KVM: x86/tdp_mmu: Morph !is_frozen_spte() check into a KVM_MMU_WARN_ON()

Remove the conditional logic for handling the setting of mirror page table
to frozen in __tdp_mmu_set_spte_atomic() and add it as a warning for both
mirror and direct cases.

The mirror page table needs to propagate PTE changes to the external page
table. This presents a problem for atomic updates which can't update both
page tables at once. So a special value, FROZEN_SPTE, is used as a
temporary state during these updates to prevent concurrent operations on
the PTE. If the TDP MMU tried to install FROZEN_SPTE as a long-term value,
it would confuse these updates.

On the other hand, it would also confuse other threads if FROZEN_SPTE is
installed as a long-term value for direct page tables (e.g., causing
another thread working on atomic zap to wait for a !FROZEN_SPTE value
endlessly).

Therefore, add the warning for installing FROZEN_SPTE as a long-term value
in __tdp_mmu_set_spte_atomic() without differentiating whether it's a
mirror or direct page table.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Link: https://patch.msgid.link/20260509075609.4242-1-yan.y.zhao@intel.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index dc455e6e7dc77c4a9d4d2db161cb4c6c9fa1b8c9..b30e33dea2653341a9521ab302a76175298b22d2 100644 (file)
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -609,7 +609,10 @@ static inline int __must_check __tdp_mmu_set_spte_atomic(struct kvm *kvm,
         */
        WARN_ON_ONCE(iter->yielded || is_frozen_spte(iter->old_spte));
 
-       if (is_mirror_sptep(iter->sptep) && !is_frozen_spte(new_spte)) {
+       /* Should not set FROZEN_SPTE as a long-term value. */
+       KVM_MMU_WARN_ON(is_frozen_spte(new_spte));
+
+       if (is_mirror_sptep(iter->sptep)) {
                int ret;
 
                /*