netdev - Re: [PATCH net-next] rtnl: Add support for netdev event to link messages

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Message-ID: <55cb61be-53af-315c-a376-e8f1d7bc78a6@redhat.com>
Date:   Thu, 30 Mar 2017 09:39:33 -0400
From:   Vlad Yasevich <vyasevic@...hat.com>
To:     David Ahern <dsa@...ulusnetworks.com>,
        Roopa Prabhu <roopa@...ulusnetworks.com>
Cc:     David Miller <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] rtnl: Add support for netdev event to link
 messages

On 03/29/2017 03:11 PM, David Ahern wrote:
> On 3/29/17 11:05 AM, Vlad Yasevich wrote:
>> On 03/29/2017 12:37 PM, Roopa Prabhu wrote:
>>> On 3/29/17, 5:23 AM, Vlad Yasevich wrote:
>>>> [ resending to list.  hit the wrong reply button last time ]
>>>>
>>>> On 03/27/2017 06:58 PM, David Miller wrote:
>>>>> From: Vladislav Yasevich <vyasevich@...il.com>
>>>>> Date: Sat, 25 Mar 2017 21:59:47 -0400
>>>>>
>>>>>> RTNL currently generates notifications on some netdev notifier events.
>>>>>> However, user space has no idea what changed.  All it sees is the
>>>>>> data and has to infer what has changed.  For some events that is not
>>>>>> possible.
>>>>>>
>>>>>> This patch adds a new field to RTM_NEWLINK message called IFLA_EVENT
>>>>>> that would have an encoding of the which event triggered this
>>>>>> notification.  Currectly, only 2 events (NETDEV_NOTIFY_PEERS and
>>>>>> NETDEV_MTUCHANGED) are supported.  These events could be interesting
>>>>>> in the virt space to trigger additional configuration commands to VMs.
>>>>>> Other events of interest may be added later.
>>>>>>
>>>>>> Signed-off-by: Vladislav Yasevich <vyasevic@...hat.com>
>>>>> At what point do we start providing the metadata for the changed
>>>>> values as well?  You'd probably need to provide both the old and
>>>>> new values to cover all cases.
>>>> I don't think if that would be possible because of when events are triggered.
>>>> We send these notifications after all the changes have already been made, so
>>>> it might be tough to carry old data.
>>>>
>>>> Looking at just the two events I am supporting in this patch, we could actually
>>>> supply the old mtu data through a NETDEV_PRECHANGEMTU event, if it is necessary.
>>>
>>> But, NETDEV_PRECHANGEMTU will be a unnecessary notification to userspace without
>>> changes. There are already enough notifications generated for links (I know you are not
>>> suggesting adding it here)
>>
>> Actually, this one already triggers a link notification to userspace.  It just has
>> no event data in it to tell you that. :)
> 
> Is it intentional or unintentional? perhaps rtnetlink_event should be a
> whitelist -- events that userspace should be notified about. Seems like
> NETDEV_ events have been added without rtnetlink_event getting updated.

I think a 'whitelist' was attempted, but as you mentioned, it hasn't been updated...
I'll defer the definitive answer to someone else.  It seems Patrick added a comment
in commit a2835763 to update the white list and it's been a few times.

> For example, does userspace care about NETDEV_UDP_TUNNEL_PUSH_INFO or
> NETDEV_CHANGE_TX_QUEUE_LEN?
> 

Probably not the first, but possibly the second.  If txquelen is changed on a device,
some apps might want to know about it.

-vlad