Re: [PATCH v3 22/28] KVM: x86/mmu: Zap defunct roots via asynchronous worker

[Paolo Bonzini]

On 3/2/22 20:33, Sean Christopherson wrote:
> What about that idea?  Put roots invalidated by "fast zap" on_another_  list?
> My very original idea of moving the roots to a separate list didn't work because
> the roots needed to be reachable by the mmu_notifier.  But we could just add
> another list_head (inside the unsync_child_bitmap union) and add the roots to
> _that_  list.

Perhaps the "separate list" idea could be extended to have a single 
worker for all kvm_tdp_mmu_put_root() work, and then indeed replace 
kvm_tdp_mmu_zap_invalidated_roots() with a flush of _that_ worker.  The 
disadvantage is a little less parallelism in zapping invalidated roots; 
but what is good for kvm_tdp_mmu_zap_invalidated_roots() is just as good 
for kvm_tdp_mmu_put_root(), I suppose.  If one wants separate work 
items, KVM could have its own workqueue, and then you flush that workqueue.

For now let's do it the simple but ugly way.  Keeping 
next_invalidated_root() does not make things worse than the status quo, 
and further work will be easier to review if it's kept separate from 
this already-complex work.

Paolo