linux-kernel - [tip: x86/microcode] x86/microcode: Introduce staging step to reduce late-loading time

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Message-ID: <176054714292.709179.17479072459906092168.tip-bot2@tip-bot2>
Date: Wed, 15 Oct 2025 16:52:22 -0000
From: "tip-bot2 for Chang S. Bae" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: "Chang S. Bae" <chang.seok.bae@...el.com>,
 "Borislav Petkov (AMD)" <bp@...en8.de>, Chao Gao <chao.gao@...el.com>,
 Tony Luck <tony.luck@...el.com>, Anselm Busse <abusse@...zon.de>,
 x86@...nel.org, linux-kernel@...r.kernel.org
Subject: [tip: x86/microcode] x86/microcode: Introduce staging step to reduce
 late-loading time

The following commit has been merged into the x86/microcode branch of tip:

Commit-ID:     7cdda85ed90c1abaccaf5f05ec217acbc102c164
Gitweb:        https://git.kernel.org/tip/7cdda85ed90c1abaccaf5f05ec217acbc102c164
Author:        Chang S. Bae <chang.seok.bae@...el.com>
AuthorDate:    Sun, 21 Sep 2025 15:48:36 -07:00
Committer:     Borislav Petkov (AMD) <bp@...en8.de>
CommitterDate: Wed, 15 Oct 2025 16:46:58 +02:00

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@...el.com>
Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
Reviewed-by: Chao Gao <chao.gao@...el.com>
Reviewed-by: Tony Luck <tony.luck@...el.com>
Tested-by: Anselm Busse <abusse@...zon.de>
Link: https://lore.kernel.org/20250320234104.8288-1-chang.seok.bae@intel.com
---
 arch/x86/kernel/cpu/microcode/core.c     | 11 +++++++++++
 arch/x86/kernel/cpu/microcode/internal.h |  4 +++-
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
index f75c140..d7baec8 100644
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -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);
diff --git a/arch/x86/kernel/cpu/microcode/internal.h b/arch/x86/kernel/cpu/microcode/internal.h
index ae8dbc2..a10b547 100644
--- a/arch/x86/kernel/cpu/microcode/internal.h
+++ b/arch/x86/kernel/cpu/microcode/internal.h
@@ -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 {