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

[Sean Christopherson <seanjc@google.com>]

On Wed, Mar 02, 2022, Paolo Bonzini wrote:
> 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.

Oof, that's not gonna work.  My approach here in v3 doesn't work either.  I finally
remembered why I had the dedicated tdp_mmu_defunct_root flag and thus the smp_mb_*()
dance.

kvm_tdp_mmu_zap_invalidated_roots() assumes that it was gifted a reference to
_all_ invalid roots by kvm_tdp_mmu_invalidate_all_roots().  This works in the
current code base only because kvm->slots_lock is held for the entire duration,
i.e. roots can't become invalid between the end of kvm_tdp_mmu_invalidate_all_roots()
and the end of kvm_tdp_mmu_zap_invalidated_roots().

Marking a root invalid in kvm_tdp_mmu_put_root() breaks that assumption, e.g. if a
new root is created and then dropped, it will be marked invalid but the "fast zap"
will not have a reference.  The "defunct" flag prevents this scenario by allowing
the "fast zap" path to identify invalid roots for which it did not take a reference.
By virtue of holding a reference, "fast zap" also guarantees that the roots it needs
to invalidate and put can't become defunct.

My preference would be to either go back to a variant of v2, or to implement my
"second list" idea.  

I also need to figure out why I didn't encounter errors in v3, because I distinctly
remember underflowing the refcount before adding the defunct flag...