* Jeremy Fitzhardinge (jeremy at goop.org) wrote: > 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. Yes, he reverted some changes because voyager boot cpu may not be cpu 0. > 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... I agree. We're right in the middle of the last bit of Xen bring up. PDA is not strictly needed, and destabilizing for this would be counter productive. thanks, -chris
