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SubjectRe: [PATCH 1/2] x86/microcode: Update late microcode in parallel
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On 8/24/19 4:53 AM, Borislav Petkov wrote:
>
> +wait_for_siblings:
> + if (__wait_for_cpus(&late_cpus_out, NSEC_PER_SEC))
> + panic("Timeout during microcode update!\n");
> +
> /*
> - * Increase the wait timeout to a safe value here since we're
> - * serializing the microcode update and that could take a while on a
> - * large number of CPUs. And that is fine as the *actual* timeout will
> - * be determined by the last CPU finished updating and thus cut short.
> + * At least one thread has completed update on each core.
> + * For others, simply call the update to make sure the
> + * per-cpu cpuinfo can be updated with right microcode
> + * revision.


What is the advantage of having those other threads go through
find_patch() and (in Intel case) intel_get_microcode_revision() (which
involves two MSR accesses) vs. having the master sibling update slaves'
microcode revisions? There are only two things that need to be updated,
uci->cpu_sig.rev and c->microcode.

-boris


> */
> - if (__wait_for_cpus(&late_cpus_out, NSEC_PER_SEC * num_online_cpus()))
> - panic("Timeout during microcode update!\n");
> + if (cpumask_first(topology_sibling_cpumask(cpu)) != cpu)
> + apply_microcode_local(&err);
>
> return ret;
> }

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