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.
