Re: [PATCH] ACPI: Add ACPI CPU hot-remove support

[Toshi Kani <toshi.kani@hp.com>]

On Tue, 2012-07-10 at 16:56 -0600, Khalid Aziz wrote:
> On Fri, 2012-07-06 at 15:00 -0600, Toshi Kani wrote:
> > Yes, offlining and eject are similar operations to a core as it alone
> > cannot be removed physically.  Ejecting a core is a logical eject
> > operation, which updates the status (_STA) of the object in ACPI after
> > offlining.  The difference from the offlining is that the ejected core
> > is no longer assigned to the partition.  Here is one example.  Say, a
> > core is assigned to a guest partition as a dedicated resource (ex. 100%
> > of its CPU time is bound to the partition).  Offlining this core saves
> > the power-consumption, but this core is still bound to the partition.
> > Ejecting the core removes it from the partition (logically), and allows
> > it to be assigned to other partition as a dedicated resource with
> > hot-add.
> > 
> 
> Ejecting a core is reasonable when eject happens from a guest. I still
> wonder what firmware would do if kernel calls eject method on a core
> when running on the native host platform. If firmware behavior is not
> well defined in this case, there might be some risk associated with
> calling eject method on core. 
> 
> Makes sense?

No, that's not the case.  The firmware only implements _EJ0 when it
supports the behavior on the environment.  It is true for both native
and virtual platforms.  Note that the presence of a CPU is abstracted
with _STA in ACPI, so it does not matter to the kernel if an eject is a
physical or logical operation.

For example, HP Superdome 2 implements _EJ0 on the native platform to
support capacity-on-demand and RAS features (which are supported by
HP-UX).  _EJ0 is still a logical eject operation in this case.

Thanks,
-Toshi