Subject: Re: [PATCH] cpu/hotplug: Fix NULL kobject warning in cpuhp_smt_enable()

From: Catalin Marinas <[email-masked]>
Date: Thu, 23 Apr 2026 21:11:16 +0100

On Thu, Apr 23, 2026 at 08:32:34PM +0800, Jinjie Ruan wrote:
> On 4/23/2026 6:08 PM, Thomas Gleixner wrote:
> > On Sat, Apr 18 2026 at 12:55, Catalin Marinas wrote:
> >> Another option would have been to avoid marking such CPUs present but I
> >> think this will break other things. Yet another option is to register
> >> all CPU devices even if they never come up (like maxcpus greater than
> >> actual CPUs).
> >>
> >> Opinions? It might be an arm64+ACPI-only thing.
> > 
> > I think so. The proper thing to do is to apply sane limits:
> > 
> >  1) The possible CPUs enumerated by firmware N_POSSIBLE_FW
> > 
> >  2) The maxcpus limit on the command line N_MAXCPUS_CL
> > 
> > So the actual possible CPUs evaluates to:
> > 
> >    num_possible = min(N_POSSIBLE_FW, N_MAXCPUS_CL, CONFIG_NR_CPUS);
> > 
> > The evaluation of the firmware should not mark CPUs present which are
> > actually not. ACPI gives you that information. See:
> > 
> >          5.2.12.14 GIC CPU Interface (GICC) Structure
> > 
> > in the ACPI spec. That has two related bits:
> > 
> > Enabled:
> > 
> >    If this bit is set, the processor is ready for use. If this bit is
> >    clear and the Online Capable bit is set, the system supports enabling
> >    this processor during OS runtime. If this bit is clear and the Online
> >    Capable bit is also clear, this processor is un- usable, and the
> >    operating system support will not attempt to use it.
> > 
> > Online Capable:
> > 
> >    The information conveyed by this bit depends on the value of the
> >    Enabled bit. If the Enabled bit is set, this bit is reserved and must
> >    be zero. Otherwise, if this bit is set, the system supports enabling
> >    this processor later during OS runtime
> > 
> > So the combination of those gives you the right answer:
> > 
> >    Enabled	Online
> >    	        Capable
> >    0            0        Not present, not possible
> >    0            1        Not present, but possible to "hotplug" layter
> >    1            0        Present
> >    1            1        Invalid
> 
> On x86, it seems that all CPUs with the ACPI_MADT_ENABLED bit set will
> be marked as present.
> 
> acpi_parse_x2apic()
>   -> enabled = processor->lapic_flags & ACPI_MADT_ENABLED
>   -> topology_register_apic(enabled)
>      -> topo_register_apic(enabled)
>         -> set_cpu_present(cpu, true)

Yes but arm64 marks all CPUs present even if !ACPI_MADT_ENABLED as we
don't have the notion of hardware CPU hotplug.

I need to dig some more into the original vCPU hotplug support and why
we ended up with all CPUs marked as present even if not calling
register_cpu():

https://lore.kernel.org/linux-arm-kernel/[email-masked]/

What's the MADT GICC provided by qemu with "-smp cpus=4,maxcpus=8"? If
it says Enabled for the first 4 and Online Capable for the rest, maybe
we can try something like below:

----------------------8<-----------------
diff --git a/arch/arm64/kernel/acpi.c b/arch/arm64/kernel/acpi.c
index 5891f92c2035..681aa2bbc399 100644
--- a/arch/arm64/kernel/acpi.c
+++ b/arch/arm64/kernel/acpi.c
@@ -448,12 +448,14 @@ int acpi_map_cpu(acpi_handle handle, phys_cpuid_t physid, u32 apci_id,
 		return *pcpu;
 	}
 
+	set_cpu_present(*pcpu, true);
 	return 0;
 }
 EXPORT_SYMBOL(acpi_map_cpu);
 
 int acpi_unmap_cpu(int cpu)
 {
+	set_cpu_present(cpu, false);
 	return 0;
 }
 EXPORT_SYMBOL(acpi_unmap_cpu);
diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index 1aa324104afb..6421027669fc 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -566,6 +566,11 @@ struct acpi_madt_generic_interrupt *acpi_cpu_get_madt_gicc(int cpu)
 }
 EXPORT_SYMBOL_GPL(acpi_cpu_get_madt_gicc);
 
+static bool acpi_cpu_is_present(int cpu)
+{
+	return acpi_cpu_get_madt_gicc(cpu)->flags & ACPI_MADT_ENABLED;
+}
+
 /*
  * acpi_map_gic_cpu_interface - parse processor MADT entry
  *
@@ -670,6 +675,11 @@ static void __init acpi_parse_and_init_cpus(void)
 		early_map_cpu_to_node(i, acpi_numa_get_nid(i));
 }
 #else
+static bool acpi_cpu_is_present(int cpu)
+{
+	return false;
+}
+
 #define acpi_parse_and_init_cpus(...)	do { } while (0)
 #endif
 
@@ -808,7 +818,8 @@ void __init smp_prepare_cpus(unsigned int max_cpus)
 		if (err)
 			continue;
 
-		set_cpu_present(cpu, true);
+		if (acpi_disabled || acpi_cpu_is_present(cpu))
+			set_cpu_present(cpu, true);
 		numa_store_cpu_info(cpu);
 	}
 }