RE: [PATCH v6 13/16] arm64: arch_register_cpu() variant to check if an ACPI handle is now available.

Salil Mehta
2024-04-17T17:03:40Z

> From: Jonathan Cameron <jonathan.cameron@huawei.com>
> Sent: Wednesday, April 17, 2024 5:55 PM
>
> On Wed, 17 Apr 2024 17:33:02 +0100
> Salil Mehta <salil.mehta@huawei.com> wrote:
>
> > Hi Jonathan,
> >
> > > From: Jonathan Cameron <jonathan.cameron@huawei.com>
> > > Sent: Wednesday, April 17, 2024 2:19 PM
> > >
> > > The ARM64 architecture does not support physical CPU HP today.
> > > To avoid any possibility of a bug against such an architecture if
> > > defined in future, check for the physical CPU HP case (not present)
> > > and return an error on any such attempt.
> > >
> > > On ARM64 virtual CPU Hotplug relies on the status value that can be
> > > queried via the AML method _STA for the CPU object.
> > >
> > > There are two conditions in which the CPU can be registered.
> > > 1) ACPI disabled.
> > > 2) ACPI enabled and the acpi_handle is available.
> > > _STA evaluates to the CPU is both enabled and present.
> > > (Note that in absence of the _STA method they are always in this
> > > state).
> > >
> > > If neither of these conditions is met the CPU is not 'yet' ready to
> > > be used and -EPROBE_DEFER is returned.
> > >
> > > Success occurs in the early attempt to register the CPUs if we are
> > > booting with DT (no concept yet of vCPU HP) if not it succeeds for
> > > already enabled CPUs when the ACPI Processor driver attaches to
> > > them. Finally it may succeed via the CPU Hotplug code indicating that
> the CPU is now enabled.
> > >
> > > For ACPI if CONFIG_ACPI_PROCESSOR the only path to get to
> > > arch_register_cpu() with that handle set is via
> > > acpi_processor_hot_add_init() which is only called from an ACPI bus
> > > scan in which _STA has already been queried there is no need to repeat
> it here.
> > > Add a comment to remind us of this in the future.
> > >
> > > Suggested-by: Rafael J. Wysocki <rafael@kernel.org>
> > > Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> > > ---
> > > v6: Add protection again Physical CPU HP to the arch specific code
> > > and don't actually check _STA
> > >
> > > Tested on arm64 with ACPI + DT build and DT only builds, booting
> > > with ACPI and DT as appropriate.
> > > ---
> > > arch/arm64/kernel/smp.c | 53
> > > +++++++++++++++++++++++++++++++++++++++++
> > > 1 file changed, 53 insertions(+)
> > >
> > > diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> > > index
> > > dc0e0b3ec2d4..ccb6ad347df9 100644
> > > --- a/arch/arm64/kernel/smp.c
> > > +++ b/arch/arm64/kernel/smp.c
> > > @@ -504,6 +504,59 @@ static int __init smp_cpu_setup(int cpu)
> > > static bool bootcpu_valid __initdata; static unsigned int
> > > cpu_count = 1;
> > >
> > > +int arch_register_cpu(int cpu)
> > > +{
> > > + acpi_handle acpi_handle = acpi_get_processor_handle(cpu);
> > > + struct cpu *c = &per_cpu(cpu_devices, cpu);
> > > +
> > > + if (!acpi_disabled && !acpi_handle &&
> > > + IS_ENABLED(CONFIG_ACPI_HOTPLUG_CPU))
> > > + return -EPROBE_DEFER;
> > > +
> > > +#ifdef CONFIG_ACPI_HOTPLUG_CPU
> > > + /* For now block anything that looks like physical CPU Hotplug */
> > > + if (invalid_logical_cpuid(cpu) || !cpu_present(cpu)) {
> > > + pr_err_once("Changing CPU present bit is not
> > > supported\n");
> > > + return -ENODEV;
> > > + }
> > > +#endif
> > > +
> > > + /*
> > > + * Availability of the acpi handle is sufficient to establish
> > > + * that _STA has aleady been checked. No need to recheck here.
> > > + */
> > > + c->hotpluggable = arch_cpu_is_hotpluggable(cpu);
> > > +
> >
> >
> > We would still need 'enabled' bitmask as applications need a way to
> > clearly get which processors are enabled and usable in case of ARM64.
> > Otherwise, they will end up scanning the entire MAX CPU space to
> > figure out which processors have been plugged or unplugged. It is
> > inefficient to bank upon errors to detect this and unnecessary to scan
> again and again.
> >
> > + set_cpu_enabled(cpu, true); // will need this change
> >
> >
> > And its corresponding additions of enabled bitmask along side the present
> masks.
> >
> > I think we had this discussion in Linaro Open Discussions group few
> > years back.
>
> Agreed - but if I understand correctly that is handled in patch 16 - which
> introduced the enabled bitmask. I tested that works and it all seems fine.
> Done for all architectures in register_cpu() and unregister_cpu() rather than
> in arch specific code.

Sorry, I missed that. Yes, this logic is already present in later patches.

Thanks
Salil.

>
> Jonathan
>
>
> >
> >
> > > + return register_cpu(c, cpu);
> > > +}
> > > +
> > > +#ifdef CONFIG_ACPI_HOTPLUG_CPU
> > > +void arch_unregister_cpu(int cpu)
> > > +{
> > > + acpi_handle acpi_handle = acpi_get_processor_handle(cpu);
> > > + struct cpu *c = &per_cpu(cpu_devices, cpu);
> > > + acpi_status status;
> > > + unsigned long long sta;
> > > +
> > > + if (!acpi_handle) {
> > > + pr_err_once("Removing a CPU without associated ACPI
> > > handle\n");
> > > + return;
> > > + }
> > > +
> > > + status = acpi_evaluate_integer(acpi_handle, "_STA", NULL, &sta);
> > > + if (ACPI_FAILURE(status))
> > > + return;
> > > +
> > > + /* For now do not allow anything that looks like physical CPU HP */
> > > + if (cpu_present(cpu) && !(sta & ACPI_STA_DEVICE_PRESENT)) {
> > > + pr_err_once("Changing CPU present bit is not
> > > supported\n");
> > > + return;
> > > + }
> > > +
> >
> > For the same reasons as above:
> >
> > + set_cpu_enabled(cpu, flase); // will need this change
> >
> >
> > > + unregister_cpu(c);
> > > +}
> > > +#endif /* CONFIG_ACPI_HOTPLUG_CPU */ +
> > > #ifdef CONFIG_ACPI
> > > static struct acpi_madt_generic_interrupt cpu_madt_gicc[NR_CPUS];
> > >
> > > --
> > > 2.39.2
> >