linux-kernel - Re: [PATCH v4 4/4] intel_idle: Add ibrs_off module parameter to force disable IBRS

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Date:   Mon, 3 Jul 2023 12:32:55 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Len Brown <lenb@...nel.org>, Jonathan Corbet <corbet@....net>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        x86@...nel.org, linux-pm@...r.kernel.org,
        Robin Jarry <rjarry@...hat.com>, Joe Mario <jmario@...hat.com>
Subject: Re: [PATCH v4 4/4] intel_idle: Add ibrs_off module parameter to
 force disable IBRS

On Tue, Jun 27, 2023 at 10:25:54PM -0400, Waiman Long wrote:

> @@ -69,6 +69,7 @@ static int max_cstate = CPUIDLE_STATE_MAX - 1;
>  static unsigned int disabled_states_mask __read_mostly;
>  static unsigned int preferred_states_mask __read_mostly;
>  static bool force_irq_on __read_mostly;
> +static bool ibrs_off __read_mostly;
>  
>  static struct cpuidle_device __percpu *intel_idle_cpuidle_devices;
>  
> @@ -1919,12 +1920,15 @@ static void state_update_enter_method(struct cpuidle_state *state, int cstate)
>  	}
>  
>  	if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS) &&
> -			   state->flags & CPUIDLE_FLAG_IBRS) {
> +			((state->flags & CPUIDLE_FLAG_IBRS) || ibrs_off)) {
>  		/*
>  		 * IBRS mitigation requires that C-states are entered
>  		 * with interrupts disabled.
>  		 */
> -		WARN_ON_ONCE(state->flags & CPUIDLE_FLAG_IRQ_ENABLE);
> +		if (ibrs_off && (state->flags & CPUIDLE_FLAG_IRQ_ENABLE))
> +			state->flags &= ~CPUIDLE_FLAG_IRQ_ENABLE;
> +		else
> +			WARN_ON_ONCE(state->flags & CPUIDLE_FLAG_IRQ_ENABLE);

If you're respinning this, you can leave out the else and avoid the
indent on the WARN:

+		if (ibrs_off && (state->flags & CPUIDLE_FLAG_IRQ_ENABLE))
+			state->flags &= ~CPUIDLE_FLAG_IRQ_ENABLE;
		WARN_ON_ONCE(state->flags & CPUIDLE_FLAG_IRQ_ENABLE);

Same effect, simpler code and all that.

>  		state->enter = intel_idle_ibrs;
>  		return;
>  	}