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

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Message-ID: <CAMkAt6q6YLBfo2RceduSXTafckEehawhD4K4hUEuB4ZNqe2kKg@mail.gmail.com>
Date:   Tue, 26 Apr 2022 13:06:57 -0600
From:   Peter Gonda <pgonda@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     John Sperbeck <jsperbeck@...gle.com>,
        kvm list <kvm@...r.kernel.org>,
        David Rientjes <rientjes@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] KVM: SEV: Mark nested locking of vcpu->lock

On Thu, Apr 21, 2022 at 9:56 AM Paolo Bonzini <pbonzini@...hat.com> 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?