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

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Date:   Wed, 2 Mar 2022 21:14:25 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        David Hildenbrand <david@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, David Matlack <dmatlack@...gle.com>,
        Ben Gardon <bgardon@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>
Subject: Re: [PATCH v3 22/28] KVM: x86/mmu: Zap defunct roots via asynchronous
 worker

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