x86/microcode: Introduce staging step to reduce late-loading time

x86/microcode: Introduce staging step to reduce late-loading time

As microcode patch sizes continue to grow, late-loading latency spikes can
lead to timeouts and disruptions in running workloads. This trend of
increasing patch sizes is expected to continue, so a foundational solution is
needed to address the issue.

To mitigate the problem, introduce a microcode staging feature. This option
processes most of the microcode update (excluding activation) on
a non-critical path, allowing CPUs to remain operational during the majority
of the update. By offloading work from the critical path, staging can
significantly reduce latency spikes.

Integrate staging as a preparatory step in late-loading. Introduce a new
callback for staging, which is invoked at the beginning of
load_late_stop_cpus(), before CPUs enter the rendezvous phase.

Staging follows an opportunistic model:

  *  If successful, it reduces CPU rendezvous time
  *  Even though it fails, the process falls back to the legacy path to
     finish the loading process but with potentially higher latency.

Extend struct microcode_ops to incorporate staging properties, which will be
implemented in the vendor code separately.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Tested-by: Anselm Busse <abusse@amazon.de>
Link: https://lore.kernel.org/20250320234104.8288-1-chang.seok.bae@intel.com