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

[Sean Christopherson <seanjc@google.com>]

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.

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);

> > > +               ret = 0;
> > > +       }
> > > +       spin_unlock(&sev->migration_lock);
> > > +
> > > +       return ret;
> > > +}
> > > +
> > > +static void svm_unlock_after_migration(struct kvm *kvm)
> > > +{
> > > +       struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> > > +
> > > +       mutex_unlock(&kvm->lock);
> > > +       WRITE_ONCE(sev->migration_in_progress, false);
> > > +}
> > > +
> >
> > This entire locking scheme seems over-complicated to me. Can we simply
> > rely on `migration_lock` and get rid of `migration_in_progress`? I was
> > chatting about these patches with Peter, while he worked on this new
> > version. But he mentioned that this locking scheme had been suggested
> > by Sean in a previous review. Sean: what do you think? My rationale
> > was that this is called via a VM-level ioctl. So serializing the
> > entire code path on `migration_lock` seems fine. But maybe I'm missing
> > something?
>
> 
> Marc I think that only having the spin lock could result in
> deadlocking. If userspace double migrated 2 VMs, A and B for
> discussion, A could grab VM_A.spin_lock then VM_A.kvm_mutex. Meanwhile
> B could grab VM_B.spin_lock and VM_B.kvm_mutex. Then A attempts to
> grab VM_B.spin_lock and we have a deadlock. If the same happens with
> the proposed scheme when A attempts to lock B, VM_B.spin_lock will be
> open but the bool will mark the VM under migration so A will unlock
> and bail. Sean originally proposed a global spin lock but I thought a
> per kvm_sev_info struct would also be safe.
 
Close.  The issue is taking kvm->lock from both VM_A and VM_B.  If userspace
double migrates we'll end up with lock ordering A->B and B-A, so we need a way
to guarantee one of those wins.  My proposed solution is to use a flag as a sort
of one-off "try lock" to detect a mean userspace.