Re: [PATCH 07/14] [POWERPC] Populate cpu_enabled_map

[Alex Chiang <achiang@hp.com>]

* Benjamin Herrenschmidt <benh@kernel.crashing.org>:
> On Mon, 2008-07-14 at 20:34 -0600, Alex Chiang wrote:
> > Populate the cpu_enabled_map correctly.
> > 
> > Note that this patch does not actually make any decisions based
> > on the contents of the map.
> > 
> > However, as the map is presented via sysfs in:
> > 
> > 	/sys/devices/system/cpu/
> > 
> > It should be populated correctly.
> 
> Care to educate me on the difference between online_map and
> enabled_map ?

enabled_map is closer conceptually to present_map than
online_map.

present_map are CPUs that are actually plugged in

online_map are CPUs that have had cpu_up() called on them; ie.
schedulable

enabled_map is somewhere inbetween -- the CPUs are plugged in,
but we don't want to cpu_up() them. On hp ia64 systems, these
CPUs are disabled by system firmware.

Currently, a user can only configure/deconfigure the CPUs from
the system firmware interface. By providing a sysfs interface for
these CPUs, we can allow the user to configure/deconfigure them
from userspace. More realistically, higher level managability
software now has an OS-level interface to interact with these
CPUs.

Might this be useful for ppc and your hypervisor based
architecture? I could imagine your hypervisor telling the kernel
about all the physically present CPUs, but then you would be able
to have finer grained control using the enabled_map.

I haven't studied your code in depth, so maybe you can just do
everything with pure online/offline, but at least on my
platforms, there are use-cases where we might want something
in-between.

Thanks.

/ac