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 20:03:02 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
cc:     Peter Zijlstra <peterz@...radead.org>,
        Jiri Kosina <jikos@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Ingo Molnar <mingo@...hat.com>,
        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

Tim,

On Mon, 19 Nov 2018, Tim Chen wrote:
> On 11/19/2018 06:57 AM, Peter Zijlstra wrote:
> > In particular, the SMT topology bits are set before we enable
> > interrrupts and similarly, are cleared after we disable interrupts for
> > the last time and die.
> 
> 
> Peter & Thomas,
> 
> Any objection if I export sched_smt_present after including
> Peter's patch and use it in spec_ctrl_update_msr instead.
> 
> Something like this?

>  
> +#ifdef CONFIG_SCHED_SMT
> +
> +extern struct static_key_false sched_smt_present;
> +
> +static inline bool cpu_smt_present(void)
> +{
> +	if (static_branch_unlikely(&sched_smt_present))
> +		return true;
> +	else
> +		return false;

What's wrong with

       return static_branch_unlikely(&sched_smt_present);

???

But that's just a stylistic nitpick. The real issue is that you prevent the
mitigation when CONFIG_SCHED_SMT=n.

Thanks,

	tglx