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Date: Tue, 23 Jan 2024 18:19:56 +0000
From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Jonathan Cameron <Jonathan.Cameron@huawei.com>,
	linux-pm@vger.kernel.org, loongarch@lists.linux.dev,
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	jianyong.wu@arm.com, justin.he@arm.com,
	James Morse <james.morse@arm.com>
Subject: Re: [PATCH RFC v3 05/21] ACPI: Rename ACPI_HOTPLUG_CPU to include
 'present'
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Sender: Russell King (Oracle) <linux@armlinux.org.uk>

On Tue, Jan 23, 2024 at 06:43:59PM +0100, Rafael J. Wysocki wrote:
> On Tue, Jan 23, 2024 at 5:36 PM Russell King (Oracle)
> <linux@armlinux.org.uk> wrote:
> >
> > On Tue, Jan 23, 2024 at 05:15:54PM +0100, Rafael J. Wysocki wrote:
> > > On Tue, Jan 23, 2024 at 2:28 PM Russell King (Oracle)
> > > <linux@armlinux.org.uk> wrote:
> > > >
> > > > On Mon, Jan 22, 2024 at 06:00:13PM +0000, Jonathan Cameron wrote:
> > > > > On Mon, 18 Dec 2023 21:35:16 +0100
> > > > > "Rafael J. Wysocki" <rafael@kernel.org> wrote:
> > > > >
> > > > > > On Wed, Dec 13, 2023 at 1:49 PM Russell King <rmk+kernel@armlinux.org.uk> wrote:
> > > > > > >
> > > > > > > From: James Morse <james.morse@arm.com>
> > > > > > >
> > > > > > > The code behind ACPI_HOTPLUG_CPU allows a not-present CPU to become
> > > > > > > present.
> > > > > >
> > > > > > Right.
> > > > > >
> > > > > > > This isn't the only use of HOTPLUG_CPU. On arm64 and riscv
> > > > > > > CPUs can be taken offline as a power saving measure.
> > > > > >
> > > > > > But still there is the case in which a non-present CPU can become
> > > > > > present, isn't it there?
> > > > >
> > > > > Not yet defined by the architectures (and I'm assuming it probably never will be).
> > > > >
> > > > > The original proposal we took to ARM was to do exactly that - they pushed
> > > > > back hard on the basis there was no architecturally safe way to implement it.
> > > > > Too much of the ARM arch has to exist from the start of time.
> > > > >
> > > > > https://lore.kernel.org/linux-arm-kernel/cbaa6d68-6143-e010-5f3c-ec62f879ad95@arm.com/
> > > > > is one of the relevant threads of the kernel side of that discussion.
> > > > >
> > > > > Not to put specific words into the ARM architects mouths, but the
> > > > > short description is that there is currently no demand for working
> > > > > out how to make physical CPU hotplug possible, as such they will not
> > > > > provide an architecturally compliant way to do it for virtual CPU hotplug and
> > > > > another means is needed (which is why this series doesn't use the present bit
> > > > > for that purpose and we have the Online capable bit in MADT/GICC)
> > > > >
> > > > > It was a 'fun' dance of several years to get to that clarification.
> > > > > As another fun fact, the same is defined for x86, but I don't think
> > > > > anyone has used it yet (GICC for ARM has an online capable bit in the flags to
> > > > > enable this, which was remarkably similar to the online capable bit in the
> > > > > flags of the Local APIC entries as added fairly recently).
> > > > >
> > > > > >
> > > > > > > On arm64 an offline CPU may be disabled by firmware, preventing it from
> > > > > > > being brought back online, but it remains present throughout.
> > > > > > >
> > > > > > > Adding code to prevent user-space trying to online these disabled CPUs
> > > > > > > needs some additional terminology.
> > > > > > >
> > > > > > > Rename the Kconfig symbol CONFIG_ACPI_HOTPLUG_PRESENT_CPU to reflect
> > > > > > > that it makes possible CPUs present.
> > > > > >
> > > > > > Honestly, I don't think that this change is necessary or even useful.
> > > > >
> > > > > Whilst it's an attempt to avoid future confusion, the rename is
> > > > > not something I really care about so my advice to Russell is drop
> > > > > it unless you are attached to it!
> > > >
> > > > While I agree that it isn't a necessity, I don't fully agree that it
> > > > isn't useful.
> > > >
> > > > One of the issues will be that while Arm64 will support hotplug vCPU,
> > > > it won't be setting ACPI_HOTPLUG_CPU because it doesn't support
> > > > the present bit changing. So I can see why James decided to rename
> > > > it - because with Arm64's hotplug vCPU, the idea that ACPI_HOTPLUG_CPU
> > > > somehow enables hotplug CPU support is now no longer true.
> > > >
> > > > Keeping it as ACPI_HOTPLUG_CPU makes the code less obvious, because it
> > > > leads one to assume that it ought to be enabled for Arm64's
> > > > implementatinon, and that could well cause issues in the future if
> > > > people make the assumption that "ACPI_HOTPLUG_CPU" means hotplug CPU
> > > > is supported in ACPI. It doesn't anymore.
> > >
> > > On x86 there is no confusion AFAICS.  It's always meant "as long as
> > > the platform supports it".
> >
> > That's x86, which supports physical CPU hotplug. We're introducing
> > support for Arm64 here which doesn't support physical CPU hotplug.
> >
> >                                                 ACPI-based      Physical        Virtual
> > Arch    HOTPLUG_CPU     ACPI_HOTPLUG_CPU        Hotplug         Hotplug         Hotplug
> > Arm64   Y               N                       Y               N               Y
> > x86     Y               Y                       Y               Y               Y
> >
> > So ACPI_HOTPLUG_CPU becomes totally misnamed with the introduction
> > of hotplug on Arm64.
> >
> > If we want to just look at stuff from an x86 perspective, then yes,
> > it remains correct to call it ACPI_HOTPLUG_CPU. It isn't correct as
> > soon as we add Arm64, as I already said.
> 
> And if you rename it, it becomes less confusing for ARM64, but more
> confusing for x86, which basically is my point.
> 
> IMO "hotplug" covers both cases well enough and "hotplug present" is
> only accurate for one of them.
> 
> > And honestly, a two line quip to my reasoned argument is not IMHO
> > an acceptable reply.
> 
> Well, I'm not even sure how to respond to this ...

The above explanation you give would have been useful...

I don't see how "hotplug" covers both cases. As I've tried to point
out many times now, ACPI_HOTPLUG_CPU is N for Arm64, yet it supports
ACPI based hotplug. How does ACPI_HOTPLUG_CPU cover Arm64 if it's
N there? IMHO it totally doesn't, and moreover, it goes against what
one would logically expect - and this is why I have a problem with
your effective NAK for this change. I believe you are basically
wrong on this for the reasons I've given - that ACPI_HOTPLUG_CPU
will be N for Arm64 despite it supporting ACPI-based CPU hotplug.

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