Re: [PATCH 2/3] KVM: use separate generations for each address space
From: Paolo Bonzini
Date: Fri Feb 17 2017 - 03:28:33 EST
> > + /*
> > + * Generations must be different for each address space.
> > + * Init kvm generation close to the maximum to easily test the
> > + * code of handling generation number wrap-around.
> > + */
> > + slots->generation = i * 2 - 150;
> > + rcu_assign_pointer(kvm->memslots[i], slots);
> > }
> I can't seem to understand why rcu_assign_pointer wasn't used before.
> kvm->memslots[i] was a rcu protected pointer even before this change,
> right ?
Actually, a better match is RCU_INIT_POINTER. Here there is no concurrent
reader because we're just initializing the struct kvm. There is something
else providing synchronization between this writer and the "first" RCU
read-side. It could be signaling a condition variable, creating a thread,
or releasing a mutex; all three of them have release semantics, which
means they imply a smp_wmb just like rcu_assign_pointer does.
> > if (init_srcu_struct(&kvm->srcu))
> > @@ -870,8 +872,14 @@ static struct kvm_memslots
> > *install_new_memslots(struct kvm *kvm,
> > * Increment the new memslot generation a second time. This prevents
> > * vm exits that race with memslot updates from caching a memslot
> > * generation that will (potentially) be valid forever.
> > + *
> > + * Generations must be unique even across address spaces. We do not need
> > + * a global counter for that, instead the generation space is evenly
> > split
> > + * across address spaces. For example, with two address spaces, address
> > + * space 0 will use generations 0, 4, 8, ... while * address space 1 will
> > + * use generations 2, 6, 10, 14, ...
> > */
> > - slots->generation++;
> > + slots->generation += KVM_ADDRESS_SPACE_NUM * 2 - 1;
> > kvm_arch_memslots_updated(kvm, slots);