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

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Message-ID: <20171013053005.cj5vhqehkwlnhyla@gmail.com>
Date:   Fri, 13 Oct 2017 07:30:05 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
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


* Thomas Gleixner <tglx@...utronix.de> wrote:

> 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.

So the code should all be __init (once that is fixed), hence data and text size 
literally does not matter - it gets freed.

So the only effect the 16-bit variables have is (marginally) worse boot times.

Thanks,

	Ingo