled, and explicitly rely on
that holding true when zapping collapsible SPTEs.  Manipulating the
CPU dirty log (PML) and write-protection also relies on this assertion,
but that's not obvious in the current code.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210213005015.1651772-11-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e89fe98..c0d22f1 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10761,13 +10761,21 @@ static void kvm_mmu_slot_apply_flags(struct kvm *kvm,
                                     enum kvm_mr_change change)
 {
        /*
-        * Nothing to do for RO slots or CREATE/MOVE/DELETE of a slot.
-        * See comments below.
+        * Nothing to do for RO slots (which can't be dirtied and can't be made
+        * writable) or CREATE/MOVE/DELETE of a slot.  See comments below.
         */
        if ((change != KVM_MR_FLAGS_ONLY) || (new->flags & KVM_MEM_READONLY))
                return;
 
        /*
+        * READONLY and non-flags changes were filtered out above, and the only
+        * other flag is LOG_DIRTY_PAGES, i.e. something is wrong if dirty
+        * logging isn't being toggled on or off.
+        */
+       if (WARN_ON_ONCE(!((old->flags ^ new->flags) & KVM_MEM_LOG_DIRTY_PAGES)))
+               return;
+
+       /*
         * Dirty logging tracks sptes in 4k granularity, meaning that large
         * sptes have to be split.  If live migration is successful, the guest
         * in the source machine will be destroyed and large sptes will be
@@ -10784,8 +10792,7 @@ static void kvm_mmu_slot_apply_flags(struct kvm *kvm,
         * MOVE/DELETE: The old mappings will already have been cleaned up by
         *              kvm_arch_flush_shadow_memslot()
         */
-       if ((old->flags & KVM_MEM_LOG_DIRTY_PAGES) &&
-           !(new->flags & KVM_MEM_LOG_DIRTY_PAGES))
+       if (!(new->flags & KVM_MEM_LOG_DIRTY_PAGES))
                kvm_mmu_zap_collapsible_sptes(kvm, new);
 
        /*