netdev - Re: [PATCH 0/7] *** IRQ offloading for vDPA ***

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Message-ID: <67c4c41d-9e95-2270-4acb-6f04668c34fa@redhat.com>
Date:   Thu, 16 Jul 2020 12:20:09 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Zhu, Lingshan" <lingshan.zhu@...el.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        alex williamson <alex.williamson@...hat.com>,
        pbonzini@...hat.com, sean.j.christopherson@...el.com,
        wanpengli@...cent.com
Cc:     "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        dan daly <dan.daly@...el.com>
Subject: Re: [PATCH 0/7] *** IRQ offloading for vDPA ***


On 2020/7/16 下午12:13, Zhu, Lingshan wrote:
>
>
> On 7/16/2020 12:02 PM, Jason Wang wrote:
>>
>> On 2020/7/16 上午11:59, Zhu, Lingshan wrote:
>>>
>>> On 7/16/2020 10:59 AM, Jason Wang wrote:
>>>>
>>>> On 2020/7/16 上午9:39, Zhu, Lingshan wrote:
>>>>>
>>>>>
>>>>> On 7/15/2020 9:43 PM, Jason Wang wrote:
>>>>>>
>>>>>> On 2020/7/12 下午10:52, Zhu Lingshan wrote:
>>>>>>> Hi All,
>>>>>>>
>>>>>>> This series intends to implement IRQ offloading for
>>>>>>> vhost_vdpa.
>>>>>>>
>>>>>>> By the feat of irq forwarding facilities like posted
>>>>>>> interrupt on X86, irq bypass can  help deliver
>>>>>>> interrupts to vCPU directly.
>>>>>>>
>>>>>>> vDPA devices have dedicated hardware backends like VFIO
>>>>>>> pass-throughed devices. So it would be possible to setup
>>>>>>> irq offloading(irq bypass) for vDPA devices and gain
>>>>>>> performance improvements.
>>>>>>>
>>>>>>> In my testing, with this feature, we can save 0.1ms
>>>>>>> in a ping between two VFs on average.
>>>>>>
>>>>>>
>>>>>> Hi Lingshan:
>>>>>>
>>>>>> During the virtio-networking meeting, Michael spots two possible 
>>>>>> issues:
>>>>>>
>>>>>> 1) do we need an new uAPI to stop the irq offloading?
>>>>>> 2) can interrupt lost during the eventfd ctx?
>>>>>>
>>>>>> For 1) I think we probably not, we can allocate an independent 
>>>>>> eventfd which does not map to MSIX. So the consumer can't match 
>>>>>> the producer and we fallback to eventfd based irq.
>>>>> Hi Jason,
>>>>>
>>>>> I wonder why we need to stop irq offloading, but if we need to do 
>>>>> so, maybe a new uAPI would be more intuitive to me,
>>>>> but why and who(user? qemu?) shall initialize this process, based 
>>>>> on what kinda of basis to make the decision?
>>>>
>>>>
>>>> The reason is we may want to fallback to software datapath for some 
>>>> reason (e.g software assisted live migration). In this case we need 
>>>> intercept device write to used ring so we can not offloading 
>>>> virtqueue interrupt in this case.
>>> so add a VHOST_VDPA_STOP_IRQ_OFFLOADING? Then do we need a 
>>> VHOST_VDPA_START_IRQ_OFFLOADING, then let userspace fully control 
>>> this? Or any better approaches? 
>>
>>
>> Probably not, it's as simple as allocating another eventfd (but not 
>> irqfd), and pass it to vhost-vdpa. Then the offloading is disabled 
>> since it doesn't have a consumer.
> OK, sounds like QEMU work, no need to take care in this series, right?


That's my understanding.

Thanks


>
> Thanks,
> BR
> Zhu Lingshan
>>
>> Thanks
>>
>>