linux-kernel - Re: [PATCH 2/2] kvm: use cpumask_var_t for cpus_hardware

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Date:	Mon, 8 Dec 2008 16:35:34 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Avi Kivity <avi@...hat.com>
Cc:	"kvm-devel" <kvm@...r.kernel.org>, linux-kernel@...r.kernel.org,
	Mike Travis <travis@....com>
Subject: Re: [PATCH 2/2] kvm: use cpumask_var_t for cpus_hardware_enabled

On Monday 08 December 2008 02:25:50 Avi Kivity wrote:
> Rusty Russell wrote:
> > This changes cpus_hardware_enabled from a cpumask_t to a cpumask_var_t:
> > equivalent for CONFIG_CPUMASKS_OFFSTACK=n, otherwise dynamically allocated.
> >
> >  
> > -static cpumask_t cpus_hardware_enabled;
> > +static cpumask_var_t cpus_hardware_enabled
> 
> This isn't on stack, so it isn't buying us anything.

It's the CONFIG_NR_CPUS=4096 but nr_cpu_ids=4 case which we win using
dynamic allocation.  Gotta love distribution kernels.

> Is the plan to drop cpumask_t?

Yes.  And undefine 'struct cpumask' if CONFIG_CPUMASK_OFFSTACK.  That
will stop assignment and on-stack declarations for all but the most
determined.

> If so, we're penalizing non-stack users 
> by forcing them to go through another pointer (and cacheline).

Not quite.  If !CONFIG_CPUMASK_OFFSTACK, cpumask_var_t == cpumask_t[1].
Blame Linus :)

Cheers,
Rusty.
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