linux-kernel - Re: [PATCH 1/1] x86/acpi: acpi_is_processor

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Message-ID: <7aff0a4a-369e-6278-873b-eaad6c51d024@amd.com>
Date:   Wed, 29 Mar 2023 14:35:41 -0500
From:   "Limonciello, Mario" <mario.limonciello@....com>
To:     Eric DeVolder <eric.devolder@...cle.com>, rafael@...nel.org,
        lenb@...nel.org, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com, linux-acpi@...r.kernel.org,
        linux-kernel@...r.kernel.org, kvijayab@....com
Cc:     miguel.luis@...cle.com, boris.ostrovsky@...cle.com
Subject: Re: [PATCH 1/1] x86/acpi: acpi_is_processor_usable() dropping
 possible cpus

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@...cle.com>
> Suggested-by: Boris Ostrovsky <boris.ovstrosky@...cle.com>
> Signed-off-by: Eric DeVolder <eric.devolder@...cle.com>
> ---

Reviewed-by: Mario Limonciello <mario.limonciello@....com>
Tested-by: David R <david@...olicited.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;