linux-kernel - RE: [v3 06/26] iommu, x86: No need to migrating irq for VT-d Posted-Interrupts

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Date:	Tue, 23 Dec 2014 00:37:46 +0000
From:	"Zhang, Yang Z" <yang.z.zhang@...el.com>
To:	Paolo Bonzini <pbonzini@...hat.com>,
	"Wu, Feng" <feng.wu@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"Gleb Natapov" <gleb@...nel.org>,
	"dwmw2@...radead.org" <dwmw2@...radead.org>,
	"joro@...tes.org" <joro@...tes.org>,
	Alex Williamson <alex.williamson@...hat.com>,
	Jiang Liu <jiang.liu@...ux.intel.com>
CC:	"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>,
	Eric Auger <eric.auger@...aro.org>
Subject: RE: [v3 06/26] iommu, x86: No need to migrating irq for VT-d
 Posted-Interrupts

Paolo Bonzini wrote on 2014-12-19:
> 
> 
> On 19/12/2014 02:46, Zhang, Yang Z wrote:
>>> If the IRQ is posted, its affinity is controlled by guest (irq
>>> <---> vCPU <----> pCPU), it has no effect when host changes its affinity.
>> 
>> That's the problem: User is able to changes it in host but it never
>> takes effect since it is actually controlled by guest. I guess it
>> will break the IRQ balance too.
> 
> I don't think that's a problem.
> 
> Controlling the affinity in the host affects which CPU in the host
> takes care of signaling the guest.
> 
> If this signaling is done directly by the chipset, there is no need to
> do anything in the host and thus the host affinity can be bypassed.

I don't quite understand it. If user set an interrupt's affinity to a CPU, but he still see the interrupt delivers to other CPUs in host. Do you think it is a right behavior?

> 
> Paolo


Best regards,
Yang

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