Re: [PATCH v2 04/19] genirq: Add new struct ipi_mask and helper functions

[Qais Yousef]

On 11/30/2015 11:20 AM, Thomas Gleixner wrote:
> On Wed, 25 Nov 2015, Qais Yousef wrote:
> > cpumask is limited to NR_CPUS. Introduce ipi_mask which allows us to address
> > cpu range that is higher than NR_CPUS which is required for drivers to send
> > IPIs for coprocessor that are outside Linux CPU range.
> I have second thoughts on this.
>
> cpumask is indeed limited to NR_CPUS or in case of CPUMASK_ON_STACK
> limited to nr_cpu_ids.
>
> But, that's not an issue for that coprocessor case. Let's assume you
> have 16 Linux CPUs and 4 coprocessors. So you set the number of
> possible cpus (NR_CPUS) to 20. That makes the cpumask sizeof 20.
>
> The boot-process sets the number of available cpus to 16. So the
> Linux side will never try to access anything beyond cpu15.
>
> But you can spare that extra mask magic and simply use cpumask. Sorry
> that I did not think about that earlier.


Yes it would be much better to reuse it but wouldn't the runtime checks 
against nr_cpu_ids create problems especially when CPUMASK_ON_STACK is 
defined?

Thanks,
Qais
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