netdev - Re: [Xen-devel] [RFC v2 1/4] bridge: enable interfaces to opt out from becoming the root bridge

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Message-ID: <CAB=NE6WnMFkH7JPNt+ROiWEvhvZ03vdGr21STf75DV9fRaK=PA@mail.gmail.com>
Date:	Thu, 20 Feb 2014 12:24:06 -0800
From:	"Luis R. Rodriguez" <mcgrof@...not-panic.com>
To:	Stephen Hemminger <stephen@...workplumber.org>
Cc:	Zoltan Kiss <zoltan.kiss@...rix.com>,
	Ian Campbell <Ian.Campbell@...rix.com>, kvm@...r.kernel.org,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	bridge@...ts.linux-foundation.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	xen-devel@...ts.xenproject.org
Subject: Re: [Xen-devel] [RFC v2 1/4] bridge: enable interfaces to opt out
 from becoming the root bridge

On Thu, Feb 20, 2014 at 9:19 AM, Stephen Hemminger
<stephen@...workplumber.org> wrote:
> On Wed, 19 Feb 2014 09:59:33 -0800 "Luis R. Rodriguez" <mcgrof@...not-panic.com> wrote:
>> On Wed, Feb 19, 2014 at 9:08 AM, Stephen Hemminger <stephen@...workplumber.org> wrote:
>> >
>> > Please only use the netlink/sysfs flags fields that already exist
>> > for new features.
>>
>> Sure, but what if we know a driver in most cases wants the root block
>> and we'd want to make it the default, thereby only requiring userspace
>> for toggling it off.
>
> Something in userspace has to put the device into the bridge.
> Fix the port setup in that tool via the netlink or sysfs flags in
> the bridge. It should not have to be handled in the bridge looking
> at magic flags in the device.

Agreed that's the best strategy and I'll work on sending patches to
brctl to enable the root_block preference. This approach however also
requires a userspace upgrade. I'm trying to see if we can get an
old-nasty-cryptic-hack practice removed from the kernel and we'd try
to prevent future drivers from using it -- without requiring userspace
upgrade. In this case the bad practice is to using a high static MAC
address for mimicking a root block default preference. In order to
remove that *without* requiring a userspace upgrade the dev->priv_flag
approach is the only thing I can think of. If this would go in we'd
replace the high static MAC address with a random MAC address to
prevent IPv6 SLAAC / DAD conflicts. I'd document this flag and
indicate with preference for userspace to be the one tuning these
knobs.

Without this we'd have to keep the high static MAC address on upstream
drivers and let userspace do the random'ization if it confirms the
userspace knob to turn the root block flag is available. Is the
priv_flag approach worth the compromise to remove the root block hack
practice?

  Luis
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