linux-kernel - Re: [v4 08/16] KVM: kvm-vfio: User API for IRQ forwarding

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Message-ID: <558AD9E6.1030902@linaro.org>
Date:	Wed, 24 Jun 2015 18:25:10 +0200
From:	Eric Auger <eric.auger@...aro.org>
To:	Joerg Roedel <joro@...tes.org>
CC:	Alex Williamson <alex.williamson@...hat.com>,
	Avi Kivity <avi.kivity@...il.com>,
	"Wu, Feng" <feng.wu@...el.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	"mtosatti@...hat.com" <mtosatti@...hat.com>
Subject: Re: [v4 08/16] KVM: kvm-vfio: User API for IRQ forwarding

Hi Joerg,

On 06/24/2015 05:50 PM, Joerg Roedel wrote:
> On Mon, Jun 15, 2015 at 06:17:03PM +0200, Eric Auger wrote:
>> I guess this discussion also is relevant wrt "[RFC v6 00/16] KVM-VFIO
>> IRQ forward control" series? Or is that "central registry maintained by
>> a posted interrupts manager" something more specific to x86?
> 
> From what I understood so far, the feature you implemented for ARM is a
> bit different from the ones that get introduced to x86.
> 
> Can you please share some details on how the ARM version works? I am
> interested in how the GICv2 is configured for IRQ forwarding. The
> question is whether the forwarding information needs to be updated from
> KVM and what information about the IRQ KVM needs for this.

The principle is that when you inject a virtual IRQ to a guest, you
program a register in the GIC, known as a list register. There you put
both the virtual IRQ you want to inject but also the physical IRQ it is
linked with (HWbit mode set = forwarding set). When the guest completes
the virtual IRQ the GIC HW automatically deactivates the physical IRQ
found in the list register. In that mode the physical IRQ deactivation
is under the ownership of the guest (actually automatically done by the HW).

If HWbit mode is *not* set (forwarding not set), you do not specify the
HW IRQ in the list register. The host deactivates the physical IRQ &
masks it before triggering the virtual IRQ. Only the virtual IRQ ID is
programmed in the list register. When the guest completes the virtual
IRQ, a physical maintenance IRQ is triggered. The hyp mode is entered
and eventually the host unmasks the IRQ.

Some illustrations can be found in
http://www.linux-kvm.org/images/a/a8/01x04-ARMdevice.pdf

Hope it helps

Eric
> 
> 
> 	Joerg
> 

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