Messages in this thread | | | From | Paolo Bonzini <> | Subject | Re: [PATCH 19/24] kvm: x86/mmu: Protect tdp_mmu_pages with a lock | Date | Tue, 26 Jan 2021 15:27:54 +0100 |
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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/
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