On Wed, Apr 27, 2022 at 10:04 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> 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.
Ah thats a great idea to allow for lockdep to work still. I'll try
that out, thanks again Paolo.
>
> Paolo
>
