linux-kernel - Re: [PATCH 1/3] KVM: APIC: remove unnecessary double checks on APIC existence

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Message-ID: <20160209135502.GA30378@potion.brq.redhat.com>
Date:	Tue, 9 Feb 2016 14:55:03 +0100
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 1/3] KVM: APIC: remove unnecessary double checks on APIC
 existence

2016-02-08 17:15+0100, Paolo Bonzini:
> Usually the in-kernel APIC's existence is checked in the caller.  Do not
> bother checking it again in lapic.c.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
>  static int __apic_accept_irq(struct kvm_lapic *apic, int delivery_mode,
> @@ -1601,8 +1595,7 @@ static int apic_mmio_write(struct kvm_vcpu *vcpu, struct kvm_io_device *this,
>  
>  void kvm_lapic_set_eoi(struct kvm_vcpu *vcpu)
>  {
> -	if (kvm_vcpu_has_lapic(vcpu))
> -		apic_reg_write(vcpu->arch.apic, APIC_EOI, 0);
> +	apic_reg_write(vcpu->arch.apic, APIC_EOI, 0);

This is most likely going to bug on the following path:
  handle_apic_access -> kvm_lapic_set_eoi

Before the change, handle_apic_access would just drop EOIs that should
have gone to user space ... I'm not sure if we tested it, or the path is
really never taken.