linux-kernel - Re: [PATCH v3 2/3] KVM: X86: Implement PV sched yield hypercall

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Date:   Mon, 10 Jun 2019 16:17:17 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Liran Alon <liran.alon@...cle.com>
Subject: Re: [PATCH v3 2/3] KVM: X86: Implement PV sched yield hypercall

2019-05-30 09:05+0800, Wanpeng Li:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> The target vCPUs are in runnable state after vcpu_kick and suitable 
> as a yield target. This patch implements the sched yield hypercall.
> 
> 17% performance increasement of ebizzy benchmark can be observed in an 
> over-subscribe environment. (w/ kvm-pv-tlb disabled, testing TLB flush 
> call-function IPI-many since call-function is not easy to be trigged 
> by userspace workload).
> 
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Cc: Liran Alon <liran.alon@...cle.com>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> @@ -7172,6 +7172,28 @@ void kvm_vcpu_deactivate_apicv(struct kvm_vcpu *vcpu)
>  	kvm_x86_ops->refresh_apicv_exec_ctrl(vcpu);
>  }
>  
> +static void kvm_sched_yield(struct kvm *kvm, unsigned long dest_id)
> +{
> +	struct kvm_vcpu *target = NULL;
> +	struct kvm_apic_map *map = NULL;
> +
> +	rcu_read_lock();
> +	map = rcu_dereference(kvm->arch.apic_map);
> +
> +	if (unlikely(!map) || dest_id > map->max_apic_id)
> +		goto out;
> +
> +	if (map->phys_map[dest_id]->vcpu) {

This should check for map->phys_map[dest_id].

> +		target = map->phys_map[dest_id]->vcpu;
> +		rcu_read_unlock();
> +		kvm_vcpu_yield_to(target);
> +	}
> +
> +out:
> +	if (!target)
> +		rcu_read_unlock();

Also, I find the following logic clearer

  {
  	struct kvm_vcpu *target = NULL;
  	struct kvm_apic_map *map;
  	
  	rcu_read_lock();
  	map = rcu_dereference(kvm->arch.apic_map);
  	
  	if (likely(map) && dest_id <= map->max_apic_id && map->phys_map[dest_id])
  		target = map->phys_map[dest_id]->vcpu;
  	
  	rcu_read_unlock();
  	
  	if (target)
  		kvm_vcpu_yield_to(target);
  }

thanks.