linux-kernel - Re: [PATCH v2] KVM: x86: avoid redundant REQ

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Date:   Wed, 14 Dec 2016 22:36:29 +0100
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Denis Plotnikov <dplotnikov@...tuozzo.com>
Cc:     rkagan@...tuozzo.com, den@...tuozzo.com, pbonzini@...hat.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] KVM: x86: avoid redundant REQ_EVENT

2016-12-14 13:59+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.
> Also add the parameter to kvm_cpu_get_interrupt to be passed through
> to apic_update_ppr to make sure the supression of KVM_REQ_EVENT is done
> in the KVM_REQ_EVENT processing path only.
> 
> 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>
> ---

It is not pretty, but desirable,

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

Thanks.