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From:   Paolo Bonzini <pbonzini@redhat.com>
To:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Cc:     seanjc@google.com, mlevitsk@redhat.com
Subject: [PATCH v2 1/4] KVM: x86/mmu: remove unnecessary "bool shared" argument from functions
Date:   Sat, 25 Nov 2023 03:33:57 -0500
Message-Id: <20231125083400.1399197-2-pbonzini@redhat.com>
In-Reply-To: <20231125083400.1399197-1-pbonzini@redhat.com>
References: <20231125083400.1399197-1-pbonzini@redhat.com>
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Neither tdp_mmu_next_root nor kvm_tdp_mmu_put_root need to know
if the lock is taken for read or write.  Either way, protection
is achieved via RCU and tdp_mmu_pages_lock.  Remove the argument
and just assert that the lock is taken.

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 arch/x86/kvm/mmu/mmu.c     |  2 +-
 arch/x86/kvm/mmu/tdp_mmu.c | 34 +++++++++++++++++++++-------------
 arch/x86/kvm/mmu/tdp_mmu.h |  3 +--
 3 files changed, 23 insertions(+), 16 deletions(-)

	v1->v2: comment tweak

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index c57e181bba21..1cb81573a60b 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3556,7 +3556,7 @@ static void mmu_free_root_page(struct kvm *kvm, hpa_t *root_hpa,
 		return;
 
 	if (is_tdp_mmu_page(sp))
-		kvm_tdp_mmu_put_root(kvm, sp, false);
+		kvm_tdp_mmu_put_root(kvm, sp);
 	else if (!--sp->root_count && sp->role.invalid)
 		kvm_mmu_prepare_zap_page(kvm, sp, invalid_list);
 
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 6cd4dd631a2f..05689c8d45b7 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -73,10 +73,13 @@ static void tdp_mmu_free_sp_rcu_callback(struct rcu_head *head)
 	tdp_mmu_free_sp(sp);
 }
 
-void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
-			  bool shared)
+void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root)
 {
-	kvm_lockdep_assert_mmu_lock_held(kvm, shared);
+	/*
+	 * Either read or write is okay, but mmu_lock must be held because
+	 * writers are not required to take tdp_mmu_pages_lock.
+	 */
+	lockdep_assert_held(&kvm->mmu_lock);
 
 	if (!refcount_dec_and_test(&root->tdp_mmu_root_count))
 		return;
@@ -106,10 +109,16 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
  */
 static struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm,
 					      struct kvm_mmu_page *prev_root,
-					      bool shared, bool only_valid)
+					      bool only_valid)
 {
 	struct kvm_mmu_page *next_root;
 
+	/*
+	 * While the roots themselves are RCU-protected, fields such as
+	 * role.invalid are protected by mmu_lock.
+	 */
+	lockdep_assert_held(&kvm->mmu_lock);
+
 	rcu_read_lock();
 
 	if (prev_root)
@@ -132,7 +141,7 @@ static struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm,
 	rcu_read_unlock();
 
 	if (prev_root)
-		kvm_tdp_mmu_put_root(kvm, prev_root, shared);
+		kvm_tdp_mmu_put_root(kvm, prev_root);
 
 	return next_root;
 }
@@ -144,13 +153,12 @@ static struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm,
  * recent root. (Unless keeping a live reference is desirable.)
  *
  * If shared is set, this function is operating under the MMU lock in read
- * mode. In the unlikely event that this thread must free a root, the lock
- * will be temporarily dropped and reacquired in write mode.
+ * mode.
  */
 #define __for_each_tdp_mmu_root_yield_safe(_kvm, _root, _as_id, _shared, _only_valid)\
-	for (_root = tdp_mmu_next_root(_kvm, NULL, _shared, _only_valid);	\
+	for (_root = tdp_mmu_next_root(_kvm, NULL, _only_valid);		\
 	     _root;								\
-	     _root = tdp_mmu_next_root(_kvm, _root, _shared, _only_valid))	\
+	     _root = tdp_mmu_next_root(_kvm, _root, _only_valid))		\
 		if (kvm_lockdep_assert_mmu_lock_held(_kvm, _shared) &&		\
 		    kvm_mmu_page_as_id(_root) != _as_id) {			\
 		} else
@@ -159,9 +167,9 @@ static struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm,
 	__for_each_tdp_mmu_root_yield_safe(_kvm, _root, _as_id, _shared, true)
 
 #define for_each_tdp_mmu_root_yield_safe(_kvm, _root, _shared)			\
-	for (_root = tdp_mmu_next_root(_kvm, NULL, _shared, false);		\
+	for (_root = tdp_mmu_next_root(_kvm, NULL, false);			\
 	     _root;								\
-	     _root = tdp_mmu_next_root(_kvm, _root, _shared, false))		\
+	     _root = tdp_mmu_next_root(_kvm, _root, false))			\
 		if (!kvm_lockdep_assert_mmu_lock_held(_kvm, _shared)) {		\
 		} else
 
@@ -891,7 +899,7 @@ void kvm_tdp_mmu_zap_invalidated_roots(struct kvm *kvm)
 		 * the root must be reachable by mmu_notifiers while it's being
 		 * zapped
 		 */
-		kvm_tdp_mmu_put_root(kvm, root, true);
+		kvm_tdp_mmu_put_root(kvm, root);
 	}
 
 	read_unlock(&kvm->mmu_lock);
@@ -1500,7 +1508,7 @@ void kvm_tdp_mmu_try_split_huge_pages(struct kvm *kvm,
 	for_each_valid_tdp_mmu_root_yield_safe(kvm, root, slot->as_id, shared) {
 		r = tdp_mmu_split_huge_pages_root(kvm, root, start, end, target_level, shared);
 		if (r) {
-			kvm_tdp_mmu_put_root(kvm, root, shared);
+			kvm_tdp_mmu_put_root(kvm, root);
 			break;
 		}
 	}
diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h
index 733a3aef3a96..20d97aa46c49 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.h
+++ b/arch/x86/kvm/mmu/tdp_mmu.h
@@ -17,8 +17,7 @@ __must_check static inline bool kvm_tdp_mmu_get_root(struct kvm_mmu_page *root)
 	return refcount_inc_not_zero(&root->tdp_mmu_root_count);
 }
 
-void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
-			  bool shared);
+void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root);
 
 bool kvm_tdp_mmu_zap_leafs(struct kvm *kvm, gfn_t start, gfn_t end, bool flush);
 bool kvm_tdp_mmu_zap_sp(struct kvm *kvm, struct kvm_mmu_page *sp);
-- 
2.39.1