[PATCH RFC v1 2/8] x86/hotplug: Implement SMT-primary selection for parallel bringup

[Chang S. Bae]

Parallel CPU bringup executes in two phases to avoid microcode loading
races between SMT siblings. The core code now supports architecture-
specific customization of the first bringup phase. Implement the x86 logic
to preserve the existing behavior.

Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
---
Note:
With this change, the generic SMT-aware two-phase parallel bringup logic
appears redundant since it was originally introduced for the x86
microcode loader path.

But, another architecture, MIPS, also adopted the primary-aware parallel
bringup model in:

    76c43eb507bc ("MIPS: SMP: Implement parallel CPU bring up for EyeQ")

So the generic fallback path still appears to have valid users.
---
 arch/x86/Kconfig                     |  1 +
 arch/x86/kernel/cpu/microcode/core.c | 18 ++++++++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 15fd9ec5ecac..cd6d4dfe3349 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1319,6 +1319,7 @@ config MICROCODE
 	def_bool y
 	depends on CPU_SUP_AMD || CPU_SUP_INTEL
 	select CRYPTO_LIB_SHA256 if CPU_SUP_AMD
+	select HOTPLUG_PARALLEL_ARCH_PRIMARY if HOTPLUG_PARALLEL
 
 config MICROCODE_INITRD32
 	def_bool y
diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
index 0dd0c7241c57..36f08e2c99fb 100644
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -869,6 +869,24 @@ static int mc_cpu_down_prep(unsigned int cpu)
 	return 0;
 }
 
+#ifdef CONFIG_HOTPLUG_PARALLEL_ARCH_PRIMARY
+
+/*
+ * Queried by the parallel bringup code to determine whether x86 provides a
+ * custom primary CPU mask for early loading.
+ */
+bool __init arch_cpuhp_primary_aware(void)
+{
+	return __max_threads_per_core > 1;
+}
+
+const struct cpumask *__init arch_cpuhp_get_primary_cpus(void)
+{
+	return cpu_primary_thread_mask;
+}
+
+#endif /* CONFIG_HOTPLUG_PARALLEL_ARCH_PRIMARY */
+
 static struct attribute *cpu_root_microcode_attrs[] = {
 #ifdef CONFIG_MICROCODE_LATE_LOADING
 	&dev_attr_reload.attr,
-- 
2.53.0