linux-kernel - Re: [PATCH v2 2/5] x86/idle: Disable IBRS when cpu is offline

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Date:   Wed, 21 Jun 2023 16:54:36 +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>, 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 v2 2/5] x86/idle: Disable IBRS when cpu is offline

On Wed, Jun 21, 2023 at 10:51:33AM -0400, Waiman Long wrote:
> 
> On 6/21/23 10:48, Peter Zijlstra wrote:
> > On Wed, Jun 21, 2023 at 10:44:23AM -0400, Waiman Long wrote:
> > 
> > > Well, hlt_play_dead() is only called if cpuidle_play_dead() returns an error
> > > which is not the typical case. My testing does confirm that this patch is
> > > able to keep the IBRS bit off when a CPU is offline via its online sysfs
> > > file.
> > The point is; your re-enable IBRS hunk at the end is dead-code. It
> > should never ever run and having it is confusing.
> 
> What I meant is that hlt_play_dead() should never be called unless there is
> some serious problem with the system and native_play_dead() does return in
> normal usage.

This is all through arch_cpu_idle_dead() which is __noreturn. And no,
none of this must ever return.

If you want an offline CPU to come back, you re-init.