Re: [Xen-devel] [PATCH v5 1/6] xen: Add convenient macro boot_cpu

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2014-05-02 at 16:57 +0100, Andrew Cooper wrote:
> On 02/05/14 16:52, Julien Grall wrote:
> > The macro boot_cpu will be used to get CPU variable from the boot CPU.
> >
> > Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>
> > Suggested-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
> > Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
> > Cc: Jan Beulich <jbeulich@xxxxxxxx>
> > Cc: Keir Fraser <keir@xxxxxxx>
> > ---
> >  xen/include/xen/percpu.h |    3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/xen/include/xen/percpu.h b/xen/include/xen/percpu.h
> > index abe0b11..0e848bf 100644
> > --- a/xen/include/xen/percpu.h
> > +++ b/xen/include/xen/percpu.h
> > @@ -16,6 +16,9 @@
> >  /* Preferred on Xen. Also see arch-defined per_cpu(). */
> >  #define this_cpu(var)    __get_cpu_var(var)
> >  
> > +/* Access variable on boot CPU */
> > +#define boot_cpu(var)   per_cpu(var, 0)
> > +
> 
> What is this actually used for?
> 
> In x86 with arbitrary cpu hotplug, we are trying to move away from the
> false assumption than cpu0 is the boot cpu.  Actual boot data is stored
> sideways as boot_$FOO variables, and not accessed with the per-cpu
> mechanism.

Presumably in that case you would want it to expand into
        per_cpu(var, somevar_with_boot_cpu_nr_in_it)
and being able to write that as boot_cpu would be preferable to open
coding, especially since people would forever be writing 0, at least
this way its in one place.

Ian.


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