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

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Date:	Tue, 13 Jan 2015 17:17:16 +0100
From:	Radim Kr?má? <rkrcmar@...hat.com>
To:	"Wu, Feng" <feng.wu@...el.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"hpa@...or.com" <hpa@...or.com>, "x86@...nel.org" <x86@...nel.org>,
	"gleb@...nel.org" <gleb@...nel.org>,
	"dwmw2@...radead.org" <dwmw2@...radead.org>,
	"joro@...tes.org" <joro@...tes.org>,
	"alex.williamson@...hat.com" <alex.williamson@...hat.com>,
	"jiang.liu@...ux.intel.com" <jiang.liu@...ux.intel.com>,
	"eric.auger@...aro.org" <eric.auger@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [v3 13/26] KVM: Define a new interface kvm_find_dest_vcpu() for
 VT-d PI

2015-01-13 00:27+0000, Wu, Feng:
> > On 09/01/2015 15:54, Radim Krčmář wrote:
> > > There are two points relevant to this patch in new KVM's implementation,
> > > ("KVM: x86: amend APIC lowest priority arbitration",
> > >  https://lkml.org/lkml/2015/1/9/362)
> > >
> > > 1) lowest priority depends on TPR
> > > 2) there is no need for balancing
> > >
> > > (1) has to be considered with PI as well.
> > 
> > The chipset doesn't support it. :(
> > 
> > > I kept (2) to avoid whining from people building on that behaviour, but
> > > lowest priority backed by PI could be transparent without it.
> > >
> > > Patch below removes the balancing, but I am not sure this is a price we
> > > allowed ourselves to pay ... what are your opinions?
> > 
> > I wouldn't mind, but it requires a lot of benchmarking.
> 
> In fact, the real hardware may do lowest priority in round robin way,

Yes, but we won't emulate round robin with PI and I think it is wrong to
have backends with significantly different guest-visible behaviors.

>                                                                       the new
> hardware even doesn't consider the TPR for lowest priority interrupts delivery.

A bold move ... what hardware was the first to do so?

> As discussed with Paolo before, I will submit a patch to support lowest priority for PI
> after this series is merged.

Sure, I see only two good solutions though
 1) don't optimize lowest priority with PI
 2) don't balance lowest priority
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