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

Commit-ID: 2066f00e5b2dc061fb6d8c88fadaebc97f11feaa Gitweb: https://git.kernel.org/tip/2066f00e5b2dc061fb6d8c88fadaebc97f11feaa Author: Thomas Gleixner tglx@linutronix.de AuthorDate: Sun, 21 Sep 2025 10:56:40 +02:00 Committer: Borislav Petkov (AMD) bp@alien8.de CommitterDate: Mon, 22 Sep 2025 21:25:36 +02:00

x86/topology: Implement topology_is_core_online() to address SMT regression

Christian reported that commit a430c11f4015 ("intel_idle: Rescan "dead" SMT siblings during initialization") broke the use case in which both 'nosmt' and 'maxcpus' are on the kernel command line because it onlines primary threads, which were offline due to the maxcpus limit.

The initially proposed fix to skip primary threads in the loop is inconsistent. While it prevents the primary thread to be onlined, it then onlines the corresponding hyperthread(s), which does not really make sense.

The CPU iterator in cpuhp_smt_enable() contains a check which excludes all threads of a core, when the primary thread is offline. The default implementation is a NOOP and therefore not effective on x86.

Implement topology_is_core_online() on x86 to address this issue. This makes the behaviour consistent between x86 and PowerPC.

Fixes: a430c11f4015 ("intel_idle: Rescan "dead" SMT siblings during initialization") Fixes: f694481b1d31 ("ACPI: processor: Rescan "dead" SMT siblings during initialization") Closes: https://lore.kernel.org/linux-pm/724616a2-6374-4ba3-8ce3-ea9c45e2ae3b@arm.co... Reported-by: Christian Loehle christian.loehle@arm.com Signed-off-by: Thomas Gleixner tglx@linutronix.de Signed-off-by: Borislav Petkov (AMD) bp@alien8.de Reviewed-by: Rafael J. Wysocki (Intel) rafael@kernel.org Tested-by: Christian Loehle christian.loehle@arm.com Cc: stable@vger.kernel.org Link: https://lore.kernel.org/12740505.O9o76ZdvQC@rafael.j.wysocki --- arch/x86/include/asm/topology.h | 10 ++++++++++ arch/x86/kernel/cpu/topology.c | 13 +++++++++++++ 2 files changed, 23 insertions(+)

diff --git a/arch/x86/include/asm/topology.h b/arch/x86/include/asm/topology.h index 6c79ee7..2104189 100644 --- a/arch/x86/include/asm/topology.h +++ b/arch/x86/include/asm/topology.h @@ -231,6 +231,16 @@ static inline bool topology_is_primary_thread(unsigned int cpu) } #define topology_is_primary_thread topology_is_primary_thread

+int topology_get_primary_thread(unsigned int cpu); + +static inline bool topology_is_core_online(unsigned int cpu) +{ + int pcpu = topology_get_primary_thread(cpu); + + return pcpu >= 0 ? cpu_online(pcpu) : false; +} +#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; } diff --git a/arch/x86/kernel/cpu/topology.c b/arch/x86/kernel/cpu/topology.c index e35ccdc..6073a16 100644 --- a/arch/x86/kernel/cpu/topology.c +++ b/arch/x86/kernel/cpu/topology.c @@ -372,6 +372,19 @@ unsigned int topology_unit_count(u32 apicid, enum x86_topology_domains which_uni return topo_unit_count(lvlid, at_level, apic_maps[which_units].map); }

+#ifdef CONFIG_SMP +int topology_get_primary_thread(unsigned int cpu) +{ + u32 apic_id = cpuid_to_apicid[cpu]; + + /* + * Get the core domain level APIC id, which is the primary thread + * and return the CPU number assigned to it. + */ + return topo_lookup_cpuid(topo_apicid(apic_id, TOPO_CORE_DOMAIN)); +} +#endif + #ifdef CONFIG_ACPI_HOTPLUG_CPU /** * topology_hotplug_apic - Handle a physical hotplugged APIC after boot