Re: [PATCH 19/24] kvm: x86/mmu: Protect tdp_mmu_pages with a lock

[Paolo Bonzini <pbonzini@redhat.com>]

On 21/01/21 22:32, Sean Christopherson wrote:
> Coming back to this series, I wonder if the RCU approach is truly necessary to
> get the desired scalability.  If both zap_collapsible_sptes() and NX huge page
> recovery zap_only_  leaf SPTEs, then the only path that can actually unlink a
> shadow page while holding the lock for read is the page fault path that installs
> a huge page over an existing shadow page.
> 
> Assuming the above analysis is correct, I think it's worth exploring alternatives
> to using RCU to defer freeing the SP memory, e.g. promoting to a write lock in
> the specific case of overwriting a SP (though that may not exist for rwlocks),
> or maybe something entirely different?

You can do the deferred freeing with a short write-side critical section 
to ensure all readers have terminated.

If the bool argument to handle_disconnected_tdp_mmu_page is true(*), the 
pages would be added to an llist, instead of being freed immediately. 
At the end of a shared critical section you would do

	if (!llist_empty(&kvm->arch.tdp_mmu_disconnected_pages)) {
		struct llist_node *first;
		kvm_mmu_lock(kvm);
		first = __list_del_all(&kvm->arch.tdp_mmu_disconnected_pages);
		kvm_mmu_unlock(kvm);

		/*
		 * All vCPUs have already stopped using the pages when
		 * their TLBs were flushed.  The exclusive critical
		 * section above means that there can be no readers
		 * either.
		 */
		tdp_mmu_free_disconnected_pages(first);
	}

So this is still deferred reclamation, but it's done by one of the vCPUs 
rather than a worker RCU thread.  This would replace patches 11/12/13 
and probably would be implemented after patch 18.

Paolo

(*) this idea is what prompted the comment about s/atomic/shared/