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

[Peter Zijlstra <peterz@infradead.org>]

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.