linux-kernel - Re: [PATCH RFC v3 3/9] KVM: remember position in kvm->vcpus array

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Date:   Tue, 29 Aug 2017 13:30:59 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     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,
        Paolo Bonzini <pbonzini@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Christoffer Dall <cdall@...aro.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        James Hogan <james.hogan@...tec.com>,
        Paul Mackerras <paulus@...abs.org>,
        Alexander Graf <agraf@...e.com>
Subject: Re: [PATCH RFC v3 3/9] KVM: remember position in kvm->vcpus array

On Mon, 21 Aug 2017 22:35:24 +0200
Radim Krčmář <rkrcmar@...hat.com> wrote:

> Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>
> ---
>  include/linux/kvm_host.h | 11 +++--------
>  virt/kvm/kvm_main.c      |  5 ++++-
>  2 files changed, 7 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 6882538eda32..a8ff956616d2 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -220,7 +220,8 @@ struct kvm_vcpu {
>  	struct preempt_notifier preempt_notifier;
>  #endif
>  	int cpu;
> -	int vcpu_id;
> +	int vcpu_id; /* id given by userspace at creation */
> +	int vcpus_idx; /* index in kvm->vcpus array */

I predict confusion in the future over which is which... maybe
s/vcpus_idx/vcpus_array_pos/ or something like that?

>  	int srcu_idx;
>  	int mode;
>  	unsigned long requests;
> @@ -516,13 +517,7 @@ static inline struct kvm_vcpu *kvm_get_vcpu_by_id(struct kvm *kvm, int id)
>  
>  static inline int kvm_vcpu_get_idx(struct kvm_vcpu *vcpu)
>  {
> -	struct kvm_vcpu *tmp;
> -	int idx;
> -
> -	kvm_for_each_vcpu(idx, tmp, vcpu->kvm)
> -		if (tmp == vcpu)
> -			return idx;
> -	BUG();
> +	return vcpu->vcpus_idx;
>  }
>  
>  #define kvm_for_each_memslot(memslot, slots)	\
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index e17c40d986f3..caf8323f7df7 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2498,7 +2498,10 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, u32 id)
>  		goto unlock_vcpu_destroy;
>  	}
>  
> -	BUG_ON(kvm->vcpus[atomic_read(&kvm->online_vcpus)]);
> +	vcpu->vcpus_idx = atomic_read(&kvm->online_vcpus);
> +
> +	BUG_ON(kvm->vcpus[vcpu->vcpus_idx]);
> +
>  
>  	/* Now it's all set up, let userspace reach it */
>  	kvm_get_kvm(kvm);

I like the concept.