linux-kernel - Re: [PATCH v2 5/9] KVM: perform a wake_up in kvm_make_all_cpus

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Message-Id: <20170427140655.40eebb34.cornelia.huck@de.ibm.com>
Date:   Thu, 27 Apr 2017 14:06:55 +0200
From:   Cornelia Huck <cornelia.huck@...ibm.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Christoffer Dall <cdall@...aro.org>,
        Andrew Jones <drjones@...hat.com>,
        Marc Zyngier <marc.zyngier@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        James Hogan <james.hogan@...tec.com>,
        Paul Mackerras <paulus@...abs.org>
Subject: Re: [PATCH v2 5/9] KVM: perform a wake_up in
 kvm_make_all_cpus_request

On Wed, 26 Apr 2017 22:32:23 +0200
Radim Krčmář <rkrcmar@...hat.com> wrote:

> We want to have kvm_make_all_cpus_request() to be an optmized version of
> 
>   kvm_for_each_vcpu(i, vcpu, kvm) {
>     kvm_make_request(vcpu, request);
>     kvm_vcpu_kick(vcpu);
>   }
> 
> and kvm_vcpu_kick() wakes up the target vcpu.  We know which requests do
> not need the wake up and use it to optimize the loop.
> 
> Thanks to that, this patch doesn't change the behavior of current users
> (the all don't need the wake up) and only prepares for future where the

s/the all/they all/

> wake up is going to be needed.
> 
> I think that most requests do not need the wake up, so we would flip the
> bit then.
> 
> kvm_vcpu_kick() will get this condition after it is merged with
> kvm_make_request() because we currently don't know which request is being
> kicked.

I find this sentence confusing: not all kicks are directly related to
requests.

> 
> Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>
> ---
>  virt/kvm/kvm_main.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index e5d52b46b531..3772f7dcc72d 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -186,6 +186,9 @@ bool kvm_make_all_cpus_request(struct kvm *kvm, unsigned int req)
>  		/* Set ->requests bit before we read ->mode. */
>  		smp_mb__after_atomic();
> 
> +		if (!(req & KVM_REQUEST_NO_WAKEUP))
> +			kvm_vcpu_wake_up(vcpu);
> +
>  		if (cpus != NULL && cpu != -1 && cpu != me &&
>  		      kvm_vcpu_exiting_guest_mode(vcpu) != OUTSIDE_GUEST_MODE)
>  			cpumask_set_cpu(cpu, cpus);

The code change looks good to me.