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From: Paolo Bonzini <pbonzini@redhat.com>
To: Ben Gardon <bgardon@google.com>
Cc: LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
	Peter Xu <peterx@redhat.com>,
	Sean Christopherson <seanjc@google.com>,
	Peter Shier <pshier@google.com>,
	Junaid Shahid <junaids@google.com>,
	Jim Mattson <jmattson@google.com>,
	Yulei Zhang <yulei.kernel@gmail.com>,
	Wanpeng Li <kernellwp@gmail.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Xiao Guangrong <xiaoguangrong.eric@gmail.com>
Subject: Re: [PATCH v2 1/7] KVM: x86/mmu: Track if shadow MMU active
Date: Tue, 4 May 2021 22:18:40 +0200	[thread overview]
Message-ID: <34fe30b6-0d4b-f1e8-9abd-6cb0a0765492@redhat.com> (raw)
In-Reply-To: <CANgfPd9O3d9b+WYgo+ke1Jx50=ep_f-ZC1gRqUET6PDsLxW+Gw@mail.gmail.com>

On 04/05/21 19:26, Ben Gardon wrote:
> On Mon, May 3, 2021 at 6:42 AM Paolo Bonzini <pbonzini@redhat.com> wrote:
>>
>> On 29/04/21 23:18, Ben Gardon wrote:
>>> +void activate_shadow_mmu(struct kvm *kvm)
>>> +{
>>> +     kvm->arch.shadow_mmu_active = true;
>>> +}
>>> +
>>
>> I think there's no lock protecting both the write and the read side.
>> Therefore this should be an smp_store_release, and all checks in
>> patch 2 should be an smp_load_acquire.
> 
> That makes sense.
> 
>>
>> Also, the assignments to slot->arch.rmap in patch 4 (alloc_memslot_rmap)
>> should be an rcu_assign_pointer, while __gfn_to_rmap must be changed like so:
>>
>> +       struct kvm_rmap_head *head;
>> ...
>> -       return &slot->arch.rmap[level - PG_LEVEL_4K][idx];
>> +       head = srcu_dereference(slot->arch.rmap[level - PG_LEVEL_4K], &kvm->srcu,
>> +                                lockdep_is_held(&kvm->slots_arch_lock));
>> +       return &head[idx];
> 
> I'm not sure I fully understand why this becomes necessary after patch
> 4. Isn't it already needed since the memslots are protected by RCU? Or
> is there already a higher level rcu dereference?
> 
> __kvm_memslots already does an srcu dereference, so is there a path
> where we aren't getting the slots from that function where this is
> needed?

There are two point of views:

1) the easier one is just CONFIG_PROVE_RCU debugging: the rmaps need to 
be accessed under RCU because the memslots can disappear as soon as 
kvm->srcu is unlocked.

2) the harder one (though at this point I'm better at figuring out these 
ordering bugs than "traditional" mutex races) is what the happens before 
relation[1] looks like.  Consider what happens if the rmaps are 
allocated by *another thread* after the slots have been fetched.

thread 1		thread 2		thread 3
allocate memslots
rcu_assign_pointer
			slots = srcu_dereference
						allocate rmap
						rcu_assign_pointer
			head = slot->arch.rmap[]

Here, thread 3 is allocating the rmaps in the SRCU-protected 
kvm_memslots; those rmaps that didn't exist at the time thread 1 did the 
rcu_assign_pointer (which synchronizes with thread 2's srcu_dereference 
that retrieves slots), hence they were not covered by the release 
semantics of that rcu_assign_pointer and the "consume" semantics of the 
corresponding srcu_dereference.  Therefore, thread 2 needs another 
srcu_dereference when retrieving them.

Paolo

[1] https://lwn.net/Articles/844224/

> I wouldn't say that the rmaps are protected by RCU in any way that
> separate from the memslots.


  reply	other threads:[~2021-05-04 20:18 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-29 21:18 [PATCH v2 0/7] Lazily allocate memslot rmaps Ben Gardon
2021-04-29 21:18 ` [PATCH v2 1/7] KVM: x86/mmu: Track if shadow MMU active Ben Gardon
2021-05-03 13:42   ` Paolo Bonzini
2021-05-04 17:26     ` Ben Gardon
2021-05-04 20:18       ` Paolo Bonzini [this message]
2021-05-04 19:55   ` Sean Christopherson
2021-05-04 20:26     ` Paolo Bonzini
2021-05-04 20:36       ` Sean Christopherson
2021-04-29 21:18 ` [PATCH v2 2/7] KVM: x86/mmu: Skip rmap operations if shadow MMU inactive Ben Gardon
2021-04-29 21:18 ` [PATCH v2 3/7] KVM: x86/mmu: Deduplicate rmap freeing Ben Gardon
2021-04-29 21:18 ` [PATCH v2 4/7] KVM: x86/mmu: Factor out allocating memslot rmap Ben Gardon
2021-04-29 21:18 ` [PATCH v2 5/7] KVM: mmu: Refactor memslot copy Ben Gardon
2021-04-29 21:18 ` [PATCH v2 6/7] KVM: mmu: Add slots_arch_lock for memslot arch fields Ben Gardon
2021-05-03 13:29   ` Paolo Bonzini
2021-04-29 21:18 ` [PATCH v2 7/7] KVM: x86/mmu: Lazily allocate memslot rmaps Ben Gardon
2021-05-03 13:42   ` Paolo Bonzini
2021-05-03 17:29     ` Ben Gardon
2021-05-04 20:13   ` Sean Christopherson
2021-05-04 20:19     ` Paolo Bonzini
2021-05-04 20:34       ` Sean Christopherson
2021-05-04 20:22   ` Paolo Bonzini
2021-05-03 13:44 ` [PATCH v2 0/7] " Paolo Bonzini
2021-05-03 17:31   ` Ben Gardon
2021-05-04  7:21     ` Paolo Bonzini
2021-05-04 17:28       ` Ben Gardon
2021-05-04 18:17         ` Sean Christopherson

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