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

[David Woodhouse]

On Sat, 2010-01-02 at 12:39 +0100, Roberto Ragusa wrote:
> 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. 

We do that on PPC64. We have 32-bit userspace by default with a 64-bit
kernel. Only applications where 64-bit is actually _worthwhile_ are
64-bit.

On PowerPC we don't have to worry about being starved of registers in
32-bit mode, so the benefit of 64-bit userspace is much more limited.
It's really only beneficial where you really need to address more than
4GiB of RAM, or do 64-bit arithmetic.

So in the _general_ case, most of the 32-bit-userspace-on-64-bit-kernel
stuff ought to be working fine. That compatibility mode in the 64-bit
kernel ought to be fairly well tested.

Obviously that doesn't count for the nVidia binary module, which doesn't
exist for ppc64. And nouveau is relatively new and not currently being
used in Fedora/PPC64 so I'm prepared to believe that their 'work in
progress' API still has some issues in that area, as you suggest.

-- 
dwmw2