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Re: [Qemu-devel] [PATCH 0/3] Add support for for GICv2m and MSIs to arm-


From: Christoffer Dall
Subject: Re: [Qemu-devel] [PATCH 0/3] Add support for for GICv2m and MSIs to arm-virt
Date: Thu, 9 Apr 2015 10:03:33 +0200

On Thu, Apr 9, 2015 at 12:01 AM, Nikolay Nikolaev
<address@hidden> wrote:
> On Thu, Apr 9, 2015 at 12:20 AM, Christoffer Dall
> <address@hidden> wrote:
>> Now when we have a host generic PCIe controller in the virt board, it
>> would be nice to be able to use MSIs so that we can eventually enable
>> VHOST with KVM.
>>
>> With these patches you can use MSIs with TCG and with KVM, but you still
>> need some fixes for the mapping of the IRQ index to the GSI number for
>> IRQFD to work.  A separate patch series will follow this one to enable
>> that.
>>
>> Tested with KVM on XGene and with TCG by configuring a virtio-pci
>> network adapter for the guest and verifying MSIs going through as
>> expected.
>
> Christofer, have you measured the network performance difference with KVM.
> Probably the next patchseries (with IRQFD) makes bigger difference.
>
I did run numbers, and it depends greatly on the workload, but for
something simple like netperf, you see a huge increase in performance
(several orders of magnitude).  Surprisingly, a large portion of that
seems to come from using ioeventfd for the mmio accesses to vhost,
possibly because it begins to use multiple queues.  For latency
measurements, irqfd also makes a big difference.

-Christoffer



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