Re: [PATCH v3] KVM: SEV: Mark nested locking of vcpu->lock

[Paolo Bonzini]

On 4/26/22 21:06, Peter Gonda wrote:
> On Thu, Apr 21, 2022 at 9:56 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> > On 4/20/22 22:14, Peter Gonda wrote:
> > > > > > svm_vm_migrate_from() uses sev_lock_vcpus_for_migration() to lock all
> > > > > > source and target vcpu->locks. Mark the nested subclasses to avoid false
> > > > > > positives from lockdep.
> > > > Nope. Good catch, I didn't realize there was a limit 8 subclasses:
> > > Does anyone have thoughts on how we can resolve this vCPU locking with
> > > the 8 subclass max?
> >
> > The documentation does not have anything.  Maybe you can call
> > mutex_release manually (and mutex_acquire before unlocking).
> >
> > Paolo
>
> Hmm this seems to be working thanks Paolo. To lock I have been using:
>
> ...
>                    if (mutex_lock_killable_nested(
>                                &vcpu->mutex, i * SEV_NR_MIGRATION_ROLES + role))
>                            goto out_unlock;
>                    mutex_release(&vcpu->mutex.dep_map, _THIS_IP_);
> ...
>
> To unlock:
> ...
>                    mutex_acquire(&vcpu->mutex.dep_map, 0, 0, _THIS_IP_);
>                    mutex_unlock(&vcpu->mutex);
> ...
>
> If I understand correctly we are fully disabling lockdep by doing
> this. If this is the case should I just remove all the '_nested' usage
> so switch to mutex_lock_killable() and remove the per vCPU subclass?

Yes, though you could also do:

	bool acquired = false;
	kvm_for_each_vcpu(...) {
		if (acquired)
			mutex_release(&vcpu->mutex.dep_map, _THIS_IP_);
		if (mutex_lock_killable_nested(&vcpu->mutex, role)
			goto out_unlock;
		acquired = true;
		...

and to unlock:

	bool acquired = true;
	kvm_for_each_vcpu(...) {
		if (!acquired)
			mutex_acquire(&vcpu->mutex.dep_map, 0, role, _THIS_IP_);
		mutex_unlock(&vcpu->mutex);
		acquired = false;
	}

where role is either 0 or SINGLE_DEPTH_NESTING and is passed to 
sev_{,un}lock_vcpus_for_migration.

That coalesces all the mutexes for a vm in a single subclass, essentially.

Paolo