Re: [PATCH RFC v3 05/21] ACPI: Rename ACPI_HOTPLUG_CPU to include 'present'

Rafael J. Wysocki
2024-01-23T16:16:07Z

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".