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SubjectRe: [PATCH v3 0/2] kvm: level irqfd and new eoifd
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On Wed, 2012-07-11 at 14:51 +0300, Avi Kivity wrote:
> On 07/11/2012 02:23 PM, Jan Kiszka wrote:
> >>
> >> I'd appreciate a couple of examples for formality's sake.
> >
> > From the top of my head: NVIDIA FX3700 (granted, legacy by now), Atheros
> > AR9287. For others, I need to check.
>
> Thanks.
>
> >>
> >>> And then there is not easily replaceable legacy hardware like old
> >>> telephony adapters or industrial I/O cards etc. that want this.
> >>
> >> I imagine legacy hardware will live with the speed of routing through
> >> qemu, when running on modern platforms.
> >
> > Just because it's "legacy" doesn't mean it's "low performance" and "low
> > interrupt rate".
>
> I meant that it was used with lower throughput hardware, so the overhead
> of routing through qemu will be masked by the improved host hardware.
> But most of the improvement in hardware in recent years is the increase
> in core/thread count.
>
> > We still have classic KVM device assignment to provide fast-path INTx.
> > But if we want to replace it midterm, I think it's necessary for VFIO to
> > be able to provide such a path as well.
>
> I would like VFIO to have no regressions vs. kvm device assignment,
> except perhaps in uncommon corner cases. So I agree.

I ran a few TCP_RR netperf tests forcing a 1Gb tg3 nic to use INTx.
Without irqchip support vfio gets a bit more than 60% of KVM device
assignment. That's a little bit of an unfair comparison since it's more
than just the I/O path. With the proposed interfaces here, enabling
irqchip, vfio is within 10% of KVM device assignment for INTx. For MSI,
I can actually make vfio come out more than 30% better than KVM device
assignment if I send the eventfd from the hard irq handler. Using a
threaded handler as the code currently does, vfio is still behind KVM.
It's hard to beat a direct call chain.

For more devices, one that seems common among the non-enterprise users
are TV capture cards, like the old PVR-250/350 devices. These don't
support MSI. Thanks,

Alex



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