linux-kernel - Re: [RFC PATCH v2 15/35] ACPI: processor: Add support for processors described as container packages

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Message-ID: <ZUTRgNtpcVtcMFqJ@shell.armlinux.org.uk>
Date:   Fri, 3 Nov 2023 10:54:56 +0000
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Gavin Shan <gshan@...hat.com>
Cc:     James Morse <james.morse@....com>, linux-pm@...r.kernel.org,
        loongarch@...ts.linux.dev, linux-acpi@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, kvmarm@...ts.linux.dev,
        x86@...nel.org, Salil Mehta <salil.mehta@...wei.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        jianyong.wu@....com, justin.he@....com
Subject: Re: [RFC PATCH v2 15/35] ACPI: processor: Add support for processors
 described as container packages

On Mon, Sep 18, 2023 at 03:02:53PM +1000, Gavin Shan wrote:
> 
> On 9/14/23 02:38, James Morse wrote:
> > ACPI has two ways of describing processors in the DSDT. Either as a device
> > object with HID ACPI0007, or as a type 'C' package inside a Processor
> > Container. The ACPI processor driver probes CPUs described as devices, but
> > not those described as packages.
> > 
> > Duplicate descriptions are not allowed, the ACPI processor driver already
> > parses the UID from both devices and containers. acpi_processor_get_info()
> > returns an error if the UID exists twice in the DSDT.
> > 
> > The missing probe for CPUs described as packages creates a problem for
> > moving the cpu_register() calls into the acpi_processor driver, as CPUs
> > described like this don't get registered, leading to errors from other
> > subsystems when they try to add new sysfs entries to the CPU node.
> > (e.g. topology_sysfs_init()'s use of topology_add_dev() via cpuhp)
> > 
> > To fix this, parse the processor container and call acpi_processor_add()
> > for each processor that is discovered like this. The processor container
> > handler is added with acpi_scan_add_handler(), so no detach call will
> > arrive.
> > 
> > Qemu TCG describes CPUs using packages in a processor container.
> > 
> > Signed-off-by: James Morse <james.morse@....com>
> > ---
> >   drivers/acpi/acpi_processor.c | 22 ++++++++++++++++++++++
> >   1 file changed, 22 insertions(+)
> > 
> 
> I don't understand the last sentence of the commit log. QEMU
> always have "ACPI0007" for the processor devices.

I think what James is referring to is the use of Processor Containers
(see
https://uefi.org/specs/ACPI/6.5/08_Processor_Configuration_and_Control.html#processor-container-device)

which are defined using HID ACPI0010. This seems to be what
build_cpus_aml() is doing. It creates:

	\_SB.CPUS - processor container with ACPI0010

and then builds the processor devices underneath that object using
ACPI0007.

I think the use of "packages" there is wrong, it should be "processor
devices" - taking the terminology from the above specification link.
As far as I can see, QEMU does not (yet) use the option of embedding
child processor containers beneath a parent.

-- 
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