Commit 7cdda85e authored by Chang S. Bae's avatar Chang S. Bae Committed by Borislav Petkov (AMD)
Browse files

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: default avatarChang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: default avatarBorislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: default avatarChao Gao <chao.gao@intel.com>
Reviewed-by: default avatarTony Luck <tony.luck@intel.com>
Tested-by: default avatarAnselm Busse <abusse@amazon.de>
Link: https://lore.kernel.org/20250320234104.8288-1-chang.seok.bae@intel.com
parent ed44a562

arch/x86/kernel/cpu/microcode/core.c

+11 −0
Original line number Diff line number Diff line
@@ -589,6 +589,17 @@ static int load_late_stop_cpus(bool is_safe) 
		pr_err("You should switch to early loading, if possible.\n");

	}



	/*

	 * Pre-load the microcode image into a staging device. This

	 * process is preemptible and does not require stopping CPUs.

	 * Successful staging simplifies the subsequent late-loading

	 * process, reducing rendezvous time.

	 *

	 * Even if the transfer fails, the update will proceed as usual.

	 */

	if (microcode_ops->use_staging)

		microcode_ops->stage_microcode();



	atomic_set(&late_cpus_in, num_online_cpus());

	atomic_set(&offline_in_nmi, 0);

	loops_per_usec = loops_per_jiffy / (TICK_NSEC / 1000);

arch/x86/kernel/cpu/microcode/internal.h

+3 −1
Original line number Diff line number Diff line
@@ -31,10 +31,12 @@ struct microcode_ops { 
	 * See also the "Synchronization" section in microcode_core.c.

	 */

	enum ucode_state	(*apply_microcode)(int cpu);

	void			(*stage_microcode)(void);

	int			(*collect_cpu_info)(int cpu, struct cpu_signature *csig);

	void			(*finalize_late_load)(int result);

	unsigned int		nmi_safe	: 1,

				use_nmi		: 1;

				use_nmi		: 1,

				use_staging	: 1;

};



struct early_load_data {