[PATCH v5 0/6] KVM: lockdep improvements

[PATCH v5 0/6] KVM: lockdep improvements

[Maxim Levitsky]

This is	a continuation of my 'extract lock_all_vcpus/unlock_all_vcpus'
patch series.

Implement the suggestion of using lockdep's "nest_lock" feature
when locking all KVM vCPUs by adding mutex_trylock_nest_lock() and
mutex_lock_killable_nest_lock() and use these functions	in the
implementation of the
kvm_trylock_all_vcpus()/kvm_lock_all_vcpus()/kvm_unlock_all_vcpus().

Those changes allow removal of a custom workaround that was needed to
silence the lockdep warning in the SEV code and also stop lockdep from
complaining in case of ARM and RISC-V code which doesn't include the above
mentioned workaround.

Finally, it's worth noting that this patch series removes a fair
amount of duplicate code by implementing the logic in one place.

V5: addressed review feedback.

Best regards,
	Maxim Levitsky

Maxim Levitsky (6):
  locking/mutex: implement mutex_trylock_nested
  locking/mutex: implement mutex_lock_killable_nest_lock
  KVM: add kvm_lock_all_vcpus and kvm_trylock_all_vcpus
  x86: KVM: SVM: use kvm_lock_all_vcpus instead of a custom
    implementation
  KVM: arm64: use kvm_trylock_all_vcpus when locking all vCPUs
  RISC-V: KVM: use kvm_trylock_all_vcpus when locking all vCPUs

 arch/arm64/include/asm/kvm_host.h     |  3 --
 arch/arm64/kvm/arch_timer.c           |  4 +-
 arch/arm64/kvm/arm.c                  | 43 ----------------
 arch/arm64/kvm/vgic/vgic-init.c       |  4 +-
 arch/arm64/kvm/vgic/vgic-its.c        |  8 +--
 arch/arm64/kvm/vgic/vgic-kvm-device.c | 12 ++---
 arch/riscv/kvm/aia_device.c           | 34 +------------
 arch/x86/kvm/svm/sev.c                | 72 ++-------------------------
 include/linux/kvm_host.h              |  4 ++
 include/linux/mutex.h                 | 32 ++++++++++--
 kernel/locking/mutex.c                | 21 +++++---
 virt/kvm/kvm_main.c                   | 59 ++++++++++++++++++++++
 12 files changed, 126 insertions(+), 170 deletions(-)

-- 
2.46.0

[PATCH v5 1/6] locking/mutex: implement mutex_trylock_nested

[Maxim Levitsky]

Despite the fact that several lockdep-related checks are skipped when
calling trylock* versions of the locking primitives, for example
mutex_trylock, each time the mutex is acquired, a held_lock is still
placed onto the lockdep stack by __lock_acquire() which is called
regardless of whether the trylock* or regular locking API was used.

This means that if the caller successfully acquires more than
MAX_LOCK_DEPTH locks of the same class, even when using mutex_trylock,
lockdep will still complain that the maximum depth of the held lock stack
has been reached and disable itself.

For example, the following error currently occurs in the ARM version
of KVM, once the code tries to lock all vCPUs of a VM configured with more
than MAX_LOCK_DEPTH vCPUs, a situation that can easily happen on modern
systems, where having more than 48 CPUs is common, and it's also common to
run VMs that have vCPU counts approaching that number:

[  328.171264] BUG: MAX_LOCK_DEPTH too low!
[  328.175227] turning off the locking correctness validator.
[  328.180726] Please attach the output of /proc/lock_stat to the bug report
[  328.187531] depth: 48  max: 48!
[  328.190678] 48 locks held by qemu-kvm/11664:
[  328.194957]  #0: ffff800086de5ba0 (&kvm->lock){+.+.}-{3:3}, at: kvm_ioctl_create_device+0x174/0x5b0
[  328.204048]  #1: ffff0800e78800b8 (&vcpu->mutex){+.+.}-{3:3}, at: lock_all_vcpus+0x16c/0x2a0
[  328.212521]  #2: ffff07ffeee51e98 (&vcpu->mutex){+.+.}-{3:3}, at: lock_all_vcpus+0x16c/0x2a0
[  328.220991]  #3: ffff0800dc7d80b8 (&vcpu->mutex){+.+.}-{3:3}, at: lock_all_vcpus+0x16c/0x2a0
[  328.229463]  #4: ffff07ffe0c980b8 (&vcpu->mutex){+.+.}-{3:3}, at: lock_all_vcpus+0x16c/0x2a0
[  328.237934]  #5: ffff0800a3883c78 (&vcpu->mutex){+.+.}-{3:3}, at: lock_all_vcpus+0x16c/0x2a0
[  328.246405]  #6: ffff07fffbe480b8 (&vcpu->mutex){+.+.}-{3:3}, at: lock_all_vcpus+0x16c/0x2a0

Luckily, in all instances that require locking all vCPUs, the
'kvm->lock' is taken a priori, and that fact makes it possible to use
the little known feature of lockdep, called a 'nest_lock', to avoid this
warning and subsequent lockdep self-disablement.

The action of 'nested lock' being provided to lockdep's lock_acquire(),
causes the lockdep to detect that the top of the held lock stack contains
a lock of the same class and then increment its reference counter instead
of pushing a new held_lock item onto that stack.

See __lock_acquire for more information.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
---
 include/linux/mutex.h  | 15 +++++++++++++++
 kernel/locking/mutex.c | 14 +++++++++++---
 2 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/include/linux/mutex.h b/include/linux/mutex.h
index 2143d05116be..da4518cfd59c 100644
--- a/include/linux/mutex.h
+++ b/include/linux/mutex.h
@@ -193,7 +193,22 @@ extern void mutex_lock_io(struct mutex *lock);
  *
  * Returns 1 if the mutex has been acquired successfully, and 0 on contention.
  */
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+extern int _mutex_trylock_nest_lock(struct mutex *lock, struct lockdep_map *nest_lock);
+
+#define mutex_trylock_nest_lock(lock, nest_lock)		\
+(								\
+	typecheck(struct lockdep_map *, &(nest_lock)->dep_map),	\
+	_mutex_trylock_nest_lock(lock, &(nest_lock)->dep_map)	\
+)
+
+#define mutex_trylock(lock) _mutex_trylock_nest_lock(lock, NULL)
+#else
 extern int mutex_trylock(struct mutex *lock);
+#define mutex_trylock_nest_lock(lock, nest_lock) mutex_trylock(lock)
+#endif
+
 extern void mutex_unlock(struct mutex *lock);
 
 extern int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock);
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 555e2b3a665a..c75a838d3bae 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -1062,6 +1062,7 @@ __ww_mutex_lock_interruptible_slowpath(struct ww_mutex *lock,
 
 #endif
 
+#ifndef CONFIG_DEBUG_LOCK_ALLOC
 /**
  * mutex_trylock - try to acquire the mutex, without waiting
  * @lock: the mutex to be acquired
@@ -1077,18 +1078,25 @@ __ww_mutex_lock_interruptible_slowpath(struct ww_mutex *lock,
  * mutex must be released by the same task that acquired it.
  */
 int __sched mutex_trylock(struct mutex *lock)
+{
+	MUTEX_WARN_ON(lock->magic != lock);
+	return __mutex_trylock(lock);
+}
+EXPORT_SYMBOL(mutex_trylock);
+#else
+int __sched _mutex_trylock_nest_lock(struct mutex *lock, struct lockdep_map *nest_lock)
 {
 	bool locked;
 
 	MUTEX_WARN_ON(lock->magic != lock);
-
 	locked = __mutex_trylock(lock);
 	if (locked)
-		mutex_acquire(&lock->dep_map, 0, 1, _RET_IP_);
+		mutex_acquire_nest(&lock->dep_map, 0, 1, nest_lock, _RET_IP_);
 
 	return locked;
 }
-EXPORT_SYMBOL(mutex_trylock);
+EXPORT_SYMBOL(_mutex_trylock_nest_lock);
+#endif
 
 #ifndef CONFIG_DEBUG_LOCK_ALLOC
 int __sched
-- 
2.46.0

[PATCH v5 2/6] locking/mutex: implement mutex_lock_killable_nest_lock

[Maxim Levitsky]

KVM's SEV intra-host migration code needs to lock all vCPUs
of the source and the target VM, before it proceeds with the migration.

The number of vCPUs that belong to each VM is not bounded by anything
except a self-imposed KVM limit of CONFIG_KVM_MAX_NR_VCPUS vCPUs which is
significantly larger than the depth of lockdep's lock stack.

Luckily, the locks in both of the cases mentioned above, are held under
the 'kvm->lock' of each VM, which means that we can use the little
known lockdep feature called a "nest_lock" to support this use case in
a cleaner way, compared to the way it's currently done.

Implement and expose 'mutex_lock_killable_nest_lock' for this
purpose.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
---
 include/linux/mutex.h  | 17 +++++++++++++----
 kernel/locking/mutex.c |  7 ++++---
 2 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/include/linux/mutex.h b/include/linux/mutex.h
index da4518cfd59c..a039fa8c1780 100644
--- a/include/linux/mutex.h
+++ b/include/linux/mutex.h
@@ -156,16 +156,15 @@ static inline int __devm_mutex_init(struct device *dev, struct mutex *lock)
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 extern void mutex_lock_nested(struct mutex *lock, unsigned int subclass);
 extern void _mutex_lock_nest_lock(struct mutex *lock, struct lockdep_map *nest_lock);
-
 extern int __must_check mutex_lock_interruptible_nested(struct mutex *lock,
 					unsigned int subclass);
-extern int __must_check mutex_lock_killable_nested(struct mutex *lock,
-					unsigned int subclass);
+extern int __must_check _mutex_lock_killable(struct mutex *lock,
+		unsigned int subclass, struct lockdep_map *nest_lock);
 extern void mutex_lock_io_nested(struct mutex *lock, unsigned int subclass);
 
 #define mutex_lock(lock) mutex_lock_nested(lock, 0)
 #define mutex_lock_interruptible(lock) mutex_lock_interruptible_nested(lock, 0)
-#define mutex_lock_killable(lock) mutex_lock_killable_nested(lock, 0)
+#define mutex_lock_killable(lock) _mutex_lock_killable(lock, 0, NULL)
 #define mutex_lock_io(lock) mutex_lock_io_nested(lock, 0)
 
 #define mutex_lock_nest_lock(lock, nest_lock)				\
@@ -174,6 +173,15 @@ do {									\
 	_mutex_lock_nest_lock(lock, &(nest_lock)->dep_map);		\
 } while (0)
 
+#define mutex_lock_killable_nest_lock(lock, nest_lock)			\
+(									\
+	typecheck(struct lockdep_map *, &(nest_lock)->dep_map),		\
+	_mutex_lock_killable(lock, 0, &(nest_lock)->dep_map)		\
+)
+
+#define mutex_lock_killable_nested(lock, subclass) \
+	_mutex_lock_killable(lock, subclass, NULL)
+
 #else
 extern void mutex_lock(struct mutex *lock);
 extern int __must_check mutex_lock_interruptible(struct mutex *lock);
@@ -183,6 +191,7 @@ extern void mutex_lock_io(struct mutex *lock);
 # define mutex_lock_nested(lock, subclass) mutex_lock(lock)
 # define mutex_lock_interruptible_nested(lock, subclass) mutex_lock_interruptible(lock)
 # define mutex_lock_killable_nested(lock, subclass) mutex_lock_killable(lock)
+# define mutex_lock_killable_nest_lock(lock, nest_lock) mutex_lock_killable(lock)
 # define mutex_lock_nest_lock(lock, nest_lock) mutex_lock(lock)
 # define mutex_lock_io_nested(lock, subclass) mutex_lock_io(lock)
 #endif
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index c75a838d3bae..234923121ff0 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -808,11 +808,12 @@ _mutex_lock_nest_lock(struct mutex *lock, struct lockdep_map *nest)
 EXPORT_SYMBOL_GPL(_mutex_lock_nest_lock);
 
 int __sched
-mutex_lock_killable_nested(struct mutex *lock, unsigned int subclass)
+_mutex_lock_killable(struct mutex *lock, unsigned int subclass,
+				      struct lockdep_map *nest)
 {
-	return __mutex_lock(lock, TASK_KILLABLE, subclass, NULL, _RET_IP_);
+	return __mutex_lock(lock, TASK_KILLABLE, subclass, nest, _RET_IP_);
 }
-EXPORT_SYMBOL_GPL(mutex_lock_killable_nested);
+EXPORT_SYMBOL_GPL(_mutex_lock_killable);
 
 int __sched
 mutex_lock_interruptible_nested(struct mutex *lock, unsigned int subclass)
-- 
2.46.0

[PATCH v5 3/6] KVM: add kvm_lock_all_vcpus and kvm_trylock_all_vcpus

[Maxim Levitsky]

In a few cases, usually in the initialization code, KVM locks all vCPUs
of a VM to ensure that userspace doesn't do funny things while KVM performs
an operation that affects the whole VM.

Until now, all these operations were implemented using custom code,
and all of them share the same problem:

Lockdep can't cope with simultaneous locking of a large number of locks of
the same class.

However if these locks are taken while another lock is already held,
which is luckily the case, it is possible to take advantage of little known
_nest_lock feature of lockdep which allows in this case to have an
unlimited number of locks of same class to be taken.

To implement this, create two functions:
kvm_lock_all_vcpus() and kvm_trylock_all_vcpus()

Both functions are needed because some code that will be replaced in
the subsequent patches, uses mutex_trylock, instead of regular mutex_lock.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
---
 include/linux/kvm_host.h |  4 +++
 virt/kvm/kvm_main.c      | 59 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 63 insertions(+)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 1dedc421b3e3..a6140415c693 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1015,6 +1015,10 @@ static inline struct kvm_vcpu *kvm_get_vcpu_by_id(struct kvm *kvm, int id)
 
 void kvm_destroy_vcpus(struct kvm *kvm);
 
+int kvm_trylock_all_vcpus(struct kvm *kvm);
+int kvm_lock_all_vcpus(struct kvm *kvm);
+void kvm_unlock_all_vcpus(struct kvm *kvm);
+
 void vcpu_load(struct kvm_vcpu *vcpu);
 void vcpu_put(struct kvm_vcpu *vcpu);
 
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 69782df3617f..d660a7da3baa 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1368,6 +1368,65 @@ static int kvm_vm_release(struct inode *inode, struct file *filp)
 	return 0;
 }
 
+int kvm_trylock_all_vcpus(struct kvm *kvm)
+{
+	struct kvm_vcpu *vcpu;
+	unsigned long i, j;
+
+	lockdep_assert_held(&kvm->lock);
+
+	kvm_for_each_vcpu(i, vcpu, kvm)
+		if (!mutex_trylock_nest_lock(&vcpu->mutex, &kvm->lock))
+			goto out_unlock;
+	return 0;
+
+out_unlock:
+	kvm_for_each_vcpu(j, vcpu, kvm) {
+		if (i == j)
+			break;
+		mutex_unlock(&vcpu->mutex);
+	}
+	return -EINTR;
+}
+EXPORT_SYMBOL_GPL(kvm_trylock_all_vcpus);
+
+int kvm_lock_all_vcpus(struct kvm *kvm)
+{
+	struct kvm_vcpu *vcpu;
+	unsigned long i, j;
+	int r;
+
+	lockdep_assert_held(&kvm->lock);
+
+	kvm_for_each_vcpu(i, vcpu, kvm) {
+		r = mutex_lock_killable_nest_lock(&vcpu->mutex, &kvm->lock);
+		if (r)
+			goto out_unlock;
+	}
+	return 0;
+
+out_unlock:
+	kvm_for_each_vcpu(j, vcpu, kvm) {
+		if (i == j)
+			break;
+		mutex_unlock(&vcpu->mutex);
+	}
+	return r;
+}
+EXPORT_SYMBOL_GPL(kvm_lock_all_vcpus);
+
+void kvm_unlock_all_vcpus(struct kvm *kvm)
+{
+	struct kvm_vcpu *vcpu;
+	unsigned long i;
+
+	lockdep_assert_held(&kvm->lock);
+
+	kvm_for_each_vcpu(i, vcpu, kvm)
+		mutex_unlock(&vcpu->mutex);
+}
+EXPORT_SYMBOL_GPL(kvm_unlock_all_vcpus);
+
 /*
  * Allocation size is twice as large as the actual dirty bitmap size.
  * See kvm_vm_ioctl_get_dirty_log() why this is needed.
-- 
2.46.0

Re: [PATCH v5 3/6] KVM: add kvm_lock_all_vcpus and kvm_trylock_all_vcpus

[Marc Zyngier]

On Mon, 12 May 2025 19:04:04 +0100,
Maxim Levitsky <mlevitsk@redhat.com> wrote:
> 
> In a few cases, usually in the initialization code, KVM locks all vCPUs
> of a VM to ensure that userspace doesn't do funny things while KVM performs
> an operation that affects the whole VM.
> 
> Until now, all these operations were implemented using custom code,
> and all of them share the same problem:
> 
> Lockdep can't cope with simultaneous locking of a large number of locks of
> the same class.
> 
> However if these locks are taken while another lock is already held,
> which is luckily the case, it is possible to take advantage of little known
> _nest_lock feature of lockdep which allows in this case to have an
> unlimited number of locks of same class to be taken.
> 
> To implement this, create two functions:
> kvm_lock_all_vcpus() and kvm_trylock_all_vcpus()
> 
> Both functions are needed because some code that will be replaced in
> the subsequent patches, uses mutex_trylock, instead of regular mutex_lock.
> 
> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>

Acked-by: Marc Zyngier <maz@kernel.org>

	M.

-- 
Without deviation from the norm, progress is not possible.

[PATCH v5 4/6] x86: KVM: SVM: use kvm_lock_all_vcpus instead of a custom implementation

[Maxim Levitsky]

Use kvm_lock_all_vcpus instead of sev's own implementation.

Because kvm_lock_all_vcpus uses the _nest_lock feature of lockdep, which
ignores subclasses, there is no longer a need to use separate subclasses
for source and target VMs.

No functional change intended.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
---
 arch/x86/kvm/svm/sev.c | 72 +++---------------------------------------
 1 file changed, 4 insertions(+), 68 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 0bc708ee2788..16db6179013d 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -1882,70 +1882,6 @@ static void sev_unlock_two_vms(struct kvm *dst_kvm, struct kvm *src_kvm)
 	atomic_set_release(&src_sev->migration_in_progress, 0);
 }
 
-/* vCPU mutex subclasses.  */
-enum sev_migration_role {
-	SEV_MIGRATION_SOURCE = 0,
-	SEV_MIGRATION_TARGET,
-	SEV_NR_MIGRATION_ROLES,
-};
-
-static int sev_lock_vcpus_for_migration(struct kvm *kvm,
-					enum sev_migration_role role)
-{
-	struct kvm_vcpu *vcpu;
-	unsigned long i, j;
-
-	kvm_for_each_vcpu(i, vcpu, kvm) {
-		if (mutex_lock_killable_nested(&vcpu->mutex, role))
-			goto out_unlock;
-
-#ifdef CONFIG_PROVE_LOCKING
-		if (!i)
-			/*
-			 * Reset the role to one that avoids colliding with
-			 * the role used for the first vcpu mutex.
-			 */
-			role = SEV_NR_MIGRATION_ROLES;
-		else
-			mutex_release(&vcpu->mutex.dep_map, _THIS_IP_);
-#endif
-	}
-
-	return 0;
-
-out_unlock:
-
-	kvm_for_each_vcpu(j, vcpu, kvm) {
-		if (i == j)
-			break;
-
-#ifdef CONFIG_PROVE_LOCKING
-		if (j)
-			mutex_acquire(&vcpu->mutex.dep_map, role, 0, _THIS_IP_);
-#endif
-
-		mutex_unlock(&vcpu->mutex);
-	}
-	return -EINTR;
-}
-
-static void sev_unlock_vcpus_for_migration(struct kvm *kvm)
-{
-	struct kvm_vcpu *vcpu;
-	unsigned long i;
-	bool first = true;
-
-	kvm_for_each_vcpu(i, vcpu, kvm) {
-		if (first)
-			first = false;
-		else
-			mutex_acquire(&vcpu->mutex.dep_map,
-				      SEV_NR_MIGRATION_ROLES, 0, _THIS_IP_);
-
-		mutex_unlock(&vcpu->mutex);
-	}
-}
-
 static void sev_migrate_from(struct kvm *dst_kvm, struct kvm *src_kvm)
 {
 	struct kvm_sev_info *dst = to_kvm_sev_info(dst_kvm);
@@ -2083,10 +2019,10 @@ int sev_vm_move_enc_context_from(struct kvm *kvm, unsigned int source_fd)
 		charged = true;
 	}
 
-	ret = sev_lock_vcpus_for_migration(kvm, SEV_MIGRATION_SOURCE);
+	ret = kvm_lock_all_vcpus(kvm);
 	if (ret)
 		goto out_dst_cgroup;
-	ret = sev_lock_vcpus_for_migration(source_kvm, SEV_MIGRATION_TARGET);
+	ret = kvm_lock_all_vcpus(source_kvm);
 	if (ret)
 		goto out_dst_vcpu;
 
@@ -2100,9 +2036,9 @@ int sev_vm_move_enc_context_from(struct kvm *kvm, unsigned int source_fd)
 	ret = 0;
 
 out_source_vcpu:
-	sev_unlock_vcpus_for_migration(source_kvm);
+	kvm_unlock_all_vcpus(source_kvm);
 out_dst_vcpu:
-	sev_unlock_vcpus_for_migration(kvm);
+	kvm_unlock_all_vcpus(kvm);
 out_dst_cgroup:
 	/* Operates on the source on success, on the destination on failure.  */
 	if (charged)
-- 
2.46.0

[PATCH v5 5/6] KVM: arm64: use kvm_trylock_all_vcpus when locking all vCPUs

[Maxim Levitsky]

Use kvm_trylock_all_vcpus instead of a custom implementation when locking
all vCPUs of a VM, to avoid triggering a lockdep warning, in the case in
which the VM is configured to have more than MAX_LOCK_DEPTH vCPUs.

This fixes the following false lockdep warning:

[  328.171264] BUG: MAX_LOCK_DEPTH too low!
[  328.175227] turning off the locking correctness validator.
[  328.180726] Please attach the output of /proc/lock_stat to the bug report
[  328.187531] depth: 48  max: 48!
[  328.190678] 48 locks held by qemu-kvm/11664:
[  328.194957]  #0: ffff800086de5ba0 (&kvm->lock){+.+.}-{3:3}, at: kvm_ioctl_create_device+0x174/0x5b0
[  328.204048]  #1: ffff0800e78800b8 (&vcpu->mutex){+.+.}-{3:3}, at: lock_all_vcpus+0x16c/0x2a0
[  328.212521]  #2: ffff07ffeee51e98 (&vcpu->mutex){+.+.}-{3:3}, at: lock_all_vcpus+0x16c/0x2a0
[  328.220991]  #3: ffff0800dc7d80b8 (&vcpu->mutex){+.+.}-{3:3}, at: lock_all_vcpus+0x16c/0x2a0
[  328.229463]  #4: ffff07ffe0c980b8 (&vcpu->mutex){+.+.}-{3:3}, at: lock_all_vcpus+0x16c/0x2a0
[  328.237934]  #5: ffff0800a3883c78 (&vcpu->mutex){+.+.}-{3:3}, at: lock_all_vcpus+0x16c/0x2a0
[  328.246405]  #6: ffff07fffbe480b8 (&vcpu->mutex){+.+.}-{3:3}, at: lock_all_vcpus+0x16c/0x2a0

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
---
 arch/arm64/include/asm/kvm_host.h     |  3 --
 arch/arm64/kvm/arch_timer.c           |  4 +--
 arch/arm64/kvm/arm.c                  | 43 ---------------------------
 arch/arm64/kvm/vgic/vgic-init.c       |  4 +--
 arch/arm64/kvm/vgic/vgic-its.c        |  8 ++---
 arch/arm64/kvm/vgic/vgic-kvm-device.c | 12 ++++----
 6 files changed, 14 insertions(+), 60 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 08ba91e6fb03..e5ddbd1ba2ca 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -1263,9 +1263,6 @@ int __init populate_sysreg_config(const struct sys_reg_desc *sr,
 				  unsigned int idx);
 int __init populate_nv_trap_config(void);
 
-bool lock_all_vcpus(struct kvm *kvm);
-void unlock_all_vcpus(struct kvm *kvm);
-
 void kvm_calculate_traps(struct kvm_vcpu *vcpu);
 
 /* MMIO helpers */
diff --git a/arch/arm64/kvm/arch_timer.c b/arch/arm64/kvm/arch_timer.c
index 5133dcbfe9f7..fdbc8beec930 100644
--- a/arch/arm64/kvm/arch_timer.c
+++ b/arch/arm64/kvm/arch_timer.c
@@ -1766,7 +1766,7 @@ int kvm_vm_ioctl_set_counter_offset(struct kvm *kvm,
 
 	mutex_lock(&kvm->lock);
 
-	if (lock_all_vcpus(kvm)) {
+	if (!kvm_trylock_all_vcpus(kvm)) {
 		set_bit(KVM_ARCH_FLAG_VM_COUNTER_OFFSET, &kvm->arch.flags);
 
 		/*
@@ -1778,7 +1778,7 @@ int kvm_vm_ioctl_set_counter_offset(struct kvm *kvm,
 		kvm->arch.timer_data.voffset = offset->counter_offset;
 		kvm->arch.timer_data.poffset = offset->counter_offset;
 
-		unlock_all_vcpus(kvm);
+		kvm_unlock_all_vcpus(kvm);
 	} else {
 		ret = -EBUSY;
 	}
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 19ca57def629..4171bd5139c8 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -1914,49 +1914,6 @@ int kvm_arch_vm_ioctl(struct file *filp, unsigned int ioctl, unsigned long arg)
 	}
 }
 
-/* unlocks vcpus from @vcpu_lock_idx and smaller */
-static void unlock_vcpus(struct kvm *kvm, int vcpu_lock_idx)
-{
-	struct kvm_vcpu *tmp_vcpu;
-
-	for (; vcpu_lock_idx >= 0; vcpu_lock_idx--) {
-		tmp_vcpu = kvm_get_vcpu(kvm, vcpu_lock_idx);
-		mutex_unlock(&tmp_vcpu->mutex);
-	}
-}
-
-void unlock_all_vcpus(struct kvm *kvm)
-{
-	lockdep_assert_held(&kvm->lock);
-
-	unlock_vcpus(kvm, atomic_read(&kvm->online_vcpus) - 1);
-}
-
-/* Returns true if all vcpus were locked, false otherwise */
-bool lock_all_vcpus(struct kvm *kvm)
-{
-	struct kvm_vcpu *tmp_vcpu;
-	unsigned long c;
-
-	lockdep_assert_held(&kvm->lock);
-
-	/*
-	 * Any time a vcpu is in an ioctl (including running), the
-	 * core KVM code tries to grab the vcpu->mutex.
-	 *
-	 * By grabbing the vcpu->mutex of all VCPUs we ensure that no
-	 * other VCPUs can fiddle with the state while we access it.
-	 */
-	kvm_for_each_vcpu(c, tmp_vcpu, kvm) {
-		if (!mutex_trylock(&tmp_vcpu->mutex)) {
-			unlock_vcpus(kvm, c - 1);
-			return false;
-		}
-	}
-
-	return true;
-}
-
 static unsigned long nvhe_percpu_size(void)
 {
 	return (unsigned long)CHOOSE_NVHE_SYM(__per_cpu_end) -
diff --git a/arch/arm64/kvm/vgic/vgic-init.c b/arch/arm64/kvm/vgic/vgic-init.c
index 1f33e71c2a73..6a426d403a6b 100644
--- a/arch/arm64/kvm/vgic/vgic-init.c
+++ b/arch/arm64/kvm/vgic/vgic-init.c
@@ -88,7 +88,7 @@ int kvm_vgic_create(struct kvm *kvm, u32 type)
 	lockdep_assert_held(&kvm->lock);
 
 	ret = -EBUSY;
-	if (!lock_all_vcpus(kvm))
+	if (kvm_trylock_all_vcpus(kvm))
 		return ret;
 
 	mutex_lock(&kvm->arch.config_lock);
@@ -142,7 +142,7 @@ int kvm_vgic_create(struct kvm *kvm, u32 type)
 
 out_unlock:
 	mutex_unlock(&kvm->arch.config_lock);
-	unlock_all_vcpus(kvm);
+	kvm_unlock_all_vcpus(kvm);
 	return ret;
 }
 
diff --git a/arch/arm64/kvm/vgic/vgic-its.c b/arch/arm64/kvm/vgic/vgic-its.c
index fb96802799c6..7454388e3646 100644
--- a/arch/arm64/kvm/vgic/vgic-its.c
+++ b/arch/arm64/kvm/vgic/vgic-its.c
@@ -1999,7 +1999,7 @@ static int vgic_its_attr_regs_access(struct kvm_device *dev,
 
 	mutex_lock(&dev->kvm->lock);
 
-	if (!lock_all_vcpus(dev->kvm)) {
+	if (kvm_trylock_all_vcpus(dev->kvm)) {
 		mutex_unlock(&dev->kvm->lock);
 		return -EBUSY;
 	}
@@ -2034,7 +2034,7 @@ static int vgic_its_attr_regs_access(struct kvm_device *dev,
 	}
 out:
 	mutex_unlock(&dev->kvm->arch.config_lock);
-	unlock_all_vcpus(dev->kvm);
+	kvm_unlock_all_vcpus(dev->kvm);
 	mutex_unlock(&dev->kvm->lock);
 	return ret;
 }
@@ -2704,7 +2704,7 @@ static int vgic_its_ctrl(struct kvm *kvm, struct vgic_its *its, u64 attr)
 
 	mutex_lock(&kvm->lock);
 
-	if (!lock_all_vcpus(kvm)) {
+	if (kvm_trylock_all_vcpus(kvm)) {
 		mutex_unlock(&kvm->lock);
 		return -EBUSY;
 	}
@@ -2726,7 +2726,7 @@ static int vgic_its_ctrl(struct kvm *kvm, struct vgic_its *its, u64 attr)
 
 	mutex_unlock(&its->its_lock);
 	mutex_unlock(&kvm->arch.config_lock);
-	unlock_all_vcpus(kvm);
+	kvm_unlock_all_vcpus(kvm);
 	mutex_unlock(&kvm->lock);
 	return ret;
 }
diff --git a/arch/arm64/kvm/vgic/vgic-kvm-device.c b/arch/arm64/kvm/vgic/vgic-kvm-device.c
index 359094f68c23..f9ae790163fb 100644
--- a/arch/arm64/kvm/vgic/vgic-kvm-device.c
+++ b/arch/arm64/kvm/vgic/vgic-kvm-device.c
@@ -268,7 +268,7 @@ static int vgic_set_common_attr(struct kvm_device *dev,
 				return -ENXIO;
 			mutex_lock(&dev->kvm->lock);
 
-			if (!lock_all_vcpus(dev->kvm)) {
+			if (kvm_trylock_all_vcpus(dev->kvm)) {
 				mutex_unlock(&dev->kvm->lock);
 				return -EBUSY;
 			}
@@ -276,7 +276,7 @@ static int vgic_set_common_attr(struct kvm_device *dev,
 			mutex_lock(&dev->kvm->arch.config_lock);
 			r = vgic_v3_save_pending_tables(dev->kvm);
 			mutex_unlock(&dev->kvm->arch.config_lock);
-			unlock_all_vcpus(dev->kvm);
+			kvm_unlock_all_vcpus(dev->kvm);
 			mutex_unlock(&dev->kvm->lock);
 			return r;
 		}
@@ -390,7 +390,7 @@ static int vgic_v2_attr_regs_access(struct kvm_device *dev,
 
 	mutex_lock(&dev->kvm->lock);
 
-	if (!lock_all_vcpus(dev->kvm)) {
+	if (kvm_trylock_all_vcpus(dev->kvm)) {
 		mutex_unlock(&dev->kvm->lock);
 		return -EBUSY;
 	}
@@ -415,7 +415,7 @@ static int vgic_v2_attr_regs_access(struct kvm_device *dev,
 
 out:
 	mutex_unlock(&dev->kvm->arch.config_lock);
-	unlock_all_vcpus(dev->kvm);
+	kvm_unlock_all_vcpus(dev->kvm);
 	mutex_unlock(&dev->kvm->lock);
 
 	if (!ret && !is_write)
@@ -554,7 +554,7 @@ static int vgic_v3_attr_regs_access(struct kvm_device *dev,
 
 	mutex_lock(&dev->kvm->lock);
 
-	if (!lock_all_vcpus(dev->kvm)) {
+	if (kvm_trylock_all_vcpus(dev->kvm)) {
 		mutex_unlock(&dev->kvm->lock);
 		return -EBUSY;
 	}
@@ -611,7 +611,7 @@ static int vgic_v3_attr_regs_access(struct kvm_device *dev,
 
 out:
 	mutex_unlock(&dev->kvm->arch.config_lock);
-	unlock_all_vcpus(dev->kvm);
+	kvm_unlock_all_vcpus(dev->kvm);
 	mutex_unlock(&dev->kvm->lock);
 
 	if (!ret && uaccess && !is_write) {
-- 
2.46.0

Re: [PATCH v5 5/6] KVM: arm64: use kvm_trylock_all_vcpus when locking all vCPUs

[Marc Zyngier]

On Mon, 12 May 2025 19:04:06 +0100,
Maxim Levitsky <mlevitsk@redhat.com> wrote:
> 
> Use kvm_trylock_all_vcpus instead of a custom implementation when locking
> all vCPUs of a VM, to avoid triggering a lockdep warning, in the case in
> which the VM is configured to have more than MAX_LOCK_DEPTH vCPUs.
> 
> This fixes the following false lockdep warning:
> 
> [  328.171264] BUG: MAX_LOCK_DEPTH too low!
> [  328.175227] turning off the locking correctness validator.
> [  328.180726] Please attach the output of /proc/lock_stat to the bug report
> [  328.187531] depth: 48  max: 48!
> [  328.190678] 48 locks held by qemu-kvm/11664:
> [  328.194957]  #0: ffff800086de5ba0 (&kvm->lock){+.+.}-{3:3}, at: kvm_ioctl_create_device+0x174/0x5b0
> [  328.204048]  #1: ffff0800e78800b8 (&vcpu->mutex){+.+.}-{3:3}, at: lock_all_vcpus+0x16c/0x2a0
> [  328.212521]  #2: ffff07ffeee51e98 (&vcpu->mutex){+.+.}-{3:3}, at: lock_all_vcpus+0x16c/0x2a0
> [  328.220991]  #3: ffff0800dc7d80b8 (&vcpu->mutex){+.+.}-{3:3}, at: lock_all_vcpus+0x16c/0x2a0
> [  328.229463]  #4: ffff07ffe0c980b8 (&vcpu->mutex){+.+.}-{3:3}, at: lock_all_vcpus+0x16c/0x2a0
> [  328.237934]  #5: ffff0800a3883c78 (&vcpu->mutex){+.+.}-{3:3}, at: lock_all_vcpus+0x16c/0x2a0
> [  328.246405]  #6: ffff07fffbe480b8 (&vcpu->mutex){+.+.}-{3:3}, at: lock_all_vcpus+0x16c/0x2a0
> 
> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>

Acked-by: Marc Zyngier <maz@kernel.org>

Paolo: if you are queuing this for 6.16, please put it on a stable
branch so that I can merge it back in case of conflicts.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

[PATCH v5 6/6] RISC-V: KVM: use kvm_trylock_all_vcpus when locking all vCPUs

[Maxim Levitsky]

Use kvm_trylock_all_vcpus instead of a custom implementation when locking
all vCPUs of a VM.

Compile tested only.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
---
 arch/riscv/kvm/aia_device.c | 34 ++--------------------------------
 1 file changed, 2 insertions(+), 32 deletions(-)

diff --git a/arch/riscv/kvm/aia_device.c b/arch/riscv/kvm/aia_device.c
index 39cd26af5a69..6315821f0d69 100644
--- a/arch/riscv/kvm/aia_device.c
+++ b/arch/riscv/kvm/aia_device.c
@@ -12,36 +12,6 @@
 #include <linux/kvm_host.h>
 #include <linux/uaccess.h>
 
-static void unlock_vcpus(struct kvm *kvm, int vcpu_lock_idx)
-{
-	struct kvm_vcpu *tmp_vcpu;
-
-	for (; vcpu_lock_idx >= 0; vcpu_lock_idx--) {
-		tmp_vcpu = kvm_get_vcpu(kvm, vcpu_lock_idx);
-		mutex_unlock(&tmp_vcpu->mutex);
-	}
-}
-
-static void unlock_all_vcpus(struct kvm *kvm)
-{
-	unlock_vcpus(kvm, atomic_read(&kvm->online_vcpus) - 1);
-}
-
-static bool lock_all_vcpus(struct kvm *kvm)
-{
-	struct kvm_vcpu *tmp_vcpu;
-	unsigned long c;
-
-	kvm_for_each_vcpu(c, tmp_vcpu, kvm) {
-		if (!mutex_trylock(&tmp_vcpu->mutex)) {
-			unlock_vcpus(kvm, c - 1);
-			return false;
-		}
-	}
-
-	return true;
-}
-
 static int aia_create(struct kvm_device *dev, u32 type)
 {
 	int ret;
@@ -53,7 +23,7 @@ static int aia_create(struct kvm_device *dev, u32 type)
 		return -EEXIST;
 
 	ret = -EBUSY;
-	if (!lock_all_vcpus(kvm))
+	if (kvm_trylock_all_vcpus(kvm))
 		return ret;
 
 	kvm_for_each_vcpu(i, vcpu, kvm) {
@@ -65,7 +35,7 @@ static int aia_create(struct kvm_device *dev, u32 type)
 	kvm->arch.aia.in_kernel = true;
 
 out_unlock:
-	unlock_all_vcpus(kvm);
+	kvm_unlock_all_vcpus(kvm);
 	return ret;
 }
 
-- 
2.46.0

Re: [PATCH v5 6/6] RISC-V: KVM: use kvm_trylock_all_vcpus when locking all vCPUs

[Anup Patel]

On Mon, May 12, 2025 at 11:35 PM Maxim Levitsky <mlevitsk@redhat.com> wrote:
>
> Use kvm_trylock_all_vcpus instead of a custom implementation when locking
> all vCPUs of a VM.
>
> Compile tested only.
>
> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>

For KVM RISC-V:
Tested-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Anup Patel <anup@brainfault.org>

Regards,
Anup

> ---
>  arch/riscv/kvm/aia_device.c | 34 ++--------------------------------
>  1 file changed, 2 insertions(+), 32 deletions(-)
>
> diff --git a/arch/riscv/kvm/aia_device.c b/arch/riscv/kvm/aia_device.c
> index 39cd26af5a69..6315821f0d69 100644
> --- a/arch/riscv/kvm/aia_device.c
> +++ b/arch/riscv/kvm/aia_device.c
> @@ -12,36 +12,6 @@
>  #include <linux/kvm_host.h>
>  #include <linux/uaccess.h>
>
> -static void unlock_vcpus(struct kvm *kvm, int vcpu_lock_idx)
> -{
> -       struct kvm_vcpu *tmp_vcpu;
> -
> -       for (; vcpu_lock_idx >= 0; vcpu_lock_idx--) {
> -               tmp_vcpu = kvm_get_vcpu(kvm, vcpu_lock_idx);
> -               mutex_unlock(&tmp_vcpu->mutex);
> -       }
> -}
> -
> -static void unlock_all_vcpus(struct kvm *kvm)
> -{
> -       unlock_vcpus(kvm, atomic_read(&kvm->online_vcpus) - 1);
> -}
> -
> -static bool lock_all_vcpus(struct kvm *kvm)
> -{
> -       struct kvm_vcpu *tmp_vcpu;
> -       unsigned long c;
> -
> -       kvm_for_each_vcpu(c, tmp_vcpu, kvm) {
> -               if (!mutex_trylock(&tmp_vcpu->mutex)) {
> -                       unlock_vcpus(kvm, c - 1);
> -                       return false;
> -               }
> -       }
> -
> -       return true;
> -}
> -
>  static int aia_create(struct kvm_device *dev, u32 type)
>  {
>         int ret;
> @@ -53,7 +23,7 @@ static int aia_create(struct kvm_device *dev, u32 type)
>                 return -EEXIST;
>
>         ret = -EBUSY;
> -       if (!lock_all_vcpus(kvm))
> +       if (kvm_trylock_all_vcpus(kvm))
>                 return ret;
>
>         kvm_for_each_vcpu(i, vcpu, kvm) {
> @@ -65,7 +35,7 @@ static int aia_create(struct kvm_device *dev, u32 type)
>         kvm->arch.aia.in_kernel = true;
>
>  out_unlock:
> -       unlock_all_vcpus(kvm);
> +       kvm_unlock_all_vcpus(kvm);
>         return ret;
>  }
>
> --
> 2.46.0
>

Re: [PATCH v5 0/6] KVM: lockdep improvements

[Peter Zijlstra]

On Mon, May 12, 2025 at 02:04:01PM -0400, Maxim Levitsky wrote:
> This is	a continuation of my 'extract lock_all_vcpus/unlock_all_vcpus'
> patch series.
> 
> Implement the suggestion of using lockdep's "nest_lock" feature
> when locking all KVM vCPUs by adding mutex_trylock_nest_lock() and
> mutex_lock_killable_nest_lock() and use these functions	in the
> implementation of the
> kvm_trylock_all_vcpus()/kvm_lock_all_vcpus()/kvm_unlock_all_vcpus().
> 
> Those changes allow removal of a custom workaround that was needed to
> silence the lockdep warning in the SEV code and also stop lockdep from
> complaining in case of ARM and RISC-V code which doesn't include the above
> mentioned workaround.
> 
> Finally, it's worth noting that this patch series removes a fair
> amount of duplicate code by implementing the logic in one place.
> 
> V5: addressed review feedback.
> 
> Best regards,
> 	Maxim Levitsky
> 
> Maxim Levitsky (6):
>   locking/mutex: implement mutex_trylock_nested
>   locking/mutex: implement mutex_lock_killable_nest_lock
>   KVM: add kvm_lock_all_vcpus and kvm_trylock_all_vcpus
>   x86: KVM: SVM: use kvm_lock_all_vcpus instead of a custom
>     implementation
>   KVM: arm64: use kvm_trylock_all_vcpus when locking all vCPUs
>   RISC-V: KVM: use kvm_trylock_all_vcpus when locking all vCPUs
> 
>  arch/arm64/include/asm/kvm_host.h     |  3 --
>  arch/arm64/kvm/arch_timer.c           |  4 +-
>  arch/arm64/kvm/arm.c                  | 43 ----------------
>  arch/arm64/kvm/vgic/vgic-init.c       |  4 +-
>  arch/arm64/kvm/vgic/vgic-its.c        |  8 +--
>  arch/arm64/kvm/vgic/vgic-kvm-device.c | 12 ++---
>  arch/riscv/kvm/aia_device.c           | 34 +------------
>  arch/x86/kvm/svm/sev.c                | 72 ++-------------------------
>  include/linux/kvm_host.h              |  4 ++
>  include/linux/mutex.h                 | 32 ++++++++++--
>  kernel/locking/mutex.c                | 21 +++++---
>  virt/kvm/kvm_main.c                   | 59 ++++++++++++++++++++++
>  12 files changed, 126 insertions(+), 170 deletions(-)

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Re: [PATCH v5 0/6] KVM: lockdep improvements

[patchwork-bot+linux-riscv]

Hello:

This series was applied to riscv/linux.git (fixes)
by Paolo Bonzini <pbonzini@redhat.com>:

On Mon, 12 May 2025 14:04:01 -0400 you wrote:
> This is	a continuation of my 'extract lock_all_vcpus/unlock_all_vcpus'
> patch series.
> 
> Implement the suggestion of using lockdep's "nest_lock" feature
> when locking all KVM vCPUs by adding mutex_trylock_nest_lock() and
> mutex_lock_killable_nest_lock() and use these functions	in the
> implementation of the
> kvm_trylock_all_vcpus()/kvm_lock_all_vcpus()/kvm_unlock_all_vcpus().
> 
> [...]

Here is the summary with links:
  - [v5,1/6] locking/mutex: implement mutex_trylock_nested
    https://git.kernel.org/riscv/c/c5b6ababd21a
  - [v5,2/6] locking/mutex: implement mutex_lock_killable_nest_lock
    https://git.kernel.org/riscv/c/fb49f07ba1d9
  - [v5,3/6] KVM: add kvm_lock_all_vcpus and kvm_trylock_all_vcpus
    https://git.kernel.org/riscv/c/e4a454ced74c
  - [v5,4/6] x86: KVM: SVM: use kvm_lock_all_vcpus instead of a custom implementation
    https://git.kernel.org/riscv/c/c560bc9286e6
  - [v5,5/6] KVM: arm64: use kvm_trylock_all_vcpus when locking all vCPUs
    https://git.kernel.org/riscv/c/b586c5d21954
  - [v5,6/6] RISC-V: KVM: use kvm_trylock_all_vcpus when locking all vCPUs
    https://git.kernel.org/riscv/c/8f56770d114b

You are awesome, thank you!
-- 
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