Re: RFC: please put back spare fields in struct ifnet (removed in svn 270870)

  Luigi,

On Tue, Sep 09, 2014 at 01:01:13PM +0200, Luigi Rizzo wrote:
L> > The harm is obvious: someone commits code that _uses_ spare field
L> > without assigning it a new name. Spare field is a placeholder. Of
L> > course you can use it while you experiment. However, I don't see
L> > problem with patching your source tree where you experiment.
L> 
L> The problem with _not_ having spares is that you have to recompile
L> everything after patching the headers. With the spares, i could
L> make netmap a simple add-on kernel module with no dependencies.
L> Same on linux for what matters (there wasn't a spare there, but
L> nobody used ax25 and i could check and reuse it).

What if another kernel hacker comes and also uses if_pspare[0] for
his own super-duper feature? What would you do, if then a user
reports that enabling netmap on his box instapanics if he had
some other feature turned on?

L> > The ABI plan for 'struct ifnet' is that the struct becomes
L> > opaque for device drivers. So its size and alignment no longer
L> > matters. Those who want to add new fields to struct ifnet,
L> > would also need to add accessor methods. Bits of this plan
L> > are already committed by Marcel, but its only first step.
L> 
L> ​spare fields <-> spare accessors​

Let me repeat: in future device drivers will not be capable
to dereference struct ifnet. So, any feature pointers would
be accessed via functions, making struct ifnet no longer part
of ABI. This hasn't yet happened, but one already can (and
should!) use accessors in any new code.

L> > I'm afraid that if fields are there back, the situation that
L> > happened with netmap (use of spare field) would repeat.
L> 
L> ​the spare pointer used by netmap was clearly indicated by a comment,
L> and giving it a dedicated name instead of if_pspare[0] was
L> expected to happen (hopefully together with a __FreeBSD_version bump,
L> which has not happened).

I don't agree that /* 1 netmap, 7 TDB */ is a clear comment. I
understood as "in future one of those is to be used for netmap".

Regarding __FreeBSD_version: the field should have been renamed
in the commit that brought in netmap, not in my commit. My commit
removed a field that must not be used, so it is not a reason
to bump.

If you need closest __FreeBSD_version for the change, then
please use 1100030, it happened next day.

L> I am not worried that the name change was missed when deleting if_pspare[].
L> Mistakes happen and the error was promptly corrected (apart from the
L> version bump).
L> 
L> What worries me is losing some flexibility in dealing with
L> out-of-tree kernel modules.

Just put a properly named placeholder for them as a quick solution.

A nicer solution would be to add an API for storing vendor-specific
pointers on ifnet, providing a cookie. I'm discussing that now with
kmacy_at_, who also thinks in that direction.

-- 
Totus tuus, Glebius.