Message-ID: <CADGSJ213f8tJpNXuOhv8qRew-Y5VZAwA+srNMrLZYnKdVGLdAA@mail.gmail.com>
Date:   Wed, 13 Jun 2018 18:02:01 -0700
From:   Siwei Liu <loseweigh@...il.com>
To:     "Samudrala, Sridhar" <sridhar.samudrala@...el.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Alexander Duyck <alexander.h.duyck@...el.com>,
        virtio-dev@...ts.oasis-open.org, qemu-devel@...gnu.org,
        Jiri Pirko <jiri@...nulli.us>, Jakub Kicinski <kubakici@...pl>,
        Netdev <netdev@...r.kernel.org>,
        "Brandeburg, Jesse" <jesse.brandeburg@...el.com>,
        virtualization@...ts.linux-foundation.org, aaron.f.brown@...el.com
Subject: Re: [virtio-dev] Re: [Qemu-devel] [PATCH] qemu: Introduce
 VIRTIO_NET_F_STANDBY feature bit to virtio_net

On Tue, Jun 12, 2018 at 5:08 PM, Samudrala, Sridhar
<sridhar.samudrala@...el.com> wrote:
> On 6/12/2018 4:34 AM, Michael S. Tsirkin wrote:
>>
>> On Mon, Jun 11, 2018 at 10:02:45PM -0700, Samudrala, Sridhar wrote:
>>>
>>> On 6/11/2018 7:17 PM, Michael S. Tsirkin wrote:
>>>>
>>>> On Tue, Jun 12, 2018 at 09:54:44AM +0800, Jason Wang wrote:
>>>>>
>>>>> On 2018年06月12日 01:26, Michael S. Tsirkin wrote:
>>>>>>
>>>>>> On Mon, May 07, 2018 at 04:09:54PM -0700, Sridhar Samudrala wrote:
>>>>>>>
>>>>>>> This feature bit can be used by hypervisor to indicate virtio_net
>>>>>>> device to
>>>>>>> act as a standby for another device with the same MAC address.
>>>>>>>
>>>>>>> I tested this with a small change to the patch to mark the STANDBY
>>>>>>> feature 'true'
>>>>>>> by default as i am using libvirt to start the VMs.
>>>>>>> Is there a way to pass the newly added feature bit 'standby' to qemu
>>>>>>> via libvirt
>>>>>>> XML file?
>>>>>>>
>>>>>>> Signed-off-by: Sridhar Samudrala <sridhar.samudrala@...el.com>
>>>>>>
>>>>>> So I do not think we can commit to this interface: we
>>>>>> really need to control visibility of the primary device.
>>>>>
>>>>> The problem is legacy guest won't use primary device at all if we do
>>>>> this.
>>>>
>>>> And that's by design - I think it's the only way to ensure the
>>>> legacy guest isn't confused.
>>>
>>> Yes. I think so. But i am not sure if Qemu is the right place to control
>>> the visibility
>>> of the primary device. The primary device may not be specified as an
>>> argument to Qemu. It
>>> may be plugged in later.
>>> The cloud service provider is providing a feature that enables low
>>> latency datapath and live
>>> migration capability.
>>> A tenant can use this feature only if he is running a VM that has
>>> virtio-net with failover support.
>>
>> Well live migration is there already. The new feature is low latency
>> data path.
>
>
> we get live migration with just virtio.  But I meant live migration with VF
> as
> primary device.
>
>>
>> And it's the guest that needs failover support not the VM.
>
>
> Isn't guest and VM synonymous?
>
>
>>
>>
>>> I think Qemu should check if guest virtio-net supports this feature and
>>> provide a mechanism for
>>> an upper layer indicating if the STANDBY feature is successfully
>>> negotiated or not.
>>> The upper layer can then decide if it should hot plug a VF with the same
>>> MAC and manage the 2 links.
>>> If VF is successfully hot plugged, virtio-net link should be disabled.
>>
>> Did you even talk to upper layer management about it?
>> Just list the steps they need to do and you will see
>> that's a lot of machinery to manage by the upper layer.
>>
>> What do we gain in flexibility? As far as I can see the
>> only gain is some resources saved for legacy VMs.
>>
>> That's not a lot as tenant of the upper layer probably already has
>> at least a hunch that it's a new guest otherwise
>> why bother specifying the feature at all - you
>> save even more resources without it.
>>
>
> I am not all that familiar with how Qemu manages network devices. If we can
> do all the
> required management of the primary/standby devices within Qemu, that is
> definitely a better
> approach without upper layer involvement.

Right. I would imagine in the extreme case the upper layer doesn't
have to be involved at all if QEMU manages all hot plug/unplug logic.
The management tool can supply passthrough device and virtio with the
same group UUID, QEMU auto-manages the presence of the primary, and
hot plug the device as needed before or after the migration.

-Siwei

>
>
>>
>>
>>>>> How about control the visibility of standby device?
>>>>>
>>>>> Thanks
>>>>
>>>> standy the always there to guarantee no downtime.
>>>>
>>>>>> However just for testing purposes, we could add a non-stable
>>>>>> interface "x-standby" with the understanding that as any
>>>>>> x- prefix it's unstable and will be changed down the road,
>>>>>> likely in the next release.
>>>>>>
>>>>>>
>>>>>>> ---
>>>>>>>     hw/net/virtio-net.c                         | 2 ++
>>>>>>>     include/standard-headers/linux/virtio_net.h | 3 +++
>>>>>>>     2 files changed, 5 insertions(+)
>>>>>>>
>>>>>>> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
>>>>>>> index 90502fca7c..38b3140670 100644
>>>>>>> --- a/hw/net/virtio-net.c
>>>>>>> +++ b/hw/net/virtio-net.c
>>>>>>> @@ -2198,6 +2198,8 @@ static Property virtio_net_properties[] = {
>>>>>>>                          true),
>>>>>>>         DEFINE_PROP_INT32("speed", VirtIONet, net_conf.speed,
>>>>>>> SPEED_UNKNOWN),
>>>>>>>         DEFINE_PROP_STRING("duplex", VirtIONet, net_conf.duplex_str),
>>>>>>> +    DEFINE_PROP_BIT64("standby", VirtIONet, host_features,
>>>>>>> VIRTIO_NET_F_STANDBY,
>>>>>>> +                      false),
>>>>>>>         DEFINE_PROP_END_OF_LIST(),
>>>>>>>     };
>>>>>>> diff --git a/include/standard-headers/linux/virtio_net.h
>>>>>>> b/include/standard-headers/linux/virtio_net.h
>>>>>>> index e9f255ea3f..01ec09684c 100644
>>>>>>> --- a/include/standard-headers/linux/virtio_net.h
>>>>>>> +++ b/include/standard-headers/linux/virtio_net.h
>>>>>>> @@ -57,6 +57,9 @@
>>>>>>>                                          * Steering */
>>>>>>>     #define VIRTIO_NET_F_CTRL_MAC_ADDR 23       /* Set MAC address */
>>>>>>> +#define VIRTIO_NET_F_STANDBY      62    /* Act as standby for
>>>>>>> another device
>>>>>>> +                                         * with the same MAC.
>>>>>>> +                                         */
>>>>>>>     #define VIRTIO_NET_F_SPEED_DUPLEX 63        /* Device set
>>>>>>> linkspeed and duplex */
>>>>>>>     #ifndef VIRTIO_NET_NO_LEGACY
>>>>>>> --
>>>>>>> 2.14.3
>>
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