linux-kernel - Re: [patch 28/30] x86/microcode: Handle "offline" CPUs correctly

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Date:   Thu, 10 Aug 2023 13:50:17 -0700
From:   Ashok Raj <ashok.raj@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Ashok Raj <ashok.raj@...el.com>
Subject: Re: [patch 28/30] x86/microcode: Handle "offline" CPUs correctly

On Thu, Aug 10, 2023 at 10:46:05PM +0200, Peter Zijlstra wrote:
> 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.

Patch 25 does that load in NMI. You are right, we just need "a" CPU in each
core online. 
> 
> >  				return false;
> >  			}
> > +			cpumask_set_cpu(cpu, &cpu_offline_mask);
> > +			per_cpu(ucode_ctrl, cpu) = ctrl;
> > +			continue;
> >  		}