Re: [Xen-devel] [PATCH v7 09/10] microcode: remove microcode_update_lock

["Raj, Ashok" <ashok.raj@xxxxxxxxx>]

On Tue, Jun 11, 2019 at 08:46:04PM +0800, Chao Gao wrote:
> On Wed, Jun 05, 2019 at 08:53:46AM -0600, Jan Beulich wrote:
> >
> >> @@ -307,8 +303,7 @@ static int apply_microcode(const struct 
> >> microcode_patch *patch)
> >>  
> >>      mc_intel = patch->mc_intel;
> >>  
> >> -    /* serialize access to the physical write to MSR 0x79 */
> >> -    spin_lock_irqsave(&microcode_update_lock, flags);
> >> +    BUG_ON(local_irq_is_enabled());
> >>  
> >>      /*
> >>       * Writeback and invalidate caches before updating microcode to avoid
> >
> >Thinking about it - what happens if we hit an NMI or #MC here?
> >watchdog_disable(), a call to which you add in an earlier patch,
> >doesn't really suppress the generation of NMIs, it only tells the
> >handler not to look at the accumulated statistics.
> 
> I think they should be suppressed. Ashok, could you confirm it?

I think the only sources would be the watchdog as you pointed out
which we already touch to keep it from expiring. The perf counters
i'm not an expert in, but i'll check. When we are in stop_machine() type
flow, its not clear if any of those would fire. (I might be wrong, but let
me check).

#MC shouldn't fire once you entered the rendezvous, if it does its usually
fatal like a 3strike or something. Machine is going down at that time
so nothing we could do to handle.

Cheers,
Ashok

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