linux-kernel - Re: [PATCH] x86/acpi/boot: Correct acpi_is_processor

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Message-ID: <20251112135618.GCaRSSAkqagSF_gr9j@fat_crate.local>
Date: Wed, 12 Nov 2025 14:56:18 +0100
From: Borislav Petkov <bp@...en8.de>
To: Yazen Ghannam <yazen.ghannam@....com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
	Michal Pecio <michal.pecio@...il.com>,
	Eric DeVolder <eric.devolder@...cle.com>,
	Mario Limonciello <mario.limonciello@....com>
Subject: Re: [PATCH] x86/acpi/boot: Correct acpi_is_processor_usable() check
 again

On Tue, Nov 11, 2025 at 02:53:57PM +0000, Yazen Ghannam wrote:
> ACPI v6.3 defined a new "Online Capable" MADT LAPIC flag. This bit is
> used in conjunction with the "Enabled" MADT LAPIC flag to determine if a
> CPU can be enabled/hotplugged by the OS after boot.
> 
> Before the new bit was defined, the "Enabled" bit was explicitly
> described like this (ACPI v6.0 wording provided):
> "If zero, this processor is unusable, and the operating system
> support will not attempt to use it"
> 
> This means that CPU hotplug (based on MADT) is not possible. Many BIOS
> implementations follow this guidance. They may include LAPIC entries in
> MADT for unavailable CPUs, but since these entries are marked with
> "Enabled=0" it is expected that the OS will completely ignore these
> entries.
> 
> However, QEMU will do the same (include entries with "Enabled=0") for
> the purpose of allowing CPU hotplug within the guest.
> 
> Comment from QEMU function pc_madt_cpu_entry():
>     /* ACPI spec says that LAPIC entry for non present
>      * CPU may be omitted from MADT or it must be marked
>      * as disabled. However omitting non present CPU from
>      * MADT breaks hotplug on linux. So possible CPUs
>      * should be put in MADT but kept disabled.
>      */
> 
> Recent Linux topology changes broke the QEMU use case. A following fix
> for the QEMU use case broke bare metal topology enumeration.
> 
> Rework the Linux MADT LAPIC flags check to allow the QEMU use case only
> for guests and to maintain the ACPI spec behavior for bare metal.
> 
> Remove an unnecessary check added to fix a bare metal case introduced by
> the QEMU "fix".
> 
> Fixes: fed8d8773b8e ("x86/acpi/boot: Correct acpi_is_processor_usable() check")
> Fixes: f0551af02130 ("x86/topology: Ignore non-present APIC IDs in a present package")
> Reported-by: Michal Pecio <michal.pecio@...il.com>
> Closes: https://lore.kernel.org/r/20251024204658.3da9bf3f.michal.pecio@gmail.com
> Signed-off-by: Yazen Ghannam <yazen.ghannam@....com>
> Cc: stable@...r.kernel.org
> Cc: Eric DeVolder <eric.devolder@...cle.com>
> Cc: Mario Limonciello <mario.limonciello@....com>
> ---
> 
> Notes:
>     Link:
>     https://lore.kernel.org/r/20251024204658.3da9bf3f.michal.pecio@gmail.com
>     
>     Hi all,
>     
>     This patch came out of the discussion above.
>     
>     A number of folks (myself included) understood the ACPI MADT LAPIC
>     "Enabled" flag to be potentially used for CPU hotplug. This is
>     explicitly false based on the wording in older revisions of the ACPI
>     spec.
>     
>     However, this understanding is used for QEMU. Hence we need a check to
>     differentiate the virtualization and bare metal use cases.

...

> +       return (lapic_flags & ACPI_MADT_ONLINE_CAPABLE);

Superfluous brackets.

In any case, yah, I'm going to queue this soon but I'll give Eric some time to
complain before I do.

I did play with:

https://www.qemu.org/docs/master/system/cpu-hotplug.html

and that works with that patch:

$ ./qmp-shell -p -v localhost:4444
Welcome to the QMP low-level shell!
Connected to QEMU 10.1.0
 
(QEMU) query-hotpluggable-cpus
{
    "arguments": {},
    "execute": "query-hotpluggable-cpus"
}
{
    "return": [
        {
            "props": {
                "core-id": 1,
                "socket-id": 0,
                "thread-id": 0
            },
            "type": "host-x86_64-cpu",
            "vcpus-count": 1
        },
        {
            "props": {
                "core-id": 0,
                "socket-id": 0,
                "thread-id": 0
            },
            "qom-path": "/machine/unattached/device[0]",
            "type": "host-x86_64-cpu",
            "vcpus-count": 1
        }
    ]
}
(QEMU) device_add id=cpu-2 driver=host-x86_64-cpu socket-id=0 core-id=1 thread-id=0
{
    "arguments": {
        "core-id": 1,
        "driver": "host-x86_64-cpu",
        "id": "cpu-2",
        "socket-id": 0,
        "thread-id": 0
    },
    "execute": "device_add"
}
{
    "return": {}
}
(QEMU)

and dmesg has:

[   33.281150] ACPI: acpi_map_cpu: cpu: 1, physid: 0x1, acpi_id: 0x1
[   33.289650] ACPI: CPU1 has been hot-added

But man oh man, if this ain't wagging the dog I don't know what is - we're
fixing the kernel so that qemu can hotplug because, oh well, "but then there
is this thing called reality^Wvirtualization which ruins everything."...

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette