[PATCH 10/14] KVM: x86: Further clarify the logic and comments for toggling log dirty

Sean Christopherson
2021-02-13T00:52:53Z

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>
---
arch/x86/kvm/x86.c | 15 +++++++++++----
1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e89fe98a0099..c0d22f19aed0 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10761,12 +10761,20 @@ 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
@@ -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);

/*
--
2.30.0.478.g8a0d178c01-goog