linux-kernel - Re: [PATCH 1/3] x86: Bugfix bit-rot in the calling of legacy_cache

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Message-ID: <542B1A74.9000909@nexus-software.ie>
Date:	Tue, 30 Sep 2014 22:02:44 +0100
From:	Bryan O'Donoghue <pure.logic@...us-software.ie>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	mingo@...hat.com, davej@...hat.com, hpa@...or.com, hmh@....eng.br,
	x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] x86: Bugfix bit-rot in the calling of legacy_cache_size

On 30/09/14 21:28, Thomas Gleixner wrote:
> On Tue, 30 Sep 2014, Bryan O'Donoghue wrote:
>>   static void default_init(struct cpuinfo_x86 *c)
>>   {
>> -#ifdef CONFIG_X86_64
>>   	cpu_detect_cache_sizes(c);
>> -#else
>> +
>>   	/* Not much we can do here... */
>>   	/* Check if at least it has cpuid */
>>   	if (c->cpuid_level == -1) {
>> @@ -79,7 +78,6 @@ static void default_init(struct cpuinfo_x86 *c)
>>   		else if (c->x86 == 3)
>>   			strcpy(c->x86_model_id, "386");
>>   	}
>> -#endif
>
> What's the exact point of this? This is the default init function
> which is for X86_VENDOR_UNKNOWN, right?

Correct

>>   }
>>
>>   static const struct cpu_dev default_cpu = {
>> @@ -874,6 +872,15 @@ static void identify_cpu(struct cpuinfo_x86 *c)
>>   #endif
>>
>>   	/*
>> +	 * If c_x86_vendor != X86_VENDOR_UNKNOWN i.e. a known vendor then
>> +	 * there's a vendor specific c_init()
>> +	 *
>> +	 * Even still Intel, AMD and VIA make use of legacy_cache_size which
>> +	 * is reachable only through default_init right now
>
> That's nonsense. It's called from cpu_detect_cache_sizes() and that is
> called from:
>
> arch/x86/kernel/cpu/amd.c:      cpu_detect_cache_sizes(c);
> arch/x86/kernel/cpu/centaur.c:  cpu_detect_cache_sizes(c);
> arch/x86/kernel/cpu/common.c:   cpu_detect_cache_sizes(c);
> arch/x86/kernel/cpu/cyrix.c:            cpu_detect_cache_sizes(c);
> arch/x86/kernel/cpu/transmeta.c:        cpu_detect_cache_sizes(c);

/uses grep

Yes I see for everybody except Intel that'll get called. I mised that.

> So we have 4 callsites outside of default_init() already. Now you're
> adding another one into identify_cpu()
>
>> +	 */
>> +	if (this_cpu->c_x86_vendor != X86_VENDOR_UNKNOWN)
>> +		default_init(c);
>
> With the consequence that for amd/centaur/cyrix/transmeta
> cpu_detect_cache_sizes() is invoked twice for nothing.

Yes - I see what's needed here is for cpu_detect_cache_sizes to get 
called for Intel only - so the bit rot is really only limited to

1. PIII Tualatin
2. New Intel additions on this path like Quark


> So the only CPU vendor c_init() callback which does not call
> cpu_detect_cache_sizes() is the Intel one. And therefor we inflict
> that call on all others twice. Brilliant solution!

Haha. When you put it like that you make it sound mad and unreasonable.

> So the proper solution is to add the call to identify_cpu()
> unconditionally and remove it from _all_ other call sites, make it
> static and be done with it.

Agree I'll make that update.

Thanks for your review.

--
BOD
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