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

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Date:   Thu, 28 Apr 2022 15:28:07 -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 Wed, Apr 27, 2022 at 2:18 PM Peter Gonda <pgonda@...gle.com> wrote:
>
> On Wed, Apr 27, 2022 at 10:04 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
> >
> > On 4/26/22 21:06, Peter Gonda wrote:
> > > 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?
> >
> > 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;
> >         }

So when actually trying this out I noticed that we are releasing the
current vcpu iterator but really we haven't actually taken that lock
yet. So we'd need to maintain a prev_* pointer and release that one.
That seems a bit more complicated than just doing this:

To lock:

         bool acquired = false;
         kvm_for_each_vcpu(...) {
                 if (!acquired) {
                    if (mutex_lock_killable_nested(&vcpu->mutex, role)
                        goto out_unlock;
                    acquired = true;
                 } else {
                    if (mutex_lock_killable(&vcpu->mutex, role)
                        goto out_unlock;
                 }
         }

To unlock:

         kvm_for_each_vcpu(...) {
            mutex_unlock(&vcpu->mutex);
         }

This way instead of mocking and releasing the lock_dep we just lock
the fist vcpu with mutex_lock_killable_nested(). I think this
maintains the property you suggested of "coalesces all the mutexes for
a vm in a single subclass".  Thoughts?

> >
> > 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
> >