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
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