Re: [PATCH 1/2 V4] KVM, SEV: Add support for SEV intra host migration

[Peter Gonda]

On Fri, Aug 20, 2021 at 2:53 PM Marc Orr <marcorr@xxxxxxxxxx> wrote:
>
> On Thu, Aug 19, 2021 at 3:58 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> >
> > On Thu, Aug 19, 2021, Peter Gonda wrote:
> > > > >
> > > > > +static int svm_sev_lock_for_migration(struct kvm *kvm)
> > > > > +{
> > > > > +       struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> > > > > +       int ret;
> > > > > +
> > > > > +       /*
> > > > > +        * Bail if this VM is already involved in a migration to avoid deadlock
> > > > > +        * between two VMs trying to migrate to/from each other.
> > > > > +        */
> > > > > +       spin_lock(&sev->migration_lock);
> > > > > +       if (sev->migration_in_progress)
> > > > > +               ret = -EBUSY;
> > > > > +       else {
> > > > > +               /*
> > > > > +                * Otherwise indicate VM is migrating and take the KVM lock.
> > > > > +                */
> > > > > +               sev->migration_in_progress = true;
> > > > > +               mutex_lock(&kvm->lock);
> >
> > Deadlock aside, mutex_lock() can sleep, which is not allowed while holding a
> > spinlock, i.e. this patch does not work.  That's my suggestion did the crazy
> > dance of "acquiring" a flag.

Ah, makes sense.

> >
> > What I don't know is why on earth I suggested a global spinlock, a simple atomic
> > should work, e.g.
> >
> >                 if (atomic_cmpxchg_acquire(&sev->migration_in_progress, 0, 1))
> >                         return -EBUSY;
> >
> >                 mutex_lock(&kvm->lock);
> >
> > and on the backend...
> >
> >                 mutex_unlock(&kvm->lock);
> >
> >                 atomic_set_release(&sev->migration_in_progress, 0);
>
> +1 to replacing the spin lock with an atomic flag. Correctness issues
> aside, I think it's also cleaner. Also, I'd suggest adding a comment
> to source code to explain that the `migration_in_progress` flag is to
> prevent deadlock due to the "double migration" discussed previously.

Thanks! I've updated these locks to use the atomic. It looks much cleaner now.