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

[Peter Zijlstra]

On Thu, Aug 10, 2023 at 01:50:17PM -0700, Ashok Raj wrote:
> 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. 

Patch 25 does it for online CPUs, offline CPUs are having a separate
code path:

  microcode_nmi_handler()

vs

  microcode_offline_nmi_handler()