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SubjectRe: [PATCH v3 2/3] KVM: X86: Implement PV sched yield hypercall
2019-05-30 09:05+0800, Wanpeng Li:
> From: Wanpeng Li <wanpengli@tencent.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@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Cc: Liran Alon <liran.alon@oracle.com>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.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.

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