Re: [PATCH RFC v3 03/21] ACPI: processor: Register CPUs that are online, but not described in the DSDT

["Russell King (Oracle)" <linux@armlinux.org.uk>]

On Mon, Jan 22, 2024 at 05:22:46PM +0100, Rafael J. Wysocki wrote:
> On Mon, Jan 22, 2024 at 5:02 PM Jonathan Cameron
> <Jonathan.Cameron@huawei.com> wrote:
> >
> > On Mon, 15 Jan 2024 11:06:29 +0000
> > "Russell King (Oracle)" <linux@armlinux.org.uk> wrote:
> >
> > > On Mon, Dec 18, 2023 at 09:22:03PM +0100, Rafael J. Wysocki 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>
> > > > >
> > > > > ACPI has two descriptions of CPUs, one in the MADT/APIC table, the other
> > > > > in the DSDT. Both are required. (ACPI 6.5's 8.4 "Declaring Processors"
> > > > > says "Each processor in the system must be declared in the ACPI
> > > > > namespace"). Having two descriptions allows firmware authors to get
> > > > > this wrong.
> > > > >
> > > > > If CPUs are described in the MADT/APIC, they will be brought online
> > > > > early during boot. Once the register_cpu() calls are moved to ACPI,
> > > > > they will be based on the DSDT description of the CPUs. When CPUs are
> > > > > missing from the DSDT description, they will end up online, but not
> > > > > registered.
> > > > >
> > > > > Add a helper that runs after acpi_init() has completed to register
> > > > > CPUs that are online, but weren't found in the DSDT. Any CPU that
> > > > > is registered by this code triggers a firmware-bug warning and kernel
> > > > > taint.
> > > > >
> > > > > Qemu TCG only describes the first CPU in the DSDT, unless cpu-hotplug
> > > > > is configured.
> > > >
> > > > So why is this a kernel problem?
> > >
> > > So what are you proposing should be the behaviour here? What this
> > > statement seems to be saying is that QEMU as it exists today only
> > > describes the first CPU in DSDT.
> >
> > This confuses me somewhat, because I'm far from sure which machines this
> > is true for in QEMU.  I'm guessing it's a legacy thing with
> > some old distro version of QEMU - so we'll have to paper over it anyway
> > but for current QEMU I'm not sure it's true.
> >
> > Helpfully there are a bunch of ACPI table tests so I've been checking
> > through all the multi CPU cases.
> >
> > CPU hotplug not enabled.
> > pc/DSDT.dimmpxm  - 4x Processor entries.  -smp 4
> > pc/DSDT.acpihmat - 2x Processor entries.  -smp 2
> > q35/DSDT.acpihmat - 2x Processor entries. -smp 2
> > virt/DSDT.acpihmatvirt - 4x ACPI0007 entries -smp 4
> > q35/DSDT.acpihmat-noinitiator - 4 x Processor () entries -smp 4
> > virt/DSDT.topology - 8x ACPI0007 entries
> >
> > I've also looked at the code and we have various types of
> > CPU hotplug on x86 but they all build appropriate numbers of
> > Processor() entries in DSDT.
> > Arm likewise seems to build the right number of ACPI0007 entries
> > (and doesn't yet have CPU HP support).
> >
> > If anyone can add a reference on why this is needed that would be very
> > helpful.
> 
> Yes, it would.
> 
> Personally, I would prefer to assume that it is not necessary until it
> turns out that (1) there is firmware with this issue actually in use
> and (2) updating the firmware in question to follow the specification
> is not practical.
> 
> Otherwise, we'd make it easier to ship non-compliant firmware for no
> good reason.

If Salil can't come up with a reason, then I'm in favour of dropping
the patch like already done for patch 2. If the code change serves no
useful purpose, there's no point in making the change.

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