Re: [PATCH v3] KVM: SEV: Mark nested locking of vcpu->lock

[Peter Gonda]

On Fri, Apr 29, 2022 at 11:21 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> On Fri, Apr 29, 2022 at 7:12 PM Peter Gonda <pgonda@xxxxxxxxxx> wrote:
> > Sounds good. Instead of doing this prev_vcpu solution we could just
> > keep the 1st vcpu for source and target. I think this could work since
> > all the vcpu->mutex.dep_maps do not point to the same string.
> >
> > Lock:
> >          bool acquired = false;
> >          kvm_for_each_vcpu(...) {
> >                  if (mutex_lock_killable_nested(&vcpu->mutex, role)
> >                      goto out_unlock;
> >                 acquired = true;
> >                  if (acquired)
> >                       mutex_release(&vcpu->mutex, role)
> >          }
>
> Almost:
>
>           bool first = true;
>           kvm_for_each_vcpu(...) {
>                   if (mutex_lock_killable_nested(&vcpu->mutex, role)
>                       goto out_unlock;
>                   if (first)
>                       ++role, first = false;
>                  else
>                       mutex_release(&vcpu->mutex, role);
>          }
>
> and to unlock:
>
>           bool first = true;
>           kvm_for_each_vcpu(...) {
>                 if (first)
>                       first = false;
>                 else
>                       mutex_acquire(&vcpu->mutex, role);
>                 mutex_unlock(&vcpu->mutex);
>                 acquired = false;
>           }
>
> because you cannot use the first vCPU's role again when locking.

Ah yes I missed that. I would suggest `role = SEV_NR_MIGRATION_ROLES`
or something else instead of role++ to avoid leaking this
implementation detail outside of the function signature / enum.


>
> Paolo
>