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,
linux-acpi@vger.kernel.org, linux-arch@vger.kernel.org,
linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.org,
linux-riscv@lists.infradead.org, kvmarm@lists.linux.dev,
x86@kernel.org, acpica-devel@lists.linuxfoundation.org,
linux-csky@vger.kernel.org, linux-doc@vger.kernel.org,
linux-ia64@vger.kernel.org, linux-parisc@vger.kernel.org,
Salil Mehta <salil.mehta@huawei.com>,
Jean-Philippe Brucker <jean-philippe@linaro.org>,
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'
Date: Tue, 23 Jan 2024 20:09:37 +0000 [thread overview]
Message-ID: <ZbAdAdqqfXRuY3Xj@shell.armlinux.org.uk> (raw)
In-Reply-To: <CAJZ5v0gt=MR1JGsPZnZG_AqudA-KMmb4BOa_A6H9B6+Rhe_+JQ@mail.gmail.com>
On Tue, Jan 23, 2024 at 08:27:05PM +0100, Rafael J. Wysocki wrote:
> On Tue, Jan 23, 2024 at 7:59 PM Russell King (Oracle)
> <linux@armlinux.org.uk> wrote:
> >
> > On Tue, Jan 23, 2024 at 07:26:57PM +0100, Rafael J. Wysocki wrote:
> > > On Tue, Jan 23, 2024 at 7:20 PM Russell King (Oracle)
> > > <linux@armlinux.org.uk> wrote:
> > > >
> > > > 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?
> > >
> > > But IIUC this change is preliminary for changing it (or equivalent
> > > option with a different name) to Y, isn't it?
> >
> > No. As I keep saying, ACPI_HOTPLUG_CPU ends up N on Arm64 even when
> > it supports hotplug CPU via ACPI.
> >
> > Even with the full Arm64 patch set here, under arch/ we still only
> > have:
> >
> > arch/loongarch/Kconfig: select ACPI_HOTPLUG_PRESENT_CPU if ACPI_PROCESSOR && HOTPLUG_CPU
> > arch/x86/Kconfig: select ACPI_HOTPLUG_PRESENT_CPU if ACPI_PROCESSOR && HOTPLUG_CPU
> >
> > To say it yet again, ACPI_HOTPLUG_(PRESENT_)CPU is *never* set on
> > Arm64.
>
> Allright, so ARM64 is not going to use the code that is conditional on
> ACPI_HOTPLUG_CPU today.
>
> Fair enough.
>
> > > > 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.
> > >
> > > So I still have to understand how renaming it for all architectures
> > > (including x86) is supposed to help.
> > >
> > > It will still be the same option under a different name. How does
> > > that change things technically?
> >
> > Do you think that it makes any sense to have support for ACPI-based
> > hotplug CPU
>
> So this is all about what you and I mean by "ACPI-based hotplug CPU".
>
> > *and* having it functional with a configuration symbol
> > named "ACPI_HOTPLUG_CPU" to be set to N ? That's essentially what
> > you are advocating for...
>
> Setting ACPI_HOTPLUG_CPU to N means that you are not going to compile
> the code that is conditional on it.
>
> That code allows the processor driver to be removed from CPUs and
> arch_unregister_cpu() to be called from within acpi_bus_trim() (among
> other things). On the way up, it allows arch_register_cpu() to be
> called from within acpi_bus_scan(). If these things are not done,
> what I mean by "ACPI-based hotplug CPU" is not supported.
Even on Arm64, arch_register_cpu() and arch_unregister_cpu() will be
called when the CPU in the VM is hot-removed or hot-added...
--
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!
next prev parent reply other threads:[~2024-01-23 20:09 UTC|newest]
Thread overview: 121+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-12-13 12:47 [RFC PATCH v3 00/21] ACPI/arm64: add support for virtual cpu hotplug Russell King (Oracle)
2023-12-13 12:49 ` [PATCH RFC v3 01/21] ACPI: Only enumerate enabled (or functional) devices Russell King
2023-12-14 17:32 ` Jonathan Cameron
2023-12-14 17:47 ` Rafael J. Wysocki
2023-12-14 18:10 ` Russell King (Oracle)
2023-12-14 18:16 ` Rafael J. Wysocki
2023-12-14 18:37 ` Rafael J. Wysocki
2023-12-15 15:31 ` Russell King (Oracle)
2023-12-15 16:15 ` Jonathan Cameron
2023-12-15 19:47 ` Rafael J. Wysocki
2024-01-02 14:39 ` Jonathan Cameron
2024-01-11 10:19 ` Jonathan Cameron
2024-01-11 10:26 ` Russell King (Oracle)
2024-01-12 11:52 ` Jonathan Cameron
2024-01-29 14:55 ` Russell King (Oracle)
2024-01-29 15:05 ` Rafael J. Wysocki
2024-01-29 15:16 ` Russell King (Oracle)
2024-01-29 15:34 ` Rafael J. Wysocki
2024-01-22 7:31 ` Gavin Shan
2023-12-14 17:55 ` Russell King (Oracle)
2023-12-13 12:49 ` [PATCH RFC v3 02/21] ACPI: processor: Add support for processors described as container packages Russell King
2023-12-14 17:36 ` Jonathan Cameron
2023-12-14 17:57 ` Russell King (Oracle)
2023-12-18 20:17 ` Rafael J. Wysocki
2024-01-09 15:49 ` Russell King (Oracle)
2024-01-09 16:05 ` Rafael J. Wysocki
2024-01-09 16:13 ` Russell King (Oracle)
2024-01-11 16:17 ` Jonathan Cameron
2024-01-11 17:59 ` Jonathan Cameron
2024-01-11 18:46 ` Russell King (Oracle)
2024-01-12 9:25 ` Jonathan Cameron
2024-01-12 15:01 ` Rafael J. Wysocki
2024-01-12 15:03 ` Jonathan Cameron
2024-01-15 10:47 ` Russell King (Oracle)
2023-12-13 12:49 ` [PATCH RFC v3 03/21] ACPI: processor: Register CPUs that are online, but not described in the DSDT Russell King
2023-12-18 20:22 ` Rafael J. Wysocki
2024-01-15 11:06 ` Russell King (Oracle)
2024-01-22 16:02 ` Jonathan Cameron
2024-01-22 16:22 ` Rafael J. Wysocki
2024-01-22 17:30 ` Russell King (Oracle)
2024-01-23 9:27 ` Jonathan Cameron
2024-01-25 13:56 ` Miguel Luis
2024-01-25 14:42 ` Rafael J. Wysocki
2024-01-29 13:03 ` Jonathan Cameron
2024-01-29 15:32 ` Russell King (Oracle)
2024-01-22 17:27 ` Russell King (Oracle)
2023-12-13 12:49 ` [PATCH RFC v3 04/21] ACPI: processor: Register all CPUs from acpi_processor_get_info() Russell King
2023-12-14 17:38 ` Jonathan Cameron
2023-12-18 20:30 ` Rafael J. Wysocki
2024-01-22 17:44 ` Jonathan Cameron
2024-01-22 18:03 ` Rafael J. Wysocki
2024-01-22 21:56 ` Russell King (Oracle)
2023-12-13 12:49 ` [PATCH RFC v3 05/21] ACPI: Rename ACPI_HOTPLUG_CPU to include 'present' Russell King
2023-12-14 17:41 ` Jonathan Cameron
2023-12-14 18:00 ` Russell King (Oracle)
2023-12-18 20:35 ` Rafael J. Wysocki
2024-01-22 18:00 ` Jonathan Cameron
2024-01-23 13:28 ` Russell King (Oracle)
2024-01-23 16:15 ` Rafael J. Wysocki
2024-01-23 16:36 ` Russell King (Oracle)
2024-01-23 17:43 ` Rafael J. Wysocki
2024-01-23 18:19 ` Russell King (Oracle)
2024-01-23 18:26 ` Rafael J. Wysocki
2024-01-23 18:59 ` Russell King (Oracle)
2024-01-23 19:27 ` Rafael J. Wysocki
2024-01-23 20:09 ` Russell King (Oracle) [this message]
2024-01-23 20:17 ` Rafael J. Wysocki
2024-01-23 20:57 ` Russell King (Oracle)
2024-01-23 21:12 ` Russell King (Oracle)
2024-01-23 22:05 ` Rafael J. Wysocki
2024-01-24 8:45 ` Russell King (Oracle)
2023-12-13 12:49 ` [PATCH RFC v3 06/21] ACPI: Move acpi_bus_trim_one() before acpi_scan_hot_remove() Russell King
2023-12-13 12:49 ` [PATCH RFC v3 07/21] ACPI: Rename acpi_processor_hotadd_init and remove pre-processor guards Russell King
2023-12-14 17:43 ` Jonathan Cameron
2023-12-14 18:03 ` Russell King (Oracle)
2023-12-13 12:49 ` [PATCH RFC v3 08/21] ACPI: Add post_eject to struct acpi_scan_handler for cpu hotplug Russell King
2023-12-13 12:49 ` [PATCH RFC v3 09/21] ACPI: convert acpi_processor_post_eject() to use IS_ENABLED() Russell King (Oracle)
2023-12-15 16:11 ` Jonathan Cameron
2023-12-13 12:50 ` [PATCH RFC v3 10/21] ACPI: Check _STA present bit before making CPUs not present Russell King
2023-12-15 16:18 ` Jonathan Cameron
2023-12-13 12:50 ` [PATCH RFC v3 11/21] ACPI: Warn when the present bit changes but the feature is not enabled Russell King
2023-12-13 12:50 ` [PATCH RFC v3 12/21] arm64: acpi: Move get_cpu_for_acpi_id() to a header Russell King
2023-12-13 12:50 ` [PATCH RFC v3 13/21] ACPICA: Add new MADT GICC flags fields Russell King
2023-12-15 16:23 ` Jonathan Cameron
2023-12-15 16:53 ` Russell King (Oracle)
2023-12-18 9:23 ` Lorenzo Pieralisi
2023-12-18 13:14 ` Rafael J. Wysocki
2023-12-18 16:28 ` Lorenzo Pieralisi
2023-12-27 11:15 ` Lorenzo Pieralisi
2023-12-13 12:50 ` [PATCH RFC v3 14/21] irqchip/gic-v3: Don't return errors from gic_acpi_match_gicc() Russell King
2023-12-15 16:33 ` Jonathan Cameron
2024-01-09 19:27 ` Russell King (Oracle)
2024-01-23 10:08 ` Jonathan Cameron
2023-12-13 12:50 ` [PATCH RFC v3 15/21] irqchip/gic-v3: Add support for ACPI's disabled but 'online capable' CPUs Russell King
2023-12-15 16:38 ` Jonathan Cameron
2023-12-13 12:50 ` [PATCH RFC v3 16/21] arm64: psci: Ignore DENIED CPUs Russell King
2023-12-15 16:40 ` Jonathan Cameron
2023-12-13 12:50 ` [PATCH RFC v3 17/21] ACPI: add support to register CPUs based on the _STA enabled bit Russell King
2023-12-18 13:03 ` Russell King (Oracle)
2024-01-02 14:53 ` Jonathan Cameron
2024-01-23 10:26 ` Jonathan Cameron
2024-01-23 13:10 ` Russell King (Oracle)
2024-01-23 14:22 ` Jonathan Cameron
2024-01-23 14:59 ` Russell King (Oracle)
2023-12-13 12:50 ` [PATCH RFC v3 18/21] ACPI: processor: Only call arch_unregister_cpu() if HOTPLUG_CPU is selected Russell King
2023-12-15 16:50 ` Jonathan Cameron
2023-12-18 12:58 ` Russell King (Oracle)
2024-01-23 10:29 ` Jonathan Cameron
2023-12-13 12:50 ` [PATCH RFC v3 19/21] arm64: document virtual CPU hotplug's expectations Russell King
2023-12-15 17:04 ` Jonathan Cameron
2023-12-13 12:50 ` [PATCH RFC v3 20/21] ACPI: Add _OSC bits to advertise OS support for toggling CPU present/enabled Russell King
2023-12-15 17:12 ` Jonathan Cameron
2024-01-02 13:07 ` Jose Marinho
2024-01-02 15:16 ` Jonathan Cameron
2024-01-02 15:35 ` Jose Marinho
2024-01-23 10:51 ` Jonathan Cameron
2023-12-13 12:50 ` [PATCH RFC v3 21/21] cpumask: Add enabled cpumask for present CPUs that can be brought online Russell King
2023-12-15 17:18 ` Jonathan Cameron
2023-12-18 12:14 ` Russell King (Oracle)
2024-01-02 15:19 ` Jonathan Cameron
2023-12-15 19:40 ` Thomas Gleixner
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