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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

~Andrew

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