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Date: Tue, 2 Jan 2024 14:53:20 +0000
From: Jonathan Cameron <Jonathan.Cameron@Huawei.com>
To: "Russell King (Oracle)" <linux@armlinux.org.uk>
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Subject: Re: [PATCH RFC v3 17/21] ACPI: add support to register CPUs based
 on the _STA enabled bit
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On Mon, 18 Dec 2023 13:03:32 +0000
"Russell King (Oracle)" <linux@armlinux.org.uk> wrote:

> On Wed, Dec 13, 2023 at 12:50:38PM +0000, Russell King wrote:
> > From: James Morse <james.morse@arm.com>
> > 
> > acpi_processor_get_info() registers all present CPUs. Registering a
> > CPU is what creates the sysfs entries and triggers the udev
> > notifications.
> > 
> > arm64 virtual machines that support 'virtual cpu hotplug' use the
> > enabled bit to indicate whether the CPU can be brought online, as
> > the existing ACPI tables require all hardware to be described and
> > present.
> > 
> > If firmware describes a CPU as present, but disabled, skip the
> > registration. Such CPUs are present, but can't be brought online for
> > whatever reason. (e.g. firmware/hypervisor policy).
> > 
> > Once firmware sets the enabled bit, the CPU can be registered and
> > brought online by user-space. Online CPUs, or CPUs that are missing
> > an _STA method must always be registered.  
> 
> ...
> 
> > @@ -526,6 +552,9 @@ static void acpi_processor_post_eject(struct acpi_device *device)
> >  		acpi_processor_make_not_present(device);
> >  		return;
> >  	}
> > +
> > +	if (cpu_present(pr->id) && !(sta & ACPI_STA_DEVICE_ENABLED))
> > +		arch_unregister_cpu(pr->id);  
> 
> This change isn't described in the commit log, but seems to be the cause
> of the build error identified by the kernel build bot that is fixed
> later in this series. I'm wondering whether this should be in a
> different patch, maybe "ACPI: Check _STA present bit before making CPUs
> not present" ?

Would seem a bit odd to call arch_unregister_cpu() way before the code
is added to call the matching arch_registers_cpu()

Mind you this eject doesn't just apply to those CPUs that are registered
later I think, but instead to all.  So we run into the spec hole that
there is no way to identify initially 'enabled' CPUs that might be disabled
later.

> 
> Or maybe my brain isn't working properly (due to being Covid positive.)
> Any thoughts, Jonathan?

I'll go with a resounding 'not sure' on where this change belongs.
I blame my non existent start of the year hangover.
Hope you have recovered!

Jonathan