Re: [RFC PATCH v4 8/8] acpi_idle: Disallow play_dead with FFH cstate on AMD platforms

From: Peter Zijlstra
Date: Mon Nov 25 2024 - 08:55:06 EST


On Mon, Nov 25, 2024 at 02:20:28PM +0100, Patryk Wlazlyn wrote:
> Signed-off-by: Patryk Wlazlyn <patryk.wlazlyn@xxxxxxxxxxxxxxx>
> Suggested-by: Gautham R. Shenoy <gautham.shenoy@xxxxxxx>
> ---
> drivers/acpi/processor_idle.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
> index 586cc7d1d8aa..4b4ac8d55b55 100644
> --- a/drivers/acpi/processor_idle.c
> +++ b/drivers/acpi/processor_idle.c
> @@ -803,7 +803,11 @@ static int acpi_processor_setup_cstates(struct acpi_processor *pr)
>
> state->flags = 0;
>
> - state->enter_dead = acpi_idle_play_dead;
> + /* AMD doesn't want to use mwait for play dead. */
> + bool amd_or_hygon = boot_cpu_data.x86_vendor == X86_VENDOR_AMD ||
> + boot_cpu_data.x86_vendor == X86_VENDOR_HYGON;
> + if (!(cx->entry_method == ACPI_CSTATE_FFH && amd_or_hygon))
> + state->enter_dead = acpi_idle_play_dead;

So I don't like this. Less exceptions is better.

This *SHOULD* never trigger on AMD anyway, because they recommend IO
port C[23]. But if their partner BIOS engineer does a wobbly and they
end up in MWAIT anyway, it *should* all work regardless.