linux-kernel - Re: [PATCH v2 32/45] KVM: Move initialization of preempt notifier to kvm_vcpu

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Date:   Fri, 20 Dec 2019 10:50:49 +0100
From:   Cornelia Huck <cohuck@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Marc Zyngier <maz@...nel.org>, James Hogan <jhogan@...nel.org>,
        Paul Mackerras <paulus@...abs.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        David Hildenbrand <david@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-mips@...r.kernel.org, kvm-ppc@...r.kernel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Greg Kurz <groug@...d.org>
Subject: Re: [PATCH v2 32/45] KVM: Move initialization of preempt notifier
 to kvm_vcpu_init()

On Wed, 18 Dec 2019 13:55:17 -0800
Sean Christopherson <sean.j.christopherson@...el.com> wrote:

> Initialize the preempt notifier immediately in kvm_vcpu_init() to pave
> the way for removing kvm_arch_vcpu_setup(), i.e. to allow arch specific
> code to call vcpu_load() during kvm_arch_vcpu_create().
> 
> Back when preemption support was added, the location of the call to init
> the preempt notifier was perfectly sane.  The overall vCPU creation flow
> featured a single arch specific hook and the preempt notifer was used
> immediately after its initialization (by vcpu_load()).  E.g.:
> 
>         vcpu = kvm_arch_ops->vcpu_create(kvm, n);
>         if (IS_ERR(vcpu))
>                 return PTR_ERR(vcpu);
> 
>         preempt_notifier_init(&vcpu->preempt_notifier, &kvm_preempt_ops);
> 
>         vcpu_load(vcpu);
>         r = kvm_mmu_setup(vcpu);
>         vcpu_put(vcpu);
>         if (r < 0)
>                 goto free_vcpu;
> 
> Today, the call to preempt_notifier_init() is sandwiched between two
> arch specific calls, kvm_arch_vcpu_create() and kvm_arch_vcpu_setup(),
> which needlessly forces x86 (and possibly others?) to split its vCPU
> creation flow.  Init the preempt notifier prior to any arch specific
> call so that each arch can independently decide how best to organize
> its creation flow.
> 
> Acked-by: Christoffer Dall <christoffer.dall@....com>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
>  virt/kvm/kvm_main.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)

Reviewed-by: Cornelia Huck <cohuck@...hat.com>