linux-kernel - Re: [PATCH v2 2/3] KVM: X86: Implement PV sched yield hypercall

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Date:   Tue, 28 May 2019 11:11:54 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v2 2/3] KVM: X86: Implement PV sched yield hypercall

On 28.05.19 02:53, Wanpeng Li wrote:
> 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% performace increase 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>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>

FWIW, we do have a similar interface in s390.

See arch/s390/kvm/diag.c  __diag_time_slice_end_directed for our implementation.
> ---
>  arch/x86/kvm/x86.c | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e7e57de..2ceef51 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7172,6 +7172,26 @@ void kvm_vcpu_deactivate_apicv(struct kvm_vcpu *vcpu)
>  	kvm_x86_ops->refresh_apicv_exec_ctrl(vcpu);
>  }
> 
> +void kvm_sched_yield(struct kvm *kvm, u64 dest_id)
> +{
> +	struct kvm_vcpu *target;
> +	struct kvm_apic_map *map;
> +
> +	rcu_read_lock();
> +	map = rcu_dereference(kvm->arch.apic_map);
> +
> +	if (unlikely(!map))
> +		goto out;
> +
> +	if (map->phys_map[dest_id]->vcpu) {
> +		target = map->phys_map[dest_id]->vcpu;
> +		kvm_vcpu_yield_to(target);
> +	}
> +
> +out:
> +	rcu_read_unlock();
> +}
> +
>  int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
>  {
>  	unsigned long nr, a0, a1, a2, a3, ret;
> @@ -7218,6 +7238,10 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
>  	case KVM_HC_SEND_IPI:
>  		ret = kvm_pv_send_ipi(vcpu->kvm, a0, a1, a2, a3, op_64_bit);
>  		break;
> +	case KVM_HC_SCHED_YIELD:
> +		kvm_sched_yield(vcpu->kvm, a0);
> +		ret = 0;
> +		break;
>  	default:
>  		ret = -KVM_ENOSYS;
>  		break;
>