linux-kernel - [tip: x86/microcode] x86/cpu/topology: Make primary thread mask available with SMP=n

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Message-ID: <176054714443.709179.14324172763516419832.tip-bot2@tip-bot2>
Date: Wed, 15 Oct 2025 16:52:24 -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>, Tony Luck <tony.luck@...el.com>,
 x86@...nel.org, linux-kernel@...r.kernel.org
Subject: [tip: x86/microcode] x86/cpu/topology: Make primary thread mask
 available with SMP=n

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

Commit-ID:     ed44a5625f304ff14d01acfa086e77b5262a842f
Gitweb:        https://git.kernel.org/tip/ed44a5625f304ff14d01acfa086e77b5262a842f
Author:        Chang S. Bae <chang.seok.bae@...el.com>
AuthorDate:    Sun, 21 Sep 2025 15:48:35 -07:00
Committer:     Borislav Petkov (AMD) <bp@...en8.de>
CommitterDate: Wed, 15 Oct 2025 16:46:11 +02:00

x86/cpu/topology: Make primary thread mask available with SMP=n

cpu_primary_thread_mask is only defined when CONFIG_SMP=y. However, even
in UP kernels there is always exactly one CPU, which can reasonably be
treated as the primary thread.

Historically, topology_is_primary_thread() always returned true with
CONFIG_SMP=n. A recent commit:

  4b455f59945aa ("cpu/SMT: Provide a default topology_is_primary_thread()")

replaced it with a generic implementation with the note:

  "When disabling SMT, the primary thread of the SMT will remain
   enabled/active. Architectures that have a special primary thread (e.g.
   x86) need to override this function. ..."

For consistency and clarity, make the primary thread mask available
regardless of SMP, similar to cpu_possible_mask and cpu_present_mask.

Move __cpu_primary_thread_mask into common code to prevent build issues.
Let cpu_mark_primary_thread() configure the mask even for UP kernels,
alongside other masks. Then, topology_is_primary_thread() can
consistently reference it.

Signed-off-by: Chang S. Bae <chang.seok.bae@...el.com>
Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
Reviewed-by: Tony Luck <tony.luck@...el.com>
Link: https://lore.kernel.org/r/20250320234104.8288-1-chang.seok.bae@intel.com
---
 arch/x86/include/asm/topology.h       | 12 ++++++------
 arch/x86/kernel/cpu/topology.c        |  4 ----
 arch/x86/kernel/cpu/topology_common.c |  3 +++
 arch/x86/kernel/smpboot.c             |  3 ---
 4 files changed, 9 insertions(+), 13 deletions(-)

diff --git a/arch/x86/include/asm/topology.h b/arch/x86/include/asm/topology.h
index 2104189..8c6354f 100644
--- a/arch/x86/include/asm/topology.h
+++ b/arch/x86/include/asm/topology.h
@@ -218,6 +218,12 @@ static inline unsigned int topology_amd_nodes_per_pkg(void)
 	return __amd_nodes_per_pkg;
 }
 
+#else /* CONFIG_SMP */
+static inline int topology_phys_to_logical_pkg(unsigned int pkg) { return 0; }
+static inline int topology_max_smt_threads(void) { return 1; }
+static inline unsigned int topology_amd_nodes_per_pkg(void) { return 1; }
+#endif /* !CONFIG_SMP */
+
 extern struct cpumask __cpu_primary_thread_mask;
 #define cpu_primary_thread_mask ((const struct cpumask *)&__cpu_primary_thread_mask)
 
@@ -241,12 +247,6 @@ static inline bool topology_is_core_online(unsigned int cpu)
 }
 #define topology_is_core_online topology_is_core_online
 
-#else /* CONFIG_SMP */
-static inline int topology_phys_to_logical_pkg(unsigned int pkg) { return 0; }
-static inline int topology_max_smt_threads(void) { return 1; }
-static inline unsigned int topology_amd_nodes_per_pkg(void) { return 1; }
-#endif /* !CONFIG_SMP */
-
 static inline void arch_fix_phys_package_id(int num, u32 slot)
 {
 }
diff --git a/arch/x86/kernel/cpu/topology.c b/arch/x86/kernel/cpu/topology.c
index 6073a16..f55ea3c 100644
--- a/arch/x86/kernel/cpu/topology.c
+++ b/arch/x86/kernel/cpu/topology.c
@@ -75,15 +75,11 @@ bool arch_match_cpu_phys_id(int cpu, u64 phys_id)
 	return phys_id == (u64)cpuid_to_apicid[cpu];
 }
 
-#ifdef CONFIG_SMP
 static void cpu_mark_primary_thread(unsigned int cpu, unsigned int apicid)
 {
 	if (!(apicid & (__max_threads_per_core - 1)))
 		cpumask_set_cpu(cpu, &__cpu_primary_thread_mask);
 }
-#else
-static inline void cpu_mark_primary_thread(unsigned int cpu, unsigned int apicid) { }
-#endif
 
 /*
  * Convert the APIC ID to a domain level ID by masking out the low bits
diff --git a/arch/x86/kernel/cpu/topology_common.c b/arch/x86/kernel/cpu/topology_common.c
index b5a5e14..7162579 100644
--- a/arch/x86/kernel/cpu/topology_common.c
+++ b/arch/x86/kernel/cpu/topology_common.c
@@ -16,6 +16,9 @@ EXPORT_SYMBOL_GPL(x86_topo_system);
 unsigned int __amd_nodes_per_pkg __ro_after_init;
 EXPORT_SYMBOL_GPL(__amd_nodes_per_pkg);
 
+/* CPUs which are the primary SMT threads */
+struct cpumask __cpu_primary_thread_mask __read_mostly;
+
 void topology_set_dom(struct topo_scan *tscan, enum x86_topology_domains dom,
 		      unsigned int shift, unsigned int ncpus)
 {
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index eb289ab..6b43417 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -103,9 +103,6 @@ EXPORT_PER_CPU_SYMBOL(cpu_core_map);
 DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_die_map);
 EXPORT_PER_CPU_SYMBOL(cpu_die_map);
 
-/* CPUs which are the primary SMT threads */
-struct cpumask __cpu_primary_thread_mask __read_mostly;
-
 /* Representing CPUs for which sibling maps can be computed */
 static cpumask_var_t cpu_sibling_setup_mask;