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

[Peter Zijlstra]

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.