linux-kernel - Re: [PATCH 6/6] KVM: perform a wake_up in kvm_make_all_cpus

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Date:   Mon, 10 Apr 2017 13:14:27 +0200
From:   Andrew Jones <drjones@...hat.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Christoffer Dall <cdall@...aro.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Cornelia Huck <cornelia.huck@...ibm.com>,
        James Hogan <james.hogan@...tec.com>,
        Paul Mackerras <paulus@...abs.org>
Subject: Re: [PATCH 6/6] KVM: perform a wake_up in kvm_make_all_cpus_request

On Thu, Apr 06, 2017 at 10:20:56PM +0200, Radim Krčmář 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.

Any reason we don't want kvm_vcpu_kick() to also get the
if (!(req & KVM_REQUEST_NO_WAKEUP)) optimization condition?  I did some
grepping, and don't see any kicks of the requests that have been marked as
NO_WAKEUP, so nothing should change by adding it now.  But the consistency
would be nice for the doc I'm writing.

Also, the condition in kvm_vcpu_kick() looks like overkill

 cpu != me && (unsigned)cpu < nr_cpu_ids && cpu_online(cpu)

How could vcpu->cpu ever be any offline/invalid cpu, other than -1?  The
condition in kvm_make_all_cpus_request() makes more sense to me

 cpu != -1 && cpu != me

I guess a lot this stuff is planned for a larger requests rework, when
kicks get integrated with requests?  I'm a bit anxious, though, as it
changes how I document stuff now, and even how I approach the ARM series.
For example, if kvm_make_request() already integrated kvm_vcpu_kick(),
which means also adding the smp_mb__after_atomic(), like
kvm_make_all_cpus_request() has, then I wouldn't need to add the smp_mb()
to kvm_arch_vcpu_should_kick().

Thanks,
drew

> 
> 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
> 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.
> 
> 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 a486c6ad27a6..1db503bab3dc 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_arch_vcpu_should_kick(vcpu))
>  			cpumask_set_cpu(cpu, cpus);
> -- 
> 2.12.0
>