vale and netmap module questions

[Marko Zec]

On Wed, 5 Sep 2018 17:42:06 -0700
John-Mark Gurney <jmg at funkthat.com> wrote:

> Marko Zec wrote this message on Wed, Sep 05, 2018 at 12:47 +0200:
> > On Wed, 5 Sep 2018 12:36:38 +0200
> > Vincenzo Maffione <v.maffione at gmail.com> wrote:
> >   
> > > Hi Marko,
> > >   Thanks a lot for identifying the problem.
> > > If I understand correctly, simply adding -D VIMAGE here
> > > https://github.com/luigirizzo/netmap/blob/master/sys/modules/netmap/Makefile#L11
> > > would at least mitigate the issue.
> > > If you think I'm right I'll just add it.  
> > 
> > Right, go for it...  
> 
> Why not just hook up netmap to the build?

I have no idea why it is on by default only on amd64...  Perhaps due to
the lack of adopters / testers on other platforms?  Perhaps the code has
some assumptions re. memory coherence model, or unaligned accesses,
which hold the water on amd64 but not necessarily elsewhere?

> Because if you add -DVIMAGE to the Makefile, you'll now break people
> who have kernels w/o VIMAGE..

No, and this is certifiable.  At least in this particular case, we have
only two macro instances which set / restore curvnet prior to calling
into the network stack.  If the network stack is of the non-VNET kind,
and thus doesn't care about curvnet, that won't do any harm.

> And the only reason to build netmap module manually is because it's not hooked up.

There's still a reason to retain the possibility to build netmap as
module, as this speeds up the development cycles for netmap-internal
hackers / experimenters.

But beyond that, running netmap without native support in device
drivers is pretty much pointless.