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SubjectRe: [PATCH 1/1] x86/acpi: acpi_is_processor_usable() dropping possible cpus
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On 3/27/2023 14:10, Eric DeVolder wrote:
> The logic in acpi_is_processor_usable() requires the Online Capable
> bit be set for hotpluggable cpus. The Online Capable bit is
> introduced in ACPI 6.3 and MADT.revision 5.
>
> However, as currently coded, for MADT.revision < 5,
> acpi_is_processor_usable() no longer allows for possible hot
> pluggable cpus, which is a regressive change in behavior.
>
> This patch restores the behavior where for MADT.revision < 5, the
> presence of the lapic/x2apic structure implies a possible hotpluggable
> cpu.
>
> Fixes: e2869bd7af60 ("x86/acpi/boot: Do not register processors that cannot be onlined for x2APIC")
> Suggested-by: Miguel Luis <miguel.luis@oracle.com>
> Suggested-by: Boris Ostrovsky <boris.ovstrosky@oracle.com>
> Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
> ---

Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Tested-by: David R <david@unsolicited.net>
Link:
https://lore.kernel.org/all/a15c6a64-dcd7-b102-9d9f-0225dfa1172b@unsolicited.net/T/

> arch/x86/kernel/acpi/boot.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/acpi/boot.c b/arch/x86/kernel/acpi/boot.c
> index 1c38174b5f01..7b5b8ed018b0 100644
> --- a/arch/x86/kernel/acpi/boot.c
> +++ b/arch/x86/kernel/acpi/boot.c
> @@ -193,7 +193,13 @@ static bool __init acpi_is_processor_usable(u32 lapic_flags)
> if (lapic_flags & ACPI_MADT_ENABLED)
> return true;
>
> - if (acpi_support_online_capable && (lapic_flags & ACPI_MADT_ONLINE_CAPABLE))
> + /*
> + * Prior to MADT.revision 5, the presence of the Local x2/APIC
> + * structure _implicitly_ noted a possible hotpluggable cpu.
> + * Starting with MADT.revision 5, the Online Capable bit
> + * _explicitly_ indicates a hotpluggable cpu.
> + */
> + if (!acpi_support_online_capable || (lapic_flags & ACPI_MADT_ONLINE_CAPABLE))
> return true;
>
> return false;

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