lkml.org 
[lkml]   [2021]   [Mar]   [31]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH 16/18] KVM: Don't take mmu_lock for range invalidation unless necessary
From
Date
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

\
 
 \ /
  Last update: 2021-03-31 18:49    [W:0.081 / U:0.944 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site