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

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Date:   Thu, 16 Jul 2020 16:06:23 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     "Zhu, Lingshan" <lingshan.zhu@...el.com>,
        alex williamson <alex.williamson@...hat.com>,
        pbonzini@...hat.com, sean.j.christopherson@...el.com,
        wanpengli@...cent.com,
        "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2:15, Michael S. Tsirkin wrote:
> On Thu, Jul 16, 2020 at 12:20:09PM +0800, Jason Wang wrote:
>> 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
> Let's confirm a switch happens atomically so each interrupt
> is sent either to eventfd to guest directly though.


I think it's safe since:

1) we don't alloc/free interrupt during the eventfd change
2) The irte is modified automatically through cmpxchg_double() in 
modify_irte(), so the interrupt is either remapping to eventfd or pi 
descriptor

Thanks


>