Re: [RFC v2 PATCH 00/21] KVM: x86: CPU isolation and direct interrupts delivery to guests

Avi Kivity
2012-09-06T11:46:35Z

On 09/06/2012 02:27 PM, Tomoki Sekiyama wrote:
> This RFC patch series provides facility to dedicate CPUs to KVM guests
> and enable the guests to handle interrupts from passed-through PCI devices
> directly (without VM exit and relay by the host).
>
> With this feature, we can improve throughput and response time of the device
> and the host's CPU usage by reducing the overhead of interrupt handling.
> This is good for the application using very high throughput/frequent
> interrupt device (e.g. 10GbE NIC).
> Real-time applicatoins also gets benefit from CPU isolation feature, which
> reduces interfare from host kernel tasks and scheduling delay.
>
> The overview of this patch series is presented in CloudOpen 2012.
> The slides are available at:
> http://events.linuxfoundation.org/images/stories/pdf/lcna_co2012_sekiyama.pdf

During Plumbers 2012, both Intel and AMD disclosed upcoming features to
their processors (APIC-V and AVIC) that allow directing device
interrupts to guest vcpus without host kernel involvement. This works
without pinning, dedicating a core to a guest, or any special measures
beyond support for the feature.

CPU isolation is still useful to improve real-time latency further, but
this is really independent of kvm.

I am inclined to reject this feature in favour of the new hardware
support. Sorry, I know this isn't nice to hear, but the extra
maintenance burden cannot be justified for a niche use case with special
limitations when generally useful feature exploiting proper hardware
support provides the same functionality.

--
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