Re: Per-cpu patches on top of PDA stuff...

[Jeremy Fitzhardinge <jeremy@goop.org>]

Rusty Russell wrote:
> 	The first patch simply changes the GDTs to be a straight per-cpu
> variable.  I notice that you did a similar thing with your patches, but
> this is simpler and avoids wasting space in the UP case.  It's a bit
> tricky since we've never referred to per-cpu vars from asm before, but
> since we're only referring to the pre-setup versions, it's ok.
>   

The current mechanism was specifically introduced by James Bottomley a 
while back; I guess to deal with Voyager strangeness.

As far as setting up the PDA in head.S goes, it turns out to be very 
easy without having to access any per-cpu data, since the whole CPU 
bringup stuff depends on static variables anyway.

> 	The second patch changes gs to be the per-cpu offset, and by
> implication, avoids using it altogether on UP.  This avoids a special
> "pda" structure, instead allowing all per-cpu variables to be accessed
> this way.  It avoids __thread, which I gave up after creating a horribly
> complicated patch which still didn't quite work, and was no more
> efficient if we want the kernel to run under Xen anyway.
>
> I really think this is the way to go, and I'll start work on merging
> now.

Hm, now is not really a good time.  I'm still trying to get Xen 
basically working, and the percpu PDA stuff isn't really necessary for 
that.  The PDA stuff was enough of a problem in itself...

Also, the PDA patches are in -mm, so that's probably a better base for 
your patches.

    J