linux-kernel - Re: [PATCH v1] KVM: x86: avoid redundant REQ

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Date:   Mon, 12 Dec 2016 17:29:43 +0100
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Denis Plotnikov <dplotnikov@...tuozzo.com>
Cc:     rkagan@...tuozzo.com, den@...tuozzo.com, pbonzini@...hat.com,
        svt-core@...ts.sw.ru, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] KVM: x86: avoid redundant REQ_EVENT

2016-12-12 17:02+0300, Denis Plotnikov:
> When processing KVM_REQ_EVENT, apic_update_ppr is called which may set
> KVM_REQ_EVENT again if the recalculated value of PPR becomes smaller
> than the previous one. This results in cancelling the guest entry and
> reiterating in vcpu_enter_guest.
> 
> However this is unnecessary because at this point KVM_REQ_EVENT is
> already being processed and there are no other changes in the lapic
> that may require full-fledged state recalculation.
> 
> This situation is often hit on systems with TPR shadow, where the
> TPR can be updated by the guest without a vmexit, so that the first
> apic_update_ppr to notice it is exactly the one called while
> processing KVM_REQ_EVENT.
> 
> To avoid it, introduce a parameter in apic_update_ppr allowing to
> suppress setting of KVM_REQ_EVENT, and use it on the paths called from
> KVM_REQ_EVENT processing.

We also call:

  kvm_cpu_get_interrupt() in nested_vmx_vmexit()
    - that path is intended without KVM_REQ_EVENT
  kvm_cpu_has_interrupt() in vmx_check_nested_events(),
    - I think it does no harm
  kvm_cpu_has_interrupt() in kvm_vcpu_has_events()
  kvm_cpu_has_interrupt() in kvm_vcpu_ready_for_interrupt_injection()
    - both seem safe as we should not have an interrupt between TPR
      threshold and the new PPR value, so the KVM_REQ_EVENT was useless.

I would prefer we made sure that only callers from KVM_REQ_EVENT used
the function we are changing -- it is really easy to make a hard-to-find
mistake in interrupt delivery.

> This microoptimization gives 10% performance increase on a synthetic
> test doing a lot of IPC in Windows using window messages.
> 
> Reviewed-by: Roman Kagan <rkagan@...tuozzo.com>
> Signed-off-by: Denis Plotnikov <dplotnikov@...tuozzo.com>
> ---

Still, there is a high possibility that this is going to work,

Reviewed-by: Radim Krčmář <rkrcmar@...hat.com>