linux-kernel - Re: [Patch v1 1/2] KVM: remember position in kvm->vcpus array

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Message-ID: <20191106144326.GA16249@linux.intel.com>
Date:   Wed, 6 Nov 2019 06:43:26 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Nitesh Narayan Lal <nitesh@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, mtosatti@...hat.com, rkrcmar@...hat.com,
        vkuznets@...hat.com, wanpengli@...cent.com, jmattson@...gle.com,
        joro@...tes.org
Subject: Re: [Patch v1 1/2] KVM: remember position in kvm->vcpus array

On Wed, Nov 06, 2019 at 08:36:37AM -0500, Nitesh Narayan Lal wrote:
> From: Radim Krčmář <rkrcmar@...hat.com>
> 
> Fetching an index for any vcpu in kvm->vcpus array by traversing
> the entire array everytime is costly.
> This patch remembers the position of each vcpu in kvm->vcpus array
> by storing it in vcpus_idx under kvm_vcpu structure.
> 
> Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>
> Signed-off-by: Nitesh Narayan Lal <nitesh@...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 719fc3e..31c4fde 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -266,7 +266,8 @@ 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'd probably prefer vcpu_idx or vcpu_index, but it's not a strong
preference by any means.

>  	int srcu_idx;
>  	int mode;
>  	u64 requests;
> @@ -571,13 +572,7 @@ 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 67ef3f2..24ab711 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2673,7 +2673,10 @@ 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);
> +

Nit: I'd omit this newline since the assignment and BUG_ON() are directly
related.

> +	BUG_ON(kvm->vcpus[vcpu->vcpus_idx]);

The assignment to kvm->vcpus a few lines below should be updated to use
the new index.

> +
>  
>  	/* Now it's all set up, let userspace reach it */
>  	kvm_get_kvm(kvm);
> -- 
> 1.8.3.1
>