From mboxrd@z Thu Jan  1 00:00:00 1970
From: Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: in-kernel interrupt controller steering
Date: Wed, 06 Mar 2013 14:41:04 +0100
Message-ID: <51374770.5040405@redhat.com>
References: <469599439.3185295.1362572055225.JavaMail.root@redhat.com> <1777B6DD-B341-4531-BE43-7B0161B1D093@suse.de> <20130306131424.GR11223@redhat.com>
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Cc: Alexander Graf <agraf@suse.de>, kvm@vger.kernel.org,
	kvm-ppc@vger.kernel.org, Stuart Yoder <stuart.yoder@freescale.com>,
	Scott Wood <scottwood@freescale.com>,
	Paul Mackerras <paulus@samba.org>,
	Peter Maydell <peter.maydell@linaro.org>
To: Gleb Natapov <gleb@redhat.com>
Return-path: <kvm-ppc-owner@vger.kernel.org>
In-Reply-To: <20130306131424.GR11223@redhat.com>
Sender: kvm-ppc-owner@vger.kernel.org
List-Id: kvm.vger.kernel.org

Il 06/03/2013 14:14, Gleb Natapov ha scritto:
>>>> The problem would only start if KVM_SET_IRQCHIP_TYPE (new name of
>>>> KVM_CREATE_IRQCHIP_ARGS) forced you to later call KVM_CREATE_DEVICE.
>>> 
>>> Ah, I see. I don't see why it would. The fact that there is a
>>> "LAPIC" doesn't mean that the per-vcpu SET_INTERRUPT ioctl stops
>>> working. So if SET_IRQCHIP_TYPE(!none) breaks user-space interrupt
>>> controller emulation I would consider that a bug.
>> 
> For x86 this is the case though. I do not see how it can't be. If
> LAPIC is emulated in userspace SET_INTERRUPT is used to pass IRQ
> vector that should be handled as a result of LAPIC emulation.

SET_IRQCHIP_TYPE creates the LAPICs; it would indeed break userspace
LAPIC emulation because the LAPICs would not cause userspace exits anymore.

However, it need not mandate the usage of an in-kernel IOAPIC or PIC
though.  KVM_INTERRUPT, the docs say, "is only useful if in-kernel local
APIC or equivalent is not used", but it is really only useful for if
in-kernel *IOAPIC* is not used.  The userspace IOAPIC can use it to
inject the interrupts to the in-kernel LAPIC.

So, it would be possible to create the IOAPIC or PIC separately with
KVM_CREATE_DEVICE, and have the userspace devices inject the interrupts
with KVM_IRQ_LINE_STATUS (PIC->IOAPIC) or KVM_INTERRUPT (IOAPIC->LAPIC).

Paolo