[PATCH v4 25/30] KVM: x86/mmu: Check for a REMOVED leaf SPTE before making the SPTE

[Paolo Bonzini <pbonzini@redhat.com>]

From: Sean Christopherson <seanjc@google.com>

Explicitly check for a REMOVED leaf SPTE prior to attempting to map
the final SPTE when handling a TDP MMU fault.  Functionally, this is a
nop as tdp_mmu_set_spte_atomic() will eventually detect the frozen SPTE.
Pre-checking for a REMOVED SPTE is a minor optmization, but the real goal
is to allow tdp_mmu_set_spte_atomic() to have an invariant that the "old"
SPTE is never a REMOVED SPTE.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Ben Gardon <bgardon@google.com>
Message-Id: <20220226001546.360188-24-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 arch/x86/kvm/mmu/tdp_mmu.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 2456f880508d..89e6eb6640fe 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -1202,7 +1202,11 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 		}
 	}
 
-	if (iter.level != fault->goal_level) {
+	/*
+	 * Force the guest to retry the access if the upper level SPTEs aren't
+	 * in place, or if the target leaf SPTE is frozen by another CPU.
+	 */
+	if (iter.level != fault->goal_level || is_removed_spte(iter.old_spte)) {
 		rcu_read_unlock();
 		return RET_PF_RETRY;
 	}
-- 
2.31.1