KVM: x86/mmu: Require mmu_lock be held for write in unyielding root iter

Assert that mmu_lock is held for write by users of the yield-unfriendly
TDP iterator.  The nature of a shared walk means that the caller needs to
play nice with other tasks modifying the page tables, which is more or
less the same thing as playing nice with yielding.  Theoretically, KVM
could gain a flow where it could legitimately take mmu_lock for read in
a non-preemptible context, but that's highly unlikely and any such case
should be viewed with a fair amount of scrutiny.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Ben Gardon <bgardon@google.com>
Message-Id: <20220226001546.360188-7-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/tdp_mmu.c

@@ -29,13 +29,16 @@ bool kvm_mmu_init_tdp_mmu(struct kvm *kvm)
 	return true;
 }
 
-static __always_inline void kvm_lockdep_assert_mmu_lock_held(struct kvm *kvm,
+/* Arbitrarily returns true so that this may be used in if statements. */
+static __always_inline bool kvm_lockdep_assert_mmu_lock_held(struct kvm *kvm,
 							     bool shared)
 {
 	if (shared)
 		lockdep_assert_held_read(&kvm->mmu_lock);
 	else
 		lockdep_assert_held_write(&kvm->mmu_lock);
+
+	return true;
 }
 
 void kvm_mmu_uninit_tdp_mmu(struct kvm *kvm)
@@ -172,11 +175,17 @@ static struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm,
 #define for_each_tdp_mmu_root_yield_safe(_kvm, _root, _as_id, _shared)		\
 	__for_each_tdp_mmu_root_yield_safe(_kvm, _root, _as_id, _shared, false)
 
-#define for_each_tdp_mmu_root(_kvm, _root, _as_id)				\
-	list_for_each_entry_rcu(_root, &_kvm->arch.tdp_mmu_roots, link,		\
-				lockdep_is_held_type(&kvm->mmu_lock, 0) ||	\
-				lockdep_is_held(&kvm->arch.tdp_mmu_pages_lock))	\
-		if (kvm_mmu_page_as_id(_root) != _as_id) {		\
+/*
+ * Iterate over all TDP MMU roots.  Requires that mmu_lock be held for write,
+ * the implication being that any flow that holds mmu_lock for read is
+ * inherently yield-friendly and should use the yield-safe variant above.
+ * Holding mmu_lock for write obviates the need for RCU protection as the list
+ * is guaranteed to be stable.
+ */
+#define for_each_tdp_mmu_root(_kvm, _root, _as_id)			\
+	list_for_each_entry(_root, &_kvm->arch.tdp_mmu_roots, link)	\
+		if (kvm_lockdep_assert_mmu_lock_held(_kvm, false) &&	\
+		    kvm_mmu_page_as_id(_root) != _as_id) {		\
 		} else
 
 static struct kvm_mmu_page *tdp_mmu_alloc_sp(struct kvm_vcpu *vcpu)