Re: [Qemu-devel] [PATCH] qemu: Introduce VIRTIO_NET_F_STANDBY feature bit to virtio_net

["Samudrala, Sridhar" <sridhar.samudrala@intel.com>]

On 6/5/2018 5:33 AM, Michael S. Tsirkin wrote:
> I don't think this is sufficient.

Sure. This is not sufficient for a complete solution, but is Qemu the right place
to manage primary/standby interfaces?

I think the other steps including plugging/unplugging the primary interface needs
to handled by some orchestration layer.


>
> If both primary and standby devices are present, a legacy guest without
> support for the feature might see two devices with same mac and get
> confused.

Yes. Isn't this possible today when a VM is started with a mis-configured domain
XML file that passes a virtio-net interface and a VF with the same MAC?


>
> I think that we should only make primary visible after guest acked the
> backup feature bit.

The primary can be plugged/unplugged at any time by the management layer. So
i guess this needs to be handled in the libvirt layer.

>
> And on reset or when backup is cleared in some other way, unplug the
> primary.
>
> Something like the below will do the job:
>
> Primary device is added with a special "primary-failover" flag.

Are you suggesting an extension to Qemu to add another flag for
primary device too?

> A virtual machine is then initialized with just a standby virtio
> device. Primary is not yet added.
>
> Later QEMU detects that guest driver device set DRIVER_OK.
> It then exposes the primary device to the guest, and triggers
> a device addition event (hot-plug event) for it.
>
> If QEMU detects guest driver removal, it initiates a hot-unplug sequence
> to remove the primary driver.  In particular, if QEMU detects guest
> re-initialization (e.g. by detecting guest reset) it immediately removes
> the primary device.
>
> We can move some of this code to management as well, architecturally it
> does not make too much sense but it might be easier implementation-wise.
>
> HTH
>
> On Mon, Jun 04, 2018 at 06:41:48PM -0700, Samudrala, Sridhar wrote:
>> Ping on this patch now that the kernel patches are accepted into davem's net-next tree.
>> https://patchwork.ozlabs.org/cover/920005/
>>
>>
>> On 5/7/2018 4:09 PM, 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@intel.com>
>>> ---
>>>    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