Re: Specifying names for tap interfaces

[Roger Pau Monne <roger.pau%citrix.com@localhost>]

Greg Troxel wrote:
>    Linux does this by passing the name of the tap interface Qemu has to
>    create, so when the interface is created its name is known and can be
>    used outside of Qemu.
>
>    From what I see, NetBSD has the option to fetch the name of the
>    interface created, using the TAPGIFNAME ioctl, but this is not really
>    helpful because we create the interface from Qemu, but the scripts
>    that attach the network interface are launched from the toolstack
>    (xl).
>
>    Will it be possible to provide a TAPSIFNAME ioctl to set the name of
>    the created interface? I will start looking into this, but I would
>    also like some feedback.
>
> So you basically want to have
>
>    in dom0, have xl tell qemu to make an interface for the guest, and to
>    name it /dev/tap-NNN where xl chooses that

Well in Linux you can give any name to tap interfaces so Xen is 
currently using vif2.0-emu for Linux specific reasons.

Anything that we can control, like tap-%domid.%devid would be good 
though. It doesn't have to follow any specific nomenclature, but it will 
be good to be able to specify two integers, one that corresponds to the 
domain id, and another one that identifies the specific network card 
inside that domain.

>    have qemu create a tap and arrange the name

Yes, basically to have a ioctl to be able to specify the name.

>    have xl in the dom0 bridge tapNNN to whatever

Yes, hotplug scripts are launched directly from xl in new versions, so 
xl decides the name and passes it to both Qemu and hotplug scripts.

> and the problem with just using what tap picks is that there's no
> communication path from qemu create back to xl?

No, not really. Another option is to try to fetch the name from Qemu and 
pass it to the hotplug script. But this will require changes on both the 
Qemu and the xl side, and I think it will be more complicated. Apart 
from the fact than having this functionality might be interesting to 
other users.

> Does qemu use /dev/tap, which picks a name, or by finding a /dev/tapN to
> open and then using create?

Qemu opens /dev/tap and creates a new /dev/tapX, then fetches the name 
of this interface using the TAPGIFNAME. In the past, Qemu had a 
parameter that you could use to set the bridge Qemu has to pass to the 
hotplug script, but this is no longer present, and the script Qemu calls 
after creating the interface only knows the name of the newly created 
interface, which is not really helpful, and it's not used in Linux 
because all hotplug scripts are launched from xl.

> Or is the problem that you need arbitrary names, rather than just to
> have xl be able to pick something?

Not really arbitrary, as explained before, something like tap-%d.%d 
should be ok.

> A few thoughts, not particularly coherent:
>
>    qemu could be given a name and then create a symlink from that name to
>    whatever tap chooses.  or use mknod to make the name actually be the
>    special file.

Yes, but the interface name will still be tap%d, and brconfig will fail 
to add that interface.

>    TAPSIFNAME seems tricky, because it seems to be about controlling the
>    whole name, whereas tap seems to be tapN for some (printed
>    representation of) number N, correpsonding to character special file
>    (169,N).  So is this just about choosing N, or choosing the rest of
>    the interface name?

I think that one way or another we need to be able to choose a tap name 
at creation, either the whole name, or just a number appended to tap-. 
Wouldn't it be possible to create a network interface with a random 
name, that still points to (169,N), even if N is not part of the name?

>    Or is it that choosing N is sufficient, but you'd like to be able to
>    do TAPSIFNAME on /dev/tap?  That seems awkward, because as I read the
>    code the interface is created on open, so it would have to swap the
>    softc, essentially closing the current tap and opening a new one.
>
>
> Overall, it seems easiest to teach qemu to use 'ifconfig tapN create' as
> an alternative to clone/set-name.

I'm not really sure using an external command is better than performing 
a set of ioctls, it's much more trickier to catch errors, and I would 
say that overall I doubt Qemu developers would ever agree to switching 
from using /dev/tap to ifconfig.

Thanks for the thoughts, Roger.