linux-kernel - Re: [RFC v2 2/4] net: enables interface option to skip IP

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Date:	Wed, 19 Feb 2014 10:45:25 -0600
From:	Dan Williams <dcbw@...hat.com>
To:	"Luis R. Rodriguez" <mcgrof@...not-panic.com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>
Subject: Re: [RFC v2 2/4] net: enables interface option to skip IP

On Tue, 2014-02-18 at 13:19 -0800, Luis R. Rodriguez wrote:
> On Mon, Feb 17, 2014 at 12:23 PM, Dan Williams <dcbw@...hat.com> wrote:
> > On Fri, 2014-02-14 at 18:59 -0800, Luis R. Rodriguez wrote:
> >> From: "Luis R. Rodriguez" <mcgrof@...e.com>
> >>
> >> Some interfaces do not need to have any IPv4 or IPv6
> >> addresses, so enable an option to specify this. One
> >> example where this is observed are virtualization
> >> backend interfaces which just use the net_device
> >> constructs to help with their respective frontends.
> >>
> >> This should optimize boot time and complexity on
> >> virtualization environments for each backend interface
> >> while also avoiding triggering SLAAC and DAD, which is
> >> simply pointless for these type of interfaces.
> >
> > Would it not be better/cleaner to use disable_ipv6 and then add a
> > disable_ipv4 sysctl, then use those with that interface?
> 
> Sure, but note that the both disable_ipv6 and accept_dada sysctl
> parameters are global. ipv4 and ipv6 interfaces are created upon
> NETDEVICE_REGISTER, which will get triggered when a driver calls
> register_netdev(). The goal of this patch was to enable an early
> optimization for drivers that have no need ever for ipv4 or ipv6
> interfaces.

Each interface gets override sysctls too though, eg:

/proc/sys/net/ipv6/conf/enp0s25/disable_ipv6

which is the one I meant; you're obviously right that the global ones
aren't what you want here.  But the specific ones should be suitable?
If you set that on a per-interface basis, then you'll get EPERM or
something whenever you try to add IPv6 addresses or do IPv6 routing.

> Zoltan has noted though some use cases of IPv4 or IPv6 addresses on
> backends though, as such this is no longer applicable as a
> requirement. The ipv4 sysctl however still seems like a reasonable
> approach to enable optimizations of the network in topologies where
> its known we won't need them but -- we'd need to consider a much more
> granular solution, not just global as it is now for disable_ipv6, and
> we'd also have to figure out a clean way to do this to not incur the
> cost of early address interface addition upon register_netdev().
> 
> Given that we have a use case for ipv4 and ipv6 addresses on
> xen-netback we no longer have an immediate use case for such early
> optimization primitives though, so I'll drop this.
> 
> > The IFF_SKIP_IP seems to duplicate at least part of what disable_ipv6 is
> > already doing.
> 
> disable_ipv6 is global, the goal was to make this granular and skip
> the cost upon early boot, but its been clarified we don't need this.

Like Stephen says, you need to make sure you set them before IFF_UP, but
beyond that, wouldn't the interface-specific sysctls work?

Dan

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