linux-kernel - Re: [PATCH v3 3/5] x86/mm: Optionally flush L1D on context switch

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Message-ID: <87lfed9sje.fsf@nanos.tec.linutronix.de>
Date:   Fri, 04 Dec 2020 22:21:09 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Balbir Singh <sblbir@...zon.com>, mingo@...hat.com
Cc:     peterz@...radead.org, linux-kernel@...r.kernel.org,
        keescook@...omium.org, jpoimboe@...hat.com, tony.luck@...el.com,
        benh@...nel.crashing.org, x86@...nel.org, dave.hansen@...el.com,
        thomas.lendacky@....com, torvalds@...ux-foundation.org,
        Balbir Singh <sblbir@...zon.com>
Subject: Re: [PATCH v3 3/5] x86/mm: Optionally flush L1D on context switch

On Fri, Nov 27 2020 at 17:59, Balbir Singh wrote:
>  
> +	/*
> +	 * Flush only if SMT is disabled as per the contract, which is checked
> +	 * when the feature is enabled.
> +	 */
> +	if (sched_smt_active() && !this_cpu_read(cpu_info.smt_active) &&
> +		(prev_mm & LAST_USER_MM_L1D_FLUSH))
> +		l1d_flush_hw();

So if SMT is completely disabled then no flush? Shouldn't the logic be:

    if ((!sched_smt_active() || !this_cpu_read(cpu_info.smt_active) &&
         (prev_mm & LAST_USER_MM_L1D_FLUSH))

Hmm?

But that's bad, because it's lot's of conditions to evaluate for every
switch_mm where most of them are not interested in it at all.

Let me read through the rest of the pile.

Thanks,

        tglx