genuine cpu I386_CPU kernel support

[John Baldwin]

On Wednesday 23 September 2009 11:54:34 am Julian H. Stacey wrote:
> Rui Paulo wrote:
> > On 22 Sep 2009, at 19:03, Nate Eldredge wrote:
> > 
> > > On Tue, 22 Sep 2009, John Baldwin wrote:
> > >
> > >> My comment is to just use 4.x (seriously).  A true 386 is going to  
> > >> be quite
> > >> slow and the overhead of many things added that work well on newer  
> > >> processors
> > >> is going to be very painful on a 386 (probably on a 486 as well).   
> > >> 4.x runs
> > >> fine on a 386 and should support all the hardware you can stick  
> > >> into a
> > >> machine with an 80386 CPU.
> > >
> > > Unless, of course, you plan to put it on a network.  I doubt that  
> > > 4.x is up to date with respect to security patches.
> > 
> > I don't know if they were all applied on 4.x, but I think at least the  
> > older ones are.
> 
> 4.11 fell out of security support some while back, but
> 	http://www.freebsd.org/security/index.html 
> only lists what's still in, not what fell out when.
> 
> Free/ Net/ Open/ Dragon etc all derive from Bill Jollitz port of
> BSD to 386.  Would be nice if we could still keep that first platform
> walking, even if speed can't be called running ;-)
> 
> Maybe I'll get time to chase down all that came before
> 	http://svn.freebsd.org/viewvc/base?view=revision&revision=137784

Other things added since then assume at least a 486.  Not having cmpxchg is a
bit of a killer.  The umtx stuff used by libthr assumes it can do a cmpxchg in
userland for example.  One idea kicked around many years ago was catching the
illegal instruction faults for userland and emulating cmpxchg, but that would
be a good bit of work.  FreeBSD now also makes liberal use of 'xadd' for
reference counts (see refcount_*()) so you would need to support that on a
386 as well.  There may be other places that I'm not aware of that have
similar assumptions.  FWIW, I would probably not be in favor of putting any
patches into the tree if you do manage to get it all working.  I suspect the
userbase of FreeBSD/80386 is even smaller than FreeBSD/alpha or 
FreeBSD/sparc64 and 80386 support would add a lot of ugly #ifdef's for 
miniscule gain.

-- 
John Baldwin