Messages in this thread | ![/](/images/icornerl.gif) | | Date | Wed, 27 Apr 2022 18:04:23 +0200 | From | Paolo Bonzini <> | Subject | Re: [PATCH v3] KVM: SEV: Mark nested locking of vcpu->lock |
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On 4/26/22 21:06, Peter Gonda wrote: > On Thu, Apr 21, 2022 at 9:56 AM Paolo Bonzini <pbonzini@redhat.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; }
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.
Paolo
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