linux-kernel - Re: [PATCH v1 1/3] KVM: VMX: Consider PID.PIR to determine if vCPU has pending interrupts

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Date:   Mon, 11 Nov 2019 16:58:33 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Joao Martins <joao.m.martins@...cle.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Liran Alon <liran.alon@...cle.com>,
        Jag Raman <jag.raman@...cle.com>
Subject: Re: [PATCH v1 1/3] KVM: VMX: Consider PID.PIR to determine if vCPU
 has pending interrupts

On 11/11/19 15:59, Joao Martins wrote:
>> Should we check the bitmap only if SN is false?
                                            ^^^^^

Of course it should be skipped if SN is false, as you correctly say below.

>> We have a precondition
>> that if SN is clear then non-empty PIR implies ON=1 (modulo the small
>> window in vmx_vcpu_pi_load of course), so that'd be a bit faster.
> Makes sense;
> 
> The bitmap check was really meant for SN=1.
> 
> Should SN=0 we would be saving ~22-27 cycles as far as I micro-benchmarked a few
> weeks ago. Now that you suggest it, it would be also good for older platforms too.

Or even newer platforms if they don't use VT-d.

Paolo