Re: [v3 13/26] KVM: Define a new interface kvm_find_dest_vcpu() for VT-d PI

[Radim Kr?mÃ?]

2015-01-13 00:27+0000, Wu, Feng:
> > On 09/01/2015 15:54, Radim KrÄmÃÅ 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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