installing 64-bit kernel on a 32-bit system (nouveau issue?)

[Roberto Ragusa]

(warning: added cross posting to fedora-devel)

Ramesh.R wrote:
> 
> You can use 32 bit OS in 64 bit processor.
> 
> 32 bit address bus will use 64 bit. MSB 32 bits will be idle..
> 
> But for the case, 64 bit OS in a 32 bit processor is not possible by theory.

No one is talking about that. You are not the only one in this thread
to have misunderstood.

Suppose you have a 64 bit processor.
You can run:

(a) 64 bit kernel + 64 bit apps: that would be a pure 64 bit system

(b) 64 bit kernel + 64 bit and 32 bit apps: that would be a multilib system,
where you keep some 32 apps for some reasons

A normal Fedora installation will give you case (a) or (b).

Now, consider this:

(c) 64 bit kernel + 32 bit apps: this is simply an extreme case of (b),
a 64/32 system where every app is 32.

Case (c) is interesting because:

- you can switch a 32 bit install to this mode by simply installing a 64
bit kernel (and switch back at grub level any time you want)

- the 64 bit kernel can handle all your memory better (faster) than 32
or 32+PAE kernel

- you avoid the increased memory consumption of 64 bit apps (pointers
are wider; there is big debate how much this impacts performance and
if it is able to demolish the other improvements of x86_64 such as
more regusters and SSE2 guaranteed avalability). Add to this that
when you run 32 and 64 bit apps together you have both versions of the
system libraries in memory, so the mem usage is higher.

Finally, the discussion is:
case (c) _SHOULD_ work perfectly in theory (see case (b)), but apparently
there are a couple of bad spots for things no one ever run in 32 bit
mode on 64 bit kernel. The first one I heard is the Nvidia closed source
driver (ok, we already know closed source = unfixable problems), but
this thread seems to suggest that nouveau has a similar issue.
My suspicion is that it is just an untested area and a fix could be done
easily. In any case, it looks like a show stopper for nvidia users.

Best regards.

-- 
   Roberto Ragusa    mail at robertoragusa.it