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Date:   Thu, 10 Aug 2023 22:46:05 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     LKML <linux-kernel@vger.kernel.org>, x86@kernel.org,
        Borislav Petkov <bp@alien8.de>,
        Ashok Raj <ashok.raj@intel.com>,
        Arjan van de Ven <arjan@linux.intel.com>
Subject: Re: [patch 28/30] x86/microcode: Handle "offline" CPUs correctly
Message-ID: <20230810204605.GF212435@hirez.programming.kicks-ass.net>
References: <20230810153317.850017756@linutronix.de>
 <20230810160806.562016788@linutronix.de>
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On Thu, Aug 10, 2023 at 08:38:07PM +0200, Thomas Gleixner wrote:

>  	for_each_cpu_and(cpu, cpu_present_mask, &cpus_booted_once_mask) {
> +		/*
> +		 * Offline CPUs sit in one of the play_dead() functions
> +		 * with interrupts disabled, but they still react on NMIs
> +		 * and execute arbitrary code. Also MWAIT being updated
> +		 * while the offline CPU sits there is not necessarily safe
> +		 * on all CPU variants.
> +		 *
> +		 * Mark them in the offline_cpus mask which will be handled
> +		 * by CPU0 later in the update process.
> +		 *
> +		 * Ensure that the primary thread is online so that it is
> +		 * guaranteed that all cores are updated.
> +		 */
>  		if (!cpu_online(cpu)) {
> +			if (topology_is_primary_thread(cpu) || !allow_smt_offline) {
> +				pr_err("CPU %u not online, loading aborted\n", cpu);

We could make the NMI handler do the ucode load, no? Also, you just need
any thread online, don't particularly care about primary thread or not
afaict.

>  				return false;
>  			}
> +			cpumask_set_cpu(cpu, &cpu_offline_mask);
> +			per_cpu(ucode_ctrl, cpu) = ctrl;
> +			continue;
>  		}