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Date:   Fri, 18 Oct 2019 09:12:06 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Zhenzhong Duan <zhenzhong.duan@oracle.com>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        tim.c.chen@linux.intel.com, Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>
Subject: Re: [PATCH] x86: Don't use MWAIT if explicitly requested
Message-ID: <20191018071206.GZ2328@hirez.programming.kicks-ass.net>
References: <1571370354-17736-1-git-send-email-zhenzhong.duan@oracle.com>
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On Fri, Oct 18, 2019 at 11:45:54AM +0800, Zhenzhong Duan wrote:
> If 'idle=nomwait' is specified or process matching what's in
> processor_idle_dmi_table, we should't use MWAIT at bootup stage before
> cpuidle driver loaded, even if it's preferred by default on Intel.
> 
> Add a check so that HALT instruction is used in those cases.

The comment in idle_setup():

	/*
	 * If the boot option of "idle=nomwait" is added,
	 * it means that mwait will be disabled for CPU C2/C3
	 * states. In such case it won't touch the variable
	 * of boot_option_idle_override.
	 */
	boot_option_idle_override = IDLE_NOMWAIT;

explicitly states this option is for C2+

> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> ---
>  arch/x86/kernel/process.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index 5e94c43..37fc577 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -667,6 +667,10 @@ static void amd_e400_idle(void)
>   */
>  static int prefer_mwait_c1_over_halt(const struct cpuinfo_x86 *c)
>  {
> +	/* Don't use MWAIT-C1 if explicitly requested */
> +	if (boot_option_idle_override == IDLE_NOMWAIT)
> +		return 0;

And this is very much about C1...

OTOH, "idle=halt" should be forcing HLT over MWAIT, so did you want to
write:

	if (boot_option_idle_override == IDLE_HALT)
		return 0;

instead?