Re: [PATCH] KVM: x86/mmu: Skip !MMU-present SPTEs when removing SP in exclusive mode

[Paolo Bonzini]

On 10/03/21 01:30, Sean Christopherson wrote:
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 50ef757c5586..f0c99fa04ef2 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -323,7 +323,18 @@ static void handle_removed_tdp_mmu_page(struct kvm *kvm, 
u64 *pt,
                                cpu_relax();
                        }
                } else {
+                       /*
+                        * If the SPTE is not MMU-present, there is no backing
+                        * page associated with the SPTE and so no side effects
+                        * that need to be recorded, and exclusive ownership of
+                        * mmu_lock ensures the SPTE can't be made present.
+                        * Note, zapping MMIO SPTEs is also unnecessary as they
+                        * are guarded by the memslots generation, not by being
+                        * unreachable.
+                        */
                        old_child_spte = READ_ONCE(*sptep);
+                       if (!is_shadow_present_pte(old_child_spte))
+                               continue;
  
  			/*
                         * Marking the SPTE as a removed SPTE is not

Ben, do you plan to make this path take mmu_lock for read?  If so, this 
wouldn't be too useful IIUC.

Paolo