From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Chen, Tiejun" Subject: Re: [PATCH] kvm: x86: lapic: remove one redundant judging condition Date: Thu, 06 Nov 2014 09:29:29 +0800 Message-ID: <545ACEF9.3000607@intel.com> References: <1415178185-11109-1-git-send-email-tiejun.chen@intel.com> <5459FA5B.8060207@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Cc: kvm@vger.kernel.org To: Paolo Bonzini Return-path: Received: from mga14.intel.com ([192.55.52.115]:11385 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750999AbaKFB34 (ORCPT ); Wed, 5 Nov 2014 20:29:56 -0500 In-Reply-To: <5459FA5B.8060207@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On 2014/11/5 18:22, Paolo Bonzini wrote: > On 05/11/2014 10:03, Tiejun Chen wrote: >> Finally we always return highest_irr so its unnecessary to return -1 >> after check if highest_irr == -1. >> >> Signed-off-by: Tiejun Chen >> --- >> arch/x86/kvm/lapic.c | 3 +-- >> 1 file changed, 1 insertion(+), 2 deletions(-) >> >> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c >> index 5f574b4..e6a7eb6 100644 >> --- a/arch/x86/kvm/lapic.c >> +++ b/arch/x86/kvm/lapic.c >> @@ -1638,8 +1638,7 @@ int kvm_apic_has_interrupt(struct kvm_vcpu *vcpu) >> >> apic_update_ppr(apic); >> highest_irr = apic_find_highest_irr(apic); >> - if ((highest_irr == -1) || >> - ((highest_irr & 0xF0) <= kvm_apic_get_reg(apic, APIC_PROCPRI))) >> + if ((highest_irr & 0xF0) <= kvm_apic_get_reg(apic, APIC_PROCPRI)) >> return -1; >> return highest_irr; >> } > > I think the code is clearer without this change. > > The two returns mean: > > - return -1: no interrupt to inject > > - return highest_irr: inject this interrupt > > With IRR equal to all zeroes (highest_irr = -1), your patch would make > the "if" always false ("current PPR is low, can inject the interrupt"), > but computing highest_irr & 0xF0 would make no sense if highest_irr == > -1. Yeah, you're right so here is just a little confusion to read. Actually what this code is doing looks like, @@ -1638,7 +1638,7 @@ int kvm_apic_has_interrupt(struct kvm_vcpu *vcpu) apic_update_ppr(apic); highest_irr = apic_find_highest_irr(apic); - if ((highest_irr == -1) || + if ((highest_irr != -1) && ((highest_irr & 0xF0) <= kvm_apic_get_reg(apic, APIC_PROCPRI))) return -1; return highest_irr; But it's really no big deal so we can keep the original alive. Thanks Tiejun > > To put it another way, imagine the code looked like this: > > static inline int int_prio(int vector) > { > WARN_ON(vector == -1); > return vector & 0xF0; > } > ... > > apic_update_ppr(apic); > highest_irr = apic_find_highest_irr(apic); > if (highest_irr == -1 || > int_prio(highest_irr) <= kvm_apic_get_reg(apic, APIC_PROCPRI)) > return -1; > return highest_irr; > > Then removing the check on highest_irr == -1 would trigger a warning. > > Paolo >