linux-kernel - Re: [Patch v5 08/16] smt: Create cpu_smt_enabled static key for SMT specific code

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Date:   Mon, 19 Nov 2018 13:58:15 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
cc:     Jiri Kosina <jikos@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Casey Schaufler <casey.schaufler@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Jon Masters <jcm@...hat.com>,
        Waiman Long <longman9394@...il.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [Patch v5 08/16] smt: Create cpu_smt_enabled static key for SMT
 specific code

On Fri, 16 Nov 2018, Tim Chen wrote:
> In later code, STIBP will be turned on/off in the context switch code
> path when SMT is enabled.  Checks for SMT is best
> avoided on such hot paths.
> 
> Create cpu_smt_enabled static key to turn on such SMT specific code
> statically.
> 
> This key is set in code under hot plug, so it is limited in
> scope to architecture supporting CPU hotplug, like x86.

The key is enabled by default and its scope has nothing to do with CPU
hotplug. It depends on CONFIG_HOTPLUG_SMT which is the extra SMT control
code which is currently only enabled on x86.

> diff --git a/include/linux/cpu.h b/include/linux/cpu.h
> index 218df7f..b54f085 100644
> --- a/include/linux/cpu.h
> +++ b/include/linux/cpu.h
> @@ -188,5 +188,6 @@ static inline void cpu_smt_disable(bool force) { }
>  static inline void cpu_smt_check_topology_early(void) { }
>  static inline void cpu_smt_check_topology(void) { }
>  #endif
> +DECLARE_STATIC_KEY_TRUE(cpu_smt_enabled);

And here it is declared unconditionally which allows code to use it despite
CONFIG_HOTPLUG_SMT=n and subsequently breaks the build.
  
Thanks,

	tglx