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

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Date:	Fri, 9 Jan 2015 16:12:59 +0100
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Feng Wu <feng.wu@...el.com>, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, x86@...nel.org, gleb@...nel.org,
	dwmw2@...radead.org, joro@...tes.org, alex.williamson@...hat.com,
	jiang.liu@...ux.intel.com, eric.auger@...aro.org,
	linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
	kvm@...r.kernel.org
Subject: Re: [v3 13/26] KVM: Define a new interface kvm_find_dest_vcpu() for
 VT-d PI

2015-01-09 15:56+0100, Paolo Bonzini:
> 
> 
> 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 meant that we need to recompute PI entries for lowest priority
interrupts every time guest's TPR changes.

Luckily, Linux doesn't use TPR, but other OS might be a reason to drop
lowest priority from PI optimizations.  (Or make it more complicated.)

> > 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.

(I was afraid it would come to that.)
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