[PATCH 16/18] KVM: Don't take mmu_lock for range invalidation unless necessary

[Paolo Bonzini]

On 31/03/21 18:41, Sean Christopherson wrote:
>> That said, the easiest way to avoid this would be to always update
>> mmu_notifier_count.
> Updating mmu_notifier_count requires taking mmu_lock, which would defeat the
> purpose of these shenanigans.

Okay; I wasn't sure if the problem was contention with page faults in 
general, or just the long critical sections from the MMU notifier 
callbacks.  Still updating mmu_notifier_count unconditionally is a good 
way to break up the patch in two and keep one commit just for the rwsem 
nastiness.

>>> +#if defined(CONFIG_MMU_NOTIFIER) && defined(KVM_ARCH_WANT_MMU_NOTIFIER)
>>> +	down_write(&kvm->mmu_notifier_slots_lock);
>>> +#endif
>>>   	rcu_assign_pointer(kvm->memslots[as_id], slots);
>>> +#if defined(CONFIG_MMU_NOTIFIER) && defined(KVM_ARCH_WANT_MMU_NOTIFIER)
>>> +	up_write(&kvm->mmu_notifier_slots_lock);
>>> +#endif
>> Please do this unconditionally, the cost is minimal if the rwsem is not
>> contended (as is the case if the architecture doesn't use MMU notifiers at
>> all).
> It's not the cost, it's that mmu_notifier_slots_lock doesn't exist.  That's an
> easily solved problem, but then the lock wouldn't be initialized since
> kvm_init_mmu_notifier() is a nop.  That's again easy to solve, but IMO would
> look rather weird.  I guess the counter argument is that __kvm_memslots()
> wouldn't need #ifdeffery.

Yep.  Less #ifdefs usually wins. :)

> These are the to ideas I've come up with:
> 
> Option 1:
> 	static int kvm_init_mmu_notifier(struct kvm *kvm)
> 	{
> 		init_rwsem(&kvm->mmu_notifier_slots_lock);
> 
> 	#if defined(CONFIG_MMU_NOTIFIER) && defined(KVM_ARCH_WANT_MMU_NOTIFIER)
> 		kvm->mmu_notifier.ops = &kvm_mmu_notifier_ops;
> 		return mmu_notifier_register(&kvm->mmu_notifier, current->mm);
> 	#else
> 		return 0;
> 	#endif
> 	}

Option 2 is also okay I guess, but the simplest is option 1 + just init 
it in kvm_create_vm.

Paolo