linux-kernel - Re: [PATCH v2 04/19] genirq: Add new struct ipi

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Date:	Mon, 30 Nov 2015 14:11:03 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Qais Yousef <qais.yousef@...tec.com>
cc:	linux-kernel@...r.kernel.org, jason@...edaemon.net,
	marc.zyngier@....com, jiang.liu@...ux.intel.com,
	ralf@...ux-mips.org, linux-mips@...ux-mips.org
Subject: Re: [PATCH v2 04/19] genirq: Add new struct ipi_mask and helper
 functions

On Mon, 30 Nov 2015, Qais Yousef wrote:
> 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?

nr_cpu_ids == find_last_bit(cpumask_bits(cpu_possible_mask),NR_CPUS) + 1;

Thanks,

	tglx
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