linux-kernel - RE: [v3 13/26] KVM: Define a new interface kvm_find_dest

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Date:	Fri, 19 Dec 2014 23:48:50 +0000
From:	"Wu, Feng" <feng.wu@...el.com>
To:	Paolo Bonzini <pbonzini@...hat.com>,
	"Zhang, Yang Z" <yang.z.zhang@...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-VuQAYsv1563Yd54FQh9/CA@...lic.gmane.org>
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>,
	"Wu, Feng" <feng.wu@...el.com>
Subject: RE: [v3 13/26] KVM: Define a new interface kvm_find_dest_vcpu() for
 VT-d PI



> -----Original Message-----
> From: Paolo Bonzini [mailto:paolo.bonzini@...il.com] On Behalf Of Paolo
> Bonzini
> Sent: Friday, December 19, 2014 7:59 PM
> To: Wu, Feng; Paolo Bonzini; Zhang, Yang Z; Thomas Gleixner; Ingo Molnar; H.
> Peter Anvin; x86@...nel.org; Gleb Natapov; dwmw2@...radead.org;
> joro@...tes.org; Alex Williamson; Jiang Liu
> Cc: iommu@...ts.linux-foundation.org; linux-kernel@...r.kernel.org; KVM list;
> Eric Auger
> Subject: Re: [v3 13/26] KVM: Define a new interface kvm_find_dest_vcpu() for
> VT-d PI
> 
> 
> 
> On 19/12/2014 02:30, Wu, Feng wrote:
> >>> How this can work well? All subsequent interrupts are delivered to
> >>> one vCPU? It shouldn't be the best solution, need more consideration.
> >>
> >> Well, it's a hardware limitation.  The alternative (which is easy to
> >> implement) is to only do PI for single-CPU interrupts.  This should work
> >> well for multiqueue NICs (and of course for UP guests :)), so perhaps
> >> it's a good idea to only support that as a first attempt.
> >>
> >> Paolo
> >
> > Paolo, what do you mean by "single-CPU interrupts"? Do you mean we don't
> > support lowest priority interrupts for PI? But Linux OS uses lowest priority
> > for most of the case? If so, we can hardly get benefit from this feature for
> > Linux guest OS.
> 
> You can post lowest priority interrupts if they are delivered to a
> single CPU, in which case they are effectively fixed priority.
> 
> If they are broadcast to multiple CPUs, do not post them.
> 
> Paolo

In my understanding, lowest priority interrupts are always delivered to a
Single CPU, we need to find the right destination CPU from the cpumask.
This is what I do in this patch. Did I misunderstanding something in your
Comments? Thanks a lot!

Actually, we don't support posting broadcast/multicast interrupts, because
the interrupt is associated with one Posted-interrupts descriptor, then one
vCPU.

Thanks,
Feng

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