Re: [RFC PATCH v4 5/8] x86/smp native_play_dead: Prefer cpuidle_play_dead() over mwait_play_dead()

[Rafael J. Wysocki]

On Mon, Nov 25, 2024 at 2:21 PM Patryk Wlazlyn
<patryk.wlazlyn@xxxxxxxxxxxxxxx> wrote:
>
> The algorithm based on cpuid leaf 0x5 in the mwait_play_dead(),
> for looking up the mwait hint for the deepest cstate may calculate the
> wrong hint on recent Intel x86 platforms.
>
> Prefer cpuidle_play_dead() over the generic mwait_play_dead() loop and
> only fallback to the later in case of missing appropriate enter_dead()
> handler.
>
> Signed-off-by: Patryk Wlazlyn <patryk.wlazlyn@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/kernel/smpboot.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index d0464c7a0af5..2627b56fb9bc 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1413,9 +1413,10 @@ void native_play_dead(void)
>         play_dead_common();
>         tboot_shutdown(TB_SHUTDOWN_WFS);
>
> +       /* The first successful play_dead() will not return */
> +       cpuidle_play_dead();
>         mwait_play_dead();
> -       if (cpuidle_play_dead())
> -               hlt_play_dead();
> +       hlt_play_dead();
>  }
>
>  #else /* ... !CONFIG_HOTPLUG_CPU */
> --

If you first make intel_idle provide :enter_dead() for all CPUs on all
platforms and implement it by calling mwait_play_dead_with_hint(), you
won't need mwait_play_dead() any more.