linux-kernel - Re: [PATCH RFC 2/2] KVM: RCU protected dynamic vcpus array

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Date:   Thu, 17 Aug 2017 13:14:27 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Radim Krčmář <rkrcmar@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-mips@...ux-mips.org, kvm-ppc@...r.kernel.org,
        linux-s390@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Christoffer Dall <cdall@...aro.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Cornelia Huck <cohuck@...hat.com>,
        James Hogan <james.hogan@...tec.com>,
        Paul Mackerras <paulus@...abs.org>,
        Alexander Graf <agraf@...e.com>
Subject: Re: [PATCH RFC 2/2] KVM: RCU protected dynamic vcpus array


>  	atomic_set(&kvm->online_vcpus, 0);
>  	mutex_unlock(&kvm->lock);
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index c8df733eed41..eb9fb5b493ac 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -386,12 +386,17 @@ struct kvm_memslots {
>  	int used_slots;
>  };
>  
> +struct kvm_vcpus {
> +	u32 online;
> +	struct kvm_vcpu *array[];

On option could be to simply chunk it:

+struct kvm_vcpus {
+       struct kvm_vcpu vcpus[32];
+};
+
 /*
  * Note:
  * memslots are not sorted by id anymore, please use id_to_memslot()
@@ -391,7 +395,7 @@ struct kvm {
        struct mutex slots_lock;
        struct mm_struct *mm; /* userspace tied to this vm */
        struct kvm_memslots __rcu *memslots[KVM_ADDRESS_SPACE_NUM];
-       struct kvm_vcpu *vcpus[KVM_MAX_VCPUS];
+       struct kvm_vcpus vcpus[(KVM_MAX_VCPUS + 31) / 32];

        /*
         * created_vcpus is protected by kvm->lock, and is incremented
@@ -483,12 +487,14 @@ static inline struct kvm_io_bus
*kvm_get_bus(struct kvm *kvm, enum kvm_bus idx)


1. make nobody access kvm->vcpus directly (factor out)
2. allocate next chunk if necessary when creating a VCPU and store
pointer using WRITE_ONCE
3. use READ_ONCE to test for availability of the current chunk

kvm_for_each_vcpu just has to use READ_ONCE to access/test for the right
chunk. Pointers never get invalid. No RCU needed. Sleeping in the loop
is possible.

-- 

Thanks,

David