linux-kernel - Re: [PATCH v9 2/5] x86/cpuid: Add generic table for cpuid dependencies

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Date:   Thu, 12 Oct 2017 17:13:34 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ingo Molnar <mingo@...nel.org>
cc:     Andi Kleen <andi@...stfloor.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>,
        Jonathan McDowell <noodles@...th.li>
Subject: Re: [PATCH v9 2/5] x86/cpuid: Add generic table for cpuid
 dependencies

On Thu, 12 Oct 2017, Ingo Molnar wrote:
> 
> * Andi Kleen <andi@...stfloor.org> wrote:
> 
> > --- /dev/null
> > +++ b/arch/x86/kernel/cpu/cpuid-deps.c
> > @@ -0,0 +1,109 @@
> > +/* Declare dependencies between CPUIDs */
> > +#include <linux/kernel.h>
> > +#include <linux/init.h>
> > +#include <linux/module.h>
> > +#include <asm/cpufeature.h>
> > +
> > +struct cpuid_dep {
> > +	unsigned short feature;
> > +	unsigned short depends;
> > +};
> 
> Why are these 16-bit fields? 16-bit data types should be avoided as much as 
> possible, as they generate suboptimal code.

I was looking at that as well and decided that we preferrably have a
compressed data structure. The code which walks the table is hardly
performance critical and the difference in text size is marginal.

Thanks,

	tglx