Subject: Re: [PATCH] KVM: x86: check for pic and ioapic presence before use
To: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org
References: <20161123202548.29324-1-rkrcmar@redhat.com>
Cc: stable@vger.kernel.org, Dmitry Vyukov <dvyukov@google.com>,
        Steve Rutherford <srutherford@google.com>
From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <06c15e9d-43fc-0d08-57e4-26440c441eb0@redhat.com>
Date: Wed, 23 Nov 2016 22:58:16 +0100
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On 23/11/2016 21:25, Radim Krčmář wrote:
> diff --git a/arch/x86/kvm/irq_comm.c b/arch/x86/kvm/irq_comm.c
> index 25810b144b58..ddd63b8b176e 100644
> --- a/arch/x86/kvm/irq_comm.c
> +++ b/arch/x86/kvm/irq_comm.c
> @@ -41,6 +41,15 @@ static int kvm_set_pic_irq(struct kvm_kernel_irq_routing_entry *e,
>  			   bool line_status)
>  {
>  	struct kvm_pic *pic = pic_irqchip(kvm);
> +
> +	/*
> +	 * XXX: rejecting pic routes when pic isn't in use would be better,
> +	 * but the default routing table is installed while kvm->arch.vpic is
> +	 * NULL and KVM_CREATE_IRQCHIP can race with KVM_SET_GSI_ROUTING.
> +	 */
> +	if (!pic)
> +		return -1;
> +
>  	return kvm_pic_set_irq(pic, e->irqchip.pin, irq_source_id, level);
>  }
>  

Can you explain the race with the default routing table better?  It
seems to me that it can only make the routing table go from invalid to
valid (there is no KVM_DESTROY_IRQCHIP) so it's benign.

Paolo