Re: [RFC PATCH 00/18] KVM: x86: CPU isolation and direct interrupts handling by guests

Avi Kivity
2012-06-28T16:58:32Z

On 06/28/2012 09:07 AM, Tomoki Sekiyama wrote:
> Hello,
>
> 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).
> CPU-intensive high performance applications and real-time applicatoins
> also gets benefit from CPU isolation feature, which reduces VM exit and
> scheduling delay.
>
> Current implementation is still just PoC and have many limitations, but
> submitted for RFC. Any comments are appreciated.
>
> * Overview
> Intel and AMD CPUs have a feature to handle interrupts by guests without
> VM Exit. However, because it cannot switch VM Exit based on IRQ vectors,
> interrupts to both the host and the guest will be routed to guests.
>
> To avoid mixture of host and guest interrupts, in this patch, some of CPUs
> are cut off from the host and dedicated to the guests. In addition, IRQ
> affinity of the passed-through devices are set to the guest CPUs only.
>
> For IPI from the host to the guest, we use NMIs, that is an only interrupts
> having another VM Exit flag.
>
> * Benefits
> This feature provides benefits of virtualization to areas where high
> performance and low latency are required, such as HPC and trading,
> and so on. It also useful for consolidation in large scale systems with
> many CPU cores and PCI devices passed-through or with SR-IOV.
> For the future, it may be used to keep the guests running even if the host
> is crashed (but that would need additional features like memory isolation).
>
> * Limitations
> Current implementation is experimental, unstable, and has a lot of limitations.
> - SMP guests don't work correctly
> - Only Linux guest is supported
> - Only Intel VT-x is supported
> - Only MSI and MSI-X pass-through; no ISA interrupts support
> - Non passed-through PCI devices (including virtio) are slower
> - Kernel space PIT emulation does not work
> - Needs a lot of cleanups
>

This is both impressive and scary. What is the target scenario here?
Partitioning? I don't see this working for generic consolidation.

--
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