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

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Message-ID: <CANRm+CyGZiEfWb4KaXU2LG1AMcFq+jmb6kfjnPkKeyG-zOQPAA@mail.gmail.com>
Date:   Tue, 11 Jun 2019 16:47:15 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <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

On Mon, 10 Jun 2019 at 22:17, Radim Krčmář <rkrcmar@...hat.com> wrote:
>
> 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].

Yeah, make a mistake here.

>
> > +             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);
>   }

More better, thanks.

Regards,
Wanpeng Li