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Subject: Re: [PATCH v2 07/33] x86/intel_rdt: Add support for Cache Allocation
 detection
To: "Luck, Tony" <tony.luck@intel.com>
References: <1473328647-33116-1-git-send-email-fenghua.yu@intel.com>
 <1473328647-33116-8-git-send-email-fenghua.yu@intel.com>
 <57D88052.60404@intel.com> <20160913225217.GA4325@intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <h.peter.anvin@intel.com>,
        Ingo Molnar <mingo@elte.hu>, Peter Zijlstra <peterz@infradead.org>,
        Tejun Heo <tj@kernel.org>, Borislav Petkov <bp@suse.de>,
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        Ravi V Shankar <ravi.v.shankar@intel.com>,
        Vikas Shivappa <vikas.shivappa@linux.intel.com>,
        Sai Prakhya <sai.praneeth.prakhya@intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>, x86 <x86@kernel.org>
From: Dave Hansen <dave.hansen@intel.com>
Message-ID: <57D8850B.9000107@intel.com>
Date: Tue, 13 Sep 2016 16:00:27 -0700
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On 09/13/2016 03:52 PM, Luck, Tony wrote:
> On Tue, Sep 13, 2016 at 03:40:18PM -0700, Dave Hansen wrote:
>> Are you sure you don't want to add RDT to disabled-features.h?  You have
>> a config option for it, so it seems like you should also be able to
>> optimize some of these checks away when the config option is off.
> 
> Makefile looks like this:
> 
> obj-$(CONFIG_INTEL_RDT)        += intel_rdt.o intel_rdt_rdtgroup.o intel_rdt_schemata.o
> 
> which seems to skip compiling all our code when the CONFIG
> option is off.
> 
> Our hooks to generic code look like:
> 
> +#ifdef CONFIG_INTEL_RDT
> +extern void rdtgroup_fork(struct task_struct *child);
> +extern void rdtgroup_exit(struct task_struct *tsk);
> +#else
> +static inline void rdtgroup_fork(struct task_struct *child) {}
> +static inline void rdtgroup_exit(struct task_struct *tsk) {}
> +#endif /* CONFIG_INTEL_RDT */
> 
> Does this disabled-features.h thing do something more?

If you have cpuid checks in common code, disabled-features.h can compile
them out if the config options are turned off.  For instance:

	if (cpu_feature_enabled(X86_FEATURE_PKU))
		foo();

is equivalent to:

#ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
	if (boot_cpu_has(X86_FEATURE_PKU))
		foo();
#endif

But, if all the cpu_has(c, X86_FEATURE_CAT_L3) checks are confined to
files only compiled under CONFIG_INTEL_RDT then it won't do you much
good.  But, it's pretty simple to add things, and would help you out if
checks spread beyond intel_rdt*.c.