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    SubjectRe: [PATCH 07/14] [POWERPC] Populate cpu_enabled_map
    * 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



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