linux-kernel - Re: [PATCH v3 1/3] x86/idle: Disable IBRS when cpu is offline

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Date:   Wed, 21 Jun 2023 22:40:53 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Len Brown <lenb@...nel.org>, linux-kernel@...r.kernel.org,
        x86@...nel.org, linux-pm@...r.kernel.org,
        Robin Jarry <rjarry@...hat.com>, Joe Mario <jmario@...hat.com>
Subject: Re: [PATCH v3 1/3] x86/idle: Disable IBRS when cpu is offline

On Wed, Jun 21, 2023 at 08:36:01PM -0400, Waiman Long wrote:
> +/*
> + * naitve_play_dead() is essentially a __noreturn function, but it can't
> + * be marked as such as the compiler may complain about it.
> + */

FWIW, we could in theory do so by marking the smp_ops.play_dead function
pointer as __noreturn, but it would be tricky to teach objtool how to
understand that.

>  void native_play_dead(void)
>  {
> +	if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS)) {
> +		this_cpu_write(x86_spec_ctrl_current, 0);
> +		native_wrmsrl(MSR_IA32_SPEC_CTRL, 0);
> +	}

Can update_spec_ctrl() be used instead?

-- 
Josh