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Re: [libvirt] Need to re-work final "peer address" patches and re-push them



On 05/12/2016 05:12 AM, Daniel P. Berrange wrote:
On Wed, May 11, 2016 at 11:57:36AM -0400, Laine Stump wrote:
I reverted these three patches that introduced and enabled a "peer"
attribute for type='ethernet' interface <ip> elements prior to the release
of 1.3.4 with the intent of fixing/re-posting them after release, but forgot
until today:

https://www.redhat.com/archives/libvir-list/2016-April/msg01995.html

I have patches for most of the bugs, but the one problem that still doesn't
have resolution is the naming of the "peer" attribute. In my opinion, having
the two address attributes named "address" and "peer" makes it ambiguous
which address is for the guest side and which for the host side (especially
since the attribute that has been named "peer" would be set to the "address"
in the netlink command, and the attribute named "address" would be set to
"peer" in the netlink command :-O).

Since "address" is an existing attribute, and already used for the guest
side IP address in lxc type='bridge' interfaces, it must remain as-is. In
order to make it obvious that the new address is for the host side of the
tap (or veth pair in the case of lxc), I propose calling it either "host",
or "hostAddress", e.g:

      <ip address='192.168.123.43' host='192.168.123.1' prefix='25'/>

or

      <ip address='192.168.123.4' hostAddress='192.168.123.1' prefix='25'/>

(Vasiliy had suggested "hostPeer", but I dislike that, since it sounds like
"the peer of the host", which is even more misleading).

Can some of you normally-opinionated people weigh in on this? I don't like
the feeling of making a unilateral decision :-)

Also, I'm realizing that, although there was a patch to support setting the
host-side address (hmm - "hostSide"? nah) for lxc type='bridge' interface,
this is not at all useful, because anything plugged into a bridge should not
have any IP on the side plugged into the bridge. The place where it would be
useful for lxc would be (just as it is for qemu) with a type='ethernet'
interface - the guest-side veth would have "address" and the host-side veth
would have "hostAddress", and it would then properly work without needing a
bridge (which I think is the entire point). Since lxc doesn't currently
support type='ethernet', I think that initial support should be made for
qemu only, and when type='ethernet is added to lxc, it can be made to
support an IP address on both sides of the veth pair from the start.

Lacking any useful responses, I'm thinking to update Vasiliy's patches to
use "hostAddress" (and fix the other bugs I had found) and re-post them.
I'm not actually convinced your host/guest distinction actually matches
what was being done with the peer attribute.

First, off the virNetDevSetIPAddress change was doing the following

  - "address" attribute is mapped to IFA_LOCAL in netlink
  - "peer" attribute is mapped to IFA_ADDRESS in netlink

What is the difference between IFA_LOCAL and IFA_ADDRESS you might
ask ? You can see that in /usr/include/linux/if_addr.h comments:

  * IFA_ADDRESS is prefix address, rather than local interface address.
  * It makes no difference for normally configured broadcast interfaces,
  * but for point-to-point IFA_ADDRESS is DESTINATION address,
  * local address is supplied in IFA_LOCAL attribute.

So we're setting the peer / IFA_ADDRESS to make point-to-point
routing work correctly.


In LXC containers, we set an IP address on the *guest* side of the
interface, based on the 'address' attribute. The patches extended
that by also setting the 'peer' address on the *guest* side. This
is true regardless of the type of <interface> backend configured

in QEMU machines, we set an IP address on the *host* side of the
interface, based on the 'address' attributee. The patches extended
that by also setting the 'peer' address on the *host* side. We only
do this for type=ethernet backends.

Yes, thanks for making me realize that - I had been treating the veth device pair as a single interface, and naively assumed (because I'd been unable to test it due to the peer attribute missing from the formatter output) that the pair was altogether treated as a single device for purposes of IP configuration. (chalk it up to my pre-history working on PPP, where the two ends of the link always agreed on what each others' addresses were).

Still, I think it's wrong that <ip address='1.2.3.4'/> on qemu should set the IP address on the host side, and the exact same element in the same place in an lxc config should set the ip address on the guest side.Why be purposefully inconsistent between hypervisors when we don't need to (and when doing so could be the cause of even further divergence in the future)?

IOW, whether we set addresses on the host or guest side of the
interface right now is being determined by whether we use QEMU
or LXC.  You can't say the existing 'address' attribute is either
host or guest - it could be for either.

and that's what I don't like.

Likewise the added 'peer'
attribute is also either for host or guest address - and will
*always* match the side used for the 'address' attribute. ie if
'address' was set on the host, then 'peer' would also be set
on the host.


So based on this understanding, I don't think your suggestion to
try and distinguish 'address' as being a guest thing and 'peer'
as being a host thing is actually correct. In fact I think that
'peer' name was in fact probably correct choice of naming.

I agree, if you're trying to exactly describe what's happening at the lowest level of the configuration of each individual device.

But if you look at it from a functional point of view, you have a single link with an IP address at both ends, and want to configure both of those addresses. At the lower level, the link is implemented with two devices not one (one on host , one on guest), and you almost certainly want to configure the two devices such that guestPeer == hostLocal and guestLocal == hostPeer (since it otherwise won't work properly). (MB: my understanding/opinion of this has changed slightly since considering Andrea's question about differing prefixes for host and guest sides - see my response in that sub-thread)

In the case of LXC:

              (link handled by veth driver)
  guest veth <============================> host veth

We can configure both of these devices in libvirt's setup code because libvirt creates both devices prior to starting the container, although currently we only configure the guest-side veth because the host side is always attached to a bridge, and you don't need/want an IP address on a device connected to a bridge (it's assumed of course that devices *beyond* the bridge-attached veth will have an IP address). If we wanted, though, we could set the IP address of the host-side veth, which would be useful in case of <interface type='ethernet'> (if that were supported on lxc, which currently isn't the case).

qemu is more problematic though:

                     (link handled by qemu or vhost-net)
  guest emulated NIC <========================> host tap

Since the NIC device in the guest has no visibility to the host (it doesn't exist until qemu is started, and is only configurable by the guest OS), libvirt can only reasonably configure the host side tap device. (I suppose we could run a dnsmasq listening on the tap that could answer dhcp queries from the guest, or maybe add the capability to set guest-side IP addresses via a guest agent (or maybe even run PPPoE on host and guest, thus creating another level of devices :-O), but at present for all practical purposes the best we can do is simply hope that the guest-side IP configuration we can't controll matches/mirrors the host side configuration that we can control.)

To come back to the point:

1) libvirt attempts to provide the same end-result (or as close as possible) for all the hypervisors for any given configuration; I think that having <ip address='blah'> set the guest-side local IP on one hypervisor, and the host side local IP on another doesn't live up to "as close as possible". Those are different entities, and should be configured separately.

2) It may be possible that there are valid configurations where guestPeer != hostLocal orguestLocal != hostPeer; such a thing doesn't come to mind at the moment. If not, then having dual config so that all 4 can be represented seems like overkill. (I'm going to try some experiments with this after I'm done typing.) If we do decide that it is overkill, then rather than changing the semantics of <ip> based on which hypervisor we're using, I think it would be better that the address attribute of <ip>, which currently has meaning only for lxc and means "the local-side IP of the guest interface" should continue to have that meaning when support for setting IP addresses for qemu is added.

3) If we do want to configure the host-side local and peer addresses separately from the guest-side, then we could consider this:

        <ip address='1.2.3.4' peer='4.3.2.1' prefix='8'/>
        <source ..... >
           <ip address='4.3.2.1' peer='1.2.3.4' prefix='32'/>
        </source>

(the ip element under "source" would be used to set the ip address on the host side. As an aside, it would have made more sense to have this IP address specified in the same place as the host-side device is named, but for some odd reason it is named in the <target> element, which has come to be the place where attributes pertaining to how the device appears on the *guest* side live :-/).




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