SMP on the MacBook

[Marco Gerards <mgerards@xs4all.nl>]

Hi,

For a while now I have been trying to debug an issue with Xen on the
MacBook, doing this is part of my Summer of Code project to get Xen
working properly on the Intel Mac.  In case some of you don't know,
the MacBook is an Intel Core Duo based laptop from Apple, which lacks
some legacy hardware.

Xen (testing) works perfectly for me when starting it with "nosmp",
however when not using it, it crashes while starting up the second
core.  There have been some bug report on this issue already.

Below I will explain the problem in more detail and with some
observations.  I will be very grateful if someone can come up with
ideas of how to fix this or give me some ideas that can lead to a
solution.  Not only Xen has this problem, but so has FreeBSD and
NetBSD, so a solution will be beneficial to these projects as well.

What happens is the following: The first core sends two IPI
messages(INIT and STARTUP) to start the second core.  When the second
core is started, it notifies the first core of this so it can continue
with the boot process.  However, the second core is not started by
these messages, the loop that waits for the second core times out,
continues and Xen crashes.

One strange thing is that Xen initializes the second core properly
when you press a key on the keyboard after the IPIs are sent.  I am
expecting this interrupt somehow wakes up the second core so it can
accept and process the IPIs.  Because it is a laptop the second core
is put in deep sleep mode to save power in case an OS does not use
this core at all.  But as I understand it, it should wake up when an
INIT IPI arrives.

The problems I am having with Xen can be reproduced with linux when
booting with "acpi=ht", which means the ACPI interpreter is not
started.

Hopefully someone has some ideas how to deal with this issue and how
to properly fix this.  If you want more information or want me to test
things, feel free to ask.

Thanks,
Marco