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

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Message-Id: <200812082220.42790.rusty@rustcorp.com.au>
Date:	Mon, 8 Dec 2008 22:20:42 +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 20:16:44 Avi Kivity wrote:
> Rusty Russell wrote:
> >> 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.
> >
> >   
> 
> What does it buy? 4096/8 = 512 bytes statically allocated?

It adds up, and 4096 seems to be only the start of the insanityH^H^Hfun.

> > Not quite.  If !CONFIG_CPUMASK_OFFSTACK, cpumask_var_t == cpumask_t[1].
> > Blame Linus :)
> >   
> 
> Hm, is there a C trick which will error out when allocating something on 
> the stack, but work when allocating statically?  I can think of 
> something to do the reverse, but that doesn't help.

We also need to prevent assignment, eg:

	*foo = *bar;

Because when we allocate them, we'll cut them to size.

Cheers,
Rusty.
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