On Tue, Sep 9, 2014 at 12:37 PM, Gleb Smirnoff <gleb...@freebsd.org> wrote:
> Luigi, > > On Tue, Sep 09, 2014 at 12:13:42PM +0200, Luigi Rizzo wrote: > L> svn 270870 removed all the if_*spare fields in struct ifnet. > L> They are replaced with the following comment > L> > L> /* > L> * Spare fields to be added before branching a stable branch, so > L> * that structure can be enhanced without changing the kernel > L> * binary interface. > L> */ > L> > L> which leaves me a bit unhappy. > L> Having a stable ABI is useful not only for stable branches, > L> but also (I would say even more) with head, so people can > L> run experimental code with limited modifications to the sources. > L> > L> Cases in point: > L> - we used one spare field extensively when experimenting > L> with netmap, and being able to just build a module without having > L> to recompile the whole kernel was a big win. > L> - we are developing some software GSO and again it was great to have > L> the spares in the tcpcb and in the ifnet so we could limit > L> modifications to headers used by multiple sources. > L> > L> I would kindly suggest to put the spares back. > L> I can't see how they can possibly harm. > > The harm is obvious: someone commits code that _uses_ spare field > without assigning it a new name. Spare field is a placeholder. Of > course you can use it while you experiment. However, I don't see > problem with patching your source tree where you experiment. > The problem with _not_ having spares is that you have to recompile everything after patching the headers. With the spares, i could make netmap a simple add-on kernel module with no dependencies. Same on linux for what matters (there wasn't a spare there, but nobody used ax25 and i could check and reuse it). Of course you can easily emulate extension fields for stuff that is not accessed frequently (tough a version number would help), but there are cases when you do need the extra info on a per-packet basis. > The ABI plan for 'struct ifnet' is that the struct becomes > opaque for device drivers. So its size and alignment no longer > matters. Those who want to add new fields to struct ifnet, > would also need to add accessor methods. Bits of this plan > are already committed by Marcel, but its only first step. > spare fields <-> spare accessors > I'm afraid that if fields are there back, the situation that > happened with netmap (use of spare field) would repeat. > the spare pointer used by netmap was clearly indicated by a comment, and giving it a dedicated name instead of if_pspare was expected to happen (hopefully together with a __FreeBSD_version bump, which has not happened). I am not worried that the name change was missed when deleting if_pspare. Mistakes happen and the error was promptly corrected (apart from the version bump). What worries me is losing some flexibility in dealing with out-of-tree kernel modules. cheers luigi _______________________________________________ firstname.lastname@example.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-current To unsubscribe, send any mail to "freebsd-current-unsubscr...@freebsd.org"