genuine cpu I386_CPU kernel support

[John Baldwin]

On Wednesday 23 September 2009 1:21:59 pm Julian Elischer wrote:
> John Baldwin wrote:
> > 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.  
> 
> I think a 386 can assume non-SMP in which case that can be simulated 
> just fine :-)
>   it also simplifies a lot of the other breakages..
> 
> #if (CPU == 80386) && defined(SMP)
> #error "can't have smp on a 386"
> #endif

No, it actually does not.  The in-kernel version of cmpset for 386 was to
disable interrupts while doing a cmp and jmp around a mov (even 386's have
preemption, so you do have to disable interrupts).  You can't do that in
userland (cli is a privileged instruction), which probably mandates doing a
cmpxchg emulator in the kernel for userland code.  That and disabling
interrupts is actually far less efficient than spl() for a UP 80386 machine.
I suspect newer kernels will run slower on an 80386 than 4.x.

-- 
John Baldwin