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

From: Peter Zijlstra
Date: Thu Aug 10 2023 - 18:30:38 EST


On Thu, Aug 10, 2023 at 02:49:39PM -0700, Ashok Raj wrote:
> On Thu, Aug 10, 2023 at 11:05:11PM +0200, Peter Zijlstra wrote:
> > 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()
>
> Since the code enforces all primary CPUs to be ONLINE, the secondaries are the
> other thread of the same core. So they automatically get the update when
> primary does it.
>
> The secondaries are parked in NMI just to avoid the risk of executing code
> that might be patched by primary.
>
> Or maybe you had something else in mind.

Yeah, not placing constraints on who is online at all. Also, if both
siblings are offline, then onlining will re-load ucode anyway, no?