Re: [PATCH] powerpc: Check that all cpu features are in the possible map

[Michael Ellerman <mpe@ellerman.id.au>]

On Tue, 2014-03-04 at 13:44 +1100, Michael Ellerman wrote:
> diff --git a/arch/powerpc/kernel/cputable.c b/arch/powerpc/kernel/cputable.c
> index 6c8dd5d..eb31713 100644
> --- a/arch/powerpc/kernel/cputable.c
> +++ b/arch/powerpc/kernel/cputable.c
> @@ -2255,6 +2255,15 @@ static struct cpu_spec * __init setup_cpu_spec(unsigned long offset,
>  	}
>  #endif /* CONFIG_PPC64 || CONFIG_BOOKE */
>  
> +	/*
> +	 * Check that all CPU features are in the possible mask. We don't want
> +	 * to WARN() because we're called very early, and doing so will kill
> +	 * the machine, so just printk() instead.
> +	 */
> +	if (t->cpu_features != (t->cpu_features & CPU_FTRS_POSSIBLE))
> +		printk("WARNING: cpu spec contains impossible features! 0x%lx\n",
> +			(t->cpu_features & ~CPU_FTRS_POSSIBLE));


Actually even this is not safe.

printk() takes the logbuf_lock, but we don't have a paca yet, so our LOCK_TOKEN
will be some random guff in low memory. And although that's OK, we're single
threaded, it's a bit fishy.

What's worse is printk() does local_irq_save(), which will _store_ to our paca
(soft_enabled), and so we risk flipping a value somewhere.

Interestingly lockdep appears to be OK, even though we haven't initialised it
yet. printk() helpfully turns lockdep off before doing any locking.

cheers