linux-kernel - Re: [PATCH v2 1/2] x86: Add cpu_detect_cache_sizes() to init_intel(), add Quark to legacy

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Date:	Mon, 06 Oct 2014 11:05:28 +0100
From:	Bryan O'Donoghue <pure.logic@...us-software.ie>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	mingo@...hat.com, hpa@...or.com, davej@...hat.com, hmh@....eng.br,
	x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] x86: Add cpu_detect_cache_sizes() to init_intel(),
 add Quark to legacy_cache()

On 06/10/14 10:55, Thomas Gleixner wrote:
>> +	 */
>> +	if (c->x86_cache_size == 0)
>> +		l2 = init_intel_cacheinfo(c);
>> +	else
>> +		l2 = c->x86_cache_size;
>
> Looking a bit deeper. This wont work because cpu_detect_cache_sizes()
> will set c->x86_cache_size for all cpus with extended_cpuid_level >=
> 0x80000005, which is the case for everything modern. So this results
> in init_intel_cacheinfo() not being called anymore.

True.

Missed that one.

> So we might need the following conditional:
>
> 	l2 = init_intel_cacheinfo(c);
> +	if (!c->x86_cache_size) {
> +		cpu_detect_cache_sizes(c);
> +		l2 = c->x86_cache_size;
> +	}

Yes - that'll work better.

Also - it only forces a conditional branch - so fewer extra cycles for 
everybody !Quark and !Tualatin - which is preferential. Will incorporate.


Best,
Bryan
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