Re: [PATCH v2 2/4] KVM: x86: move sev_lock/unlock_vcpus_for_migration to kvm_main.c

Peter Zijlstra
2025-04-10T08:16:56Z

On Tue, Apr 08, 2025 at 09:41:34PM -0400, Maxim Levitsky wrote:
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 69782df3617f..71c0d8c35b4b 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1368,6 +1368,77 @@ static int kvm_vm_release(struct inode *inode, struct file *filp)
> return 0;
> }
>
> +
> +/*
> + * Lock all VM vCPUs.
> + * Can be used nested (to lock vCPUS of two VMs for example)
> + */
> +int kvm_lock_all_vcpus_nested(struct kvm *kvm, bool trylock, unsigned int role)
> +{
> + struct kvm_vcpu *vcpu;
> + unsigned long i, j;
> +
> + lockdep_assert_held(&kvm->lock);
> +
> + kvm_for_each_vcpu(i, vcpu, kvm) {
> +
> + if (trylock && !mutex_trylock_nested(&vcpu->mutex, role))
> + goto out_unlock;
> + else if (!trylock && 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 = MAX_LOCK_DEPTH - 1;
> + else
> + mutex_release(&vcpu->mutex.dep_map, _THIS_IP_);
> +#endif
> + }

This code is all sorts of terrible.

Per the lockdep_assert_held() above, you serialize all these locks by
holding that lock, this means you can be using the _nest_lock()
annotation.

Also, the original code didn't have this trylock nonsense, and the
Changelog doesn't mention this -- in fact the Changelog claims no
change, which is patently false.

Anyway, please write like:

kvm_for_each_vcpu(i, vcpu, kvm) {
if (mutex_lock_killable_nest_lock(&vcpu->mutex, &kvm->lock))
goto unlock;
}

return 0;

unlock:

kvm_for_each_vcpu(j, vcpu, kvm) {
if (j == i)
break;

mutex_unlock(&vcpu->mutex);
}
return -EINTR;

And yes, you'll have to add mutex_lock_killable_nest_lock(), but that
should be trivial.