Re: [RFC PATCH 1/4] KVM: x86: Split the APIC from the rest of IRQCHIP.

[Steve Rutherford <srutherford@google.com>]

On Fri, May 15, 2015 at 01:03:02PM +0800, Wanpeng Li wrote:
> On Thu, May 14, 2015 at 12:29:21PM -0700, Andrew Honig wrote:
> >>
> >> BTW, what is the purpose of this series. If I understand it correctly, you only want to
> >> use the in-kernel lapic and leave the others (pic, ioapic) in userspace, what is the
> >> benefit of it?
> >
> >The purpose is to achieve the security benefit of removing some of the
> >interrupt handling into userspace, without incurring a significant
> >performance penalty.  If you move the entire IRQCHIP into userspace,
> >we've seen perf impacts from 15-200% depending on the workload.  With
> >this patch series, we're seeing perf penalty <1% on our tests (TCP_RR
> 
> Why keep pic and ioapic in kernel space not get obvious benefit, what's
> the bottleneck?

It's the other way around. The PIC and IOAPIC are going up to userspace, and the APICs are staying in the kernel.

> 
> Regards,
> Wanpeng Li 
> 
> >latency, TCP throughput, and Disk I/O).  See
> >(https://lwn.net/Articles/619332/)
> >
> >>
> >> Thanks,
> >> Feng
> >>
> >>>
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