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

[Dan Williams <dcbw@xxxxxxxxxx>]

On Wed, 2014-02-19 at 09:20 -0800, Luis R. Rodriguez wrote:
> On Wed, Feb 19, 2014 at 8:45 AM, Dan Williams <dcbw@xxxxxxxxxx> wrote:
> > 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@xxxxxxxxxx> wrote:
> >> > On Fri, 2014-02-14 at 18:59 -0800, Luis R. Rodriguez wrote:
> >> >> From: "Luis R. Rodriguez" <mcgrof@xxxxxxxx>
> >> >>
> >> >> 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
> 
> I hadn't seen those, thanks!

Note that there isn't yet a disable_ipv4 knob though, I was
perhaps-too-subtly trying to get you to send a patch for it, since I can
use it too :)

Dan

> > 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?
> 
> Under the approach Stephen mentioned by first ensuring the interface
> is down yes. There's one use case I can consider to still want the
> patch though, more on that below.
> 
> > 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.
> 
> Neat, thanks.
> 
> >> 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 backends though, as such this is no longer applicable as a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?
> 
> Yeah that'll do it, unless there is a measurable run time benefit cost
> to never even add these in the first place. Consider a host with tons
> of guests, not sure how many is 'a lot' these days. One would have to
> measure the cost of reducing the amount of time it takes to boot these
> up. As discussed in the other threads though there *is* some use cases
> of assigning IPv4 or IPv6 addresses to the backend interfaces though:
> routing them (although its unclear to me if iptables can be used
> instead, Zoltan?). So at least now there no clear requirement to
> remove these interfaces or not have them at all. The boot time cost
> savings should be considered though if this is ultimately desirable. I
> saw tons of timers and events that'd get triggered with any IPv4 or
> IPv6 interface laying around.
> 
>   Luis
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