Date:   Sat, 5 Mar 2022 20:53:42 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        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>,
        David Matlack <dmatlack@...gle.com>,
        Ben Gardon <bgardon@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>
Subject: Re: [PATCH v4 21/30] KVM: x86/mmu: Zap invalidated roots via
 asynchronous worker

On 3/5/22 01:34, Sean Christopherson wrote:
> On Fri, Mar 04, 2022, Paolo Bonzini wrote:
>> On 3/4/22 17:02, Sean Christopherson wrote:
>>> On Fri, Mar 04, 2022, Paolo Bonzini wrote:
>>>> On 3/3/22 22:32, Sean Christopherson wrote:
>>>> I didn't remove the paragraph from the commit message, but I think it's
>>>> unnecessary now.  The workqueue is flushed in kvm_mmu_zap_all_fast() and
>>>> kvm_mmu_uninit_tdp_mmu(), unlike the buggy patch, so it doesn't need to take
>>>> a reference to the VM.
>>>>
>>>> I think I don't even need to check kvm->users_count in the defunct root
>>>> case, as long as kvm_mmu_uninit_tdp_mmu() flushes and destroys the workqueue
>>>> before it checks that the lists are empty.
>>>
>>> Yes, that should work.  IIRC, the WARN_ONs will tell us/you quite quickly if
>>> we're wrong :-)  mmu_notifier_unregister() will call the "slow" kvm_mmu_zap_all()
>>> and thus ensure all non-root pages zapped, but "leaking" a worker will trigger
>>> the WARN_ON that there are no roots on the list.
>>
>> Good, for the record these are the commit messages I have:

I'm seeing some hangs in ~50% of installation jobs, both Windows and 
Linux.  I have not yet tried to reproduce outside the automated tests, 
or to bisect, but I'll try to push at least the first part of the series 
for 5.18.

Paolo

>>      KVM: x86/mmu: Zap invalidated roots via asynchronous worker
>>      Use the system worker threads to zap the roots invalidated
>>      by the TDP MMU's "fast zap" mechanism, implemented by
>>      kvm_tdp_mmu_invalidate_all_roots().
>>      At this point, apart from allowing some parallelism in the zapping of
>>      roots, the workqueue is a glorified linked list: work items are added and
>>      flushed entirely within a single kvm->slots_lock critical section.  However,
>>      the workqueue fixes a latent issue where kvm_mmu_zap_all_invalidated_roots()
>>      assumes that it owns a reference to all invalid roots; therefore, no
>>      one can set the invalid bit outside kvm_mmu_zap_all_fast().  Putting the
>>      invalidated roots on a linked list... erm, on a workqueue ensures that
>>      tdp_mmu_zap_root_work() only puts back those extra references that
>>      kvm_mmu_zap_all_invalidated_roots() had gifted to it.
>>
>> and
>>
>>      KVM: x86/mmu: Zap defunct roots via asynchronous worker
>>      Zap defunct roots, a.k.a. roots that have been invalidated after their
>>      last reference was initially dropped, asynchronously via the existing work
>>      queue instead of forcing the work upon the unfortunate task that happened
>>      to drop the last reference.
>>      If a vCPU task drops the last reference, the vCPU is effectively blocked
>>      by the host for the entire duration of the zap.  If the root being zapped
>>      happens be fully populated with 4kb leaf SPTEs, e.g. due to dirty logging
>>      being active, the zap can take several hundred seconds.  Unsurprisingly,
>>      most guests are unhappy if a vCPU disappears for hundreds of seconds.
>>      E.g. running a synthetic selftest that triggers a vCPU root zap with
>>      ~64tb of guest memory and 4kb SPTEs blocks the vCPU for 900+ seconds.
>>      Offloading the zap to a worker drops the block time to <100ms.
>>      There is an important nuance to this change.  If the same work item
>>      was queued twice before the work function has run, it would only
>>      execute once and one reference would be leaked.  Therefore, now that
>>      queueing items is not anymore protected by write_lock(&kvm->mmu_lock),
>>      kvm_tdp_mmu_invalidate_all_roots() has to check root->role.invalid and
>>      skip already invalid roots.  On the other hand, kvm_mmu_zap_all_fast()
>>      must return only after those skipped roots have been zapped as well.
>>      These two requirements can be satisfied only if _all_ places that
>>      change invalid to true now schedule the worker before releasing the
>>      mmu_lock.  There are just two, kvm_tdp_mmu_put_root() and
>>      kvm_tdp_mmu_invalidate_all_roots().
> 
> Very nice!
> 

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