HEADS UP Re: cvs commit: src/sys/conf options.i386 src/sys/i386/i386 bios.c locore.s machdep.c mpboot.s pmap.c vm86bios.s vm_machdep.c src/sys/i386/include _types.h bus_at386.h param.h pmap.

[Peter Jeremy]

On Sun, Mar 30, 2003 at 06:20:30PM -0500, Jake Burkholder wrote:
>Apparently, On Sun, Mar 30, 2003 at 03:30:52PM -0600,
>	Mike Silbersack said words to the effect of;
>> Is it practically possible with PAE and busdma'd drivers that such a
>> configuration could work?
>
>I'm not sure I understand the question, you mean is it possible to use
>separate address spaces for the kernel and userland, giving a full 4G each?
>Yes it is possible, but it is not practical.

Why do you say "not practical"?  Unix spent most of its formative
years with kernel and userland in separate address spaces.  I don't
think the code exists in 4BSD but it's definitely still functional
in 2BSD (which is under a BSD license now-a-days).

>  It would be prohibitively expensive and ugly.

I'll accept "ugly" and "expensive".  The "prohibitively" is more of a
value judgement.  Currently FreeBSD needs to trade KVA against UVA for
large RAM configurations.  If you have an application that needs lots
of KVA and lots of UVA then FreeBSD on x86 isn't currently an option.
Wearing the overheads on system calls, copyin and copyout may be
cheaper than the alternatives.

Peter