linux-kernel - Re: [PATCH v4 1/6] x86/microcode: Introduce staging step to reduce late-loading time

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Message-ID: <aKLaIxtuobcwOMb2@intel.com>
Date: Mon, 18 Aug 2025 15:45:39 +0800
From: Chao Gao <chao.gao@...el.com>
To: "Chang S. Bae" <chang.seok.bae@...el.com>
CC: <x86@...nel.org>, <tglx@...utronix.de>, <mingo@...hat.com>,
	<bp@...en8.de>, <dave.hansen@...ux.intel.com>, <colinmitchell@...gle.com>,
	<abusse@...zon.de>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 1/6] x86/microcode: Introduce staging step to reduce
 late-loading time

>+	/*
>+	 * Pre-load the microcode image into a staging device. This
>+	 * process is preemptible and does not require stopping CPUs.
>+	 * Successful staging simplifies the subsequent late-loading
>+	 * process, reducing rendezvous time.
>+	 *
>+	 * Even if the transfer fails, the update will proceed as usual.
>+	 */
>+	if (microcode_ops->use_staging)
>+		microcode_ops->stage_microcode();

->use_staging is not necessary. we can directly check if ->stage_microcode()
is NULL, or even add a stub function and then call ->stage_microcode()
unconditionally here. But since this pattern occurs only once in the series,
I don't have a strong preference.

If my review counts for anything,

Reviewed-by: Chao Gao <chao.gao@...el.com>

>+
> 	atomic_set(&late_cpus_in, num_online_cpus());
> 	atomic_set(&offline_in_nmi, 0);
> 	loops_per_usec = loops_per_jiffy / (TICK_NSEC / 1000);
>diff --git a/arch/x86/kernel/cpu/microcode/internal.h b/arch/x86/kernel/cpu/microcode/internal.h
>index 50a9702ae4e2..adf02ebbf7a3 100644
>--- a/arch/x86/kernel/cpu/microcode/internal.h
>+++ b/arch/x86/kernel/cpu/microcode/internal.h
>@@ -31,10 +31,12 @@ struct microcode_ops {
> 	 * See also the "Synchronization" section in microcode_core.c.
> 	 */
> 	enum ucode_state	(*apply_microcode)(int cpu);
>+	void			(*stage_microcode)(void);
> 	int			(*collect_cpu_info)(int cpu, struct cpu_signature *csig);
> 	void			(*finalize_late_load)(int result);
> 	unsigned int		nmi_safe	: 1,
>-				use_nmi		: 1;
>+				use_nmi		: 1,
>+				use_staging	: 1;
> };
> 
> struct early_load_data {
>-- 
>2.48.1
>