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 19:33:53 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.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 Wed, Mar 02, 2022, Paolo Bonzini wrote:
> On 3/2/22 19:01, Sean Christopherson wrote:
> > > It passes a smoke test, and also resolves the debate on the fate of patch 1.
> > +1000, I love this approach.  Do you want me to work on a v3, or shall I let you
> > have the honors?
> 
> I'm already running the usual battery of tests, so I should be able to post
> it either tomorrow (early in my evening) or Friday morning.

Gah, now I remember why I didn't use an async worker.  kvm_mmu_zap_all_fast()
must ensure all SPTEs are zapped and their dirty/accessed data written back to
the primary MMU prior to returning.  Once the memslot update completes, the old
deleted/moved memslot is no longer reachable by the mmu_notifier.  If an mmu_notifier
zaps pfns reachable via the root, KVM will do nothing because there's no relevant
memslot.

So we can use workers, but kvm_mmu_zap_all_fast() would need to flush all workers
before returning, which ends up being no different than putting the invalid roots
on a different list.

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.

Let me go resurrect that patch from v1 and tweak it to keep the roots on the old
list, but add them to a new list as well.  That would get rid of the invalid
root iterator stuff.