[PATCH v3 1/4] arm64: KVM: use mutex_trylock_nest_lock when locking all vCPUs

Maxim Levitsky
2025-04-30T21:31:39Z

Use mutex_trylock_nest_lock instead of mutex_trylock when locking all vCPUs
of a VM, to avoid triggering a lockdep warning, if 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

Since the locking of all vCPUs is a primitive that can be useful in other
architectures that are supported by KVM, also move the code to kvm_main.c

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 ++++----
include/linux/kvm_host.h | 3 ++
virt/kvm/kvm_main.c | 34 +++++++++++++++++++++
8 files changed, 51 insertions(+), 60 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index e98cfe7855a6..96ce0b01a61e 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 68fec8c95fee..d31f42a71bdc 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) {
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 1dedc421b3e3..10d6652c7aa0 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1015,6 +1015,9 @@ 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);
+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..834f08dfa24c 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1368,6 +1368,40 @@ static int kvm_vm_release(struct inode *inode, struct file *filp)
return 0;
}

+/*
+ * Try to lock all of the VM's vCPUs.
+ * Assumes that the kvm->lock is held.
+ */
+int kvm_trylock_all_vcpus(struct kvm *kvm)
+{
+ struct kvm_vcpu *vcpu;
+ unsigned long i, j;
+
+ 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);
+
+void kvm_unlock_all_vcpus(struct kvm *kvm)
+{
+ struct kvm_vcpu *vcpu;
+ unsigned long i;
+
+ 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