netdev - Re: [RFC PATCH] virtio_net: Extend virtio to use VF datapath when available

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Message-ID: <ec3471ba-7255-d7a2-b79a-f5590cd26b59@intel.com>
Date:   Tue, 19 Dec 2017 16:26:02 -0800
From:   "Samudrala, Sridhar" <sridhar.samudrala@...el.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     David Miller <davem@...emloft.net>, mst@...hat.com,
        netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        alexander.duyck@...il.com, jesse.brandeburg@...el.com
Subject: Re: [RFC PATCH] virtio_net: Extend virtio to use VF datapath when
 available

On 12/19/2017 2:53 PM, Stephen Hemminger wrote:
> On Tue, 19 Dec 2017 14:37:50 -0800
> "Samudrala, Sridhar" <sridhar.samudrala@...el.com> wrote:
>
>> On 12/19/2017 11:46 AM, Stephen Hemminger wrote:
>>> On Tue, 19 Dec 2017 11:42:33 -0800
>>> "Samudrala, Sridhar" <sridhar.samudrala@...el.com> wrote:
>>>   
>>>> On 12/19/2017 10:41 AM, Stephen Hemminger wrote:
>>>>> On Tue, 19 Dec 2017 13:21:17 -0500 (EST)
>>>>> David Miller <davem@...emloft.net> wrote:
>>>>>      
>>>>>> From: Stephen Hemminger <stephen@...workplumber.org>
>>>>>> Date: Tue, 19 Dec 2017 09:55:48 -0800
>>>>>>      
>>>>>>> could be 10ms, just enough to let udev do its renaming
>>>>>> Please, move to some kind of notification or event based handling of
>>>>>> this problem.
>>>>>>
>>>>>> No delay is safe, what if userspace gets swapped out or whatever
>>>>>> else might make userspace stall unexpectedly?
>>>>>>      
>>>>> The plan is to remove the delay and do the naming in the kernel.
>>>>> This was suggested by Lennart since udev is only doing naming policy
>>>>> because kernel names were not repeatable.
>>>>>
>>>>> This makes the VF show up as "ethN_vf" on Hyper-V which is user friendly.
>>>>>
>>>>> Patch is pending.
>>>> Do we really need to delay the setup until the name is changed?
>>>> Can't we call dev_set_mtu() and dev_open() until dev_change_name() is done?
>>>>
>>>> Thanks
>>>> Sridhar
>>> You can call dev_set_mtu, but when dev_open is done the device name
>>> can not be changed by userspace.
>> I did a quick test to remove the delay and also the dev_open() call and
>> i don't see
>> any issues with virtio taking over the VF datapath.
>> Only the netdev_info() messages may show old device name.
>>
>> Any specific scenario where we need to explicitly call  VF's dev_open()
>> in the VF setup process?
>> I tried i40evf driver loaded after virtio_net  AND  virtio_net loading
>> after i40evf.
>>
> It happens with hotplug. It is possible on Hyper-V to hotplug SR-IOV on
> and off while guest is running. If SR-IOV is disabled in host then the
> VF device is removed (hotplug) and the inverse. If the master device is
> up then the VF device should be brought up by the master device.

Even with KVM, we need to hot unplug SR-IOV device on the source host 
and  plug it
back on the destination host to enable live migration. It all works for 
me even without
the dev_open() of the lower device  from the VF setup routine. Will send 
out a v2 version
of this patch without the delayed VF setup after some more testing next 
week.

Thanks
Sridhar