On 4/26/22 21:06, Peter Gonda wrote:
On Thu, Apr 21, 2022 at 9:56 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> 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
