Subject: Re: bcopy, bzero, copypage, and zeropage
To: David S. Miller <>
From: Chris G Demetriou <>
List: tech-kern
Date: 12/14/1996 13:52:29
> Why not include all of the routines, set aside enough space in the
> "real" routines for the largest of all the possible routines, which is
> just full of nops, and self modify that code space with the routine
> you want to use?  Zero overhead execution time wise.  If space is a
> concern, put each possible routine in a page aligned full page of
> space, once the routine you want has been copied into it's final
> destination you can give back all that space to the pager.

And all of a sudden, tracebacks and post-mortem debugging become so
much less useful and/or harder...

(Something like this was actually suggested in another forum
yesterday(?) by jonathan stone, as well...  And i still dislike it.  8-)

Note that this affects only GENERIC kernels.  For machine-specific
kernels, which people should be using, anyway, in my opinion, there is
no overhead whatsoever.  That philosophy, and the fact that it's been
part of BSD "forever," is a good reason to run specific kernels.

GENERIC kernels are used e.g. for distributions.  You don't
particularly want trickery there.  Also, if somebody's trying to run
them on a new machine, and gets a crash, you want that crash to be
absolutely as debuggable as possible, so that if it's a stupid mistake
you can fix it and not have them testing kernels ad infinitum...

While self-modifying code may have advantages, in my opinion this
isn't a place where the advantages outweigh the disadvantages.