Re: [PATCH 1/1] x86/acpi: acpi_is_processor_usable() dropping possible cpus

From: "Limonciello, Mario" <mario.limonciello@amd.com>
To: Eric DeVolder <eric.devolder@oracle.com>,
	rafael@kernel.org, lenb@kernel.org, tglx@linutronix.de,
	mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com,
	x86@kernel.org, hpa@zytor.com, linux-acpi@vger.kernel.org,
	linux-kernel@vger.kernel.org, kvijayab@amd.com
Cc: miguel.luis@oracle.com, boris.ostrovsky@oracle.com
Subject: Re: [PATCH 1/1] x86/acpi: acpi_is_processor_usable() dropping possible cpus
Date: Wed, 29 Mar 2023 14:35:41 -0500	[thread overview]
Message-ID: <7aff0a4a-369e-6278-873b-eaad6c51d024@amd.com> (raw)
In-Reply-To: <20230327191026.3454-2-eric.devolder@oracle.com>

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;