linux-kernel - Re: [PATCH v1 2/4] KVM: arm/arm64: vgic: Improve comment on kvm_vgic_inject

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Message-ID: <20190225000917.GA4001@leoy-ThinkPad-X240s>
Date:   Mon, 25 Feb 2019 08:09:17 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Marc Zyngier <marc.zyngier@....com>
Cc:     Christoffer Dall <christoffer.dall@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        James Morse <james.morse@....com>,
        Andre Przywara <andre.przywara@....com>,
        Jun Yao <yaojun8558363@...il.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/4] KVM: arm/arm64: vgic: Improve comment on
 kvm_vgic_inject_irq

On Fri, Feb 22, 2019 at 03:40:50PM +0000, Marc Zyngier wrote:

[...]

> > > The interrupt affinity is either defined by the distributor
> > > configuration (SPIs) or the ITS configuration (LPIs).
> > 
> > Given to the up example, I am struggling to understand how you can set
> > the interrupt affinity for virtio device.
> > 
> > Do you set the physical interrupt affinity to CPU0/1 in host OS and
> > forward it to guest OS?  Or set interrupt affinity in guest OS (I
> > tried on Juno board to set irq affinity in guest OS from
> > '/proc/irq/xxx/smp_affinity' but failed)?   Or this is accomplished by
> > user space tool (lkvm or qemu)?
> 
> virtio interrupts are purely virtual, and the host plays no part in
> their routing (nor does userspace). As for their affinity, that
> depends on the virtio driver. Some virtio devices allow their affinity
> to be changed, some don't. Here, this is a virtio-net device, which is
> perfectly happy to see its queue interrupts moved to a different vcpu.
> 
> I tend to run irqbalance in my guests so that it actually exercises
> the affinity setting in the background.
> 
> > Sorry if I am asking a stupid question :)
> 
> It's not stupid. You're simply confusing multiple independent layers.

Thanks a lot for explaination, Marc.

Thanks,
Leo Yan