Re: [PATCH 2/2] kvm: use cpumask_var_t for cpus_hardware_enabled

Rusty Russell
2008-12-08T11:55:28Z

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.