Re: [PATCH v4 21/30] KVM: x86/mmu: Zap invalidated roots via asynchronous worker

[Sean Christopherson]

On Fri, Mar 04, 2022, Paolo Bonzini wrote:
> On 3/3/22 22:32, Sean Christopherson wrote:
> I didn't remove the paragraph from the commit message, but I think it's
> unnecessary now.  The workqueue is flushed in kvm_mmu_zap_all_fast() and
> kvm_mmu_uninit_tdp_mmu(), unlike the buggy patch, so it doesn't need to take
> a reference to the VM.
> 
> I think I don't even need to check kvm->users_count in the defunct root
> case, as long as kvm_mmu_uninit_tdp_mmu() flushes and destroys the workqueue
> before it checks that the lists are empty.

Yes, that should work.  IIRC, the WARN_ONs will tell us/you quite quickly if
we're wrong :-)  mmu_notifier_unregister() will call the "slow" kvm_mmu_zap_all()
and thus ensure all non-root pages zapped, but "leaking" a worker will trigger
the WARN_ON that there are no roots on the list.