linux-kernel - Re: [PATCH v2] x86/acpi/boot: Do not register processors that cannot be onlined for x2apic

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Date:   Thu, 5 Jan 2023 18:09:59 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Kishon Vijay Abraham I <kvijayab@....com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Santosh Shukla <santosh.shukla@....com>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Borislav Petkov <bpetkov@....com>,
        Leo Duran <leo.duran@....com>
Subject: Re: [PATCH v2] x86/acpi/boot: Do not register processors that cannot
 be onlined for x2apic

On Thu, Jan 5, 2023 at 5:11 AM Kishon Vijay Abraham I <kvijayab@....com> wrote:
>
> Section 5.2.12.12 Processor Local x2APIC Structure in the ACPI v6.5
> spec mandates that both "enabled" and "online capable" Local APIC Flags
> should be used to determine if the processor is usable or not.
>
> However, Linux doesn't use the "online capable" flag for x2APIC to
> determine if the processor is usable. As a result, cpu_possible_mask has
> incorrect value and results in more memory getting allocated for per_cpu
> variables than it is going to be used.
>
> Make sure Linux parses both "enabled" and "online capable" flags for
> x2APIC to correctly determine if the processor is usable.
>
> Fixes: aa06e20f1be6 ("x86/ACPI: Don't add CPUs that are not online capable")
> Reviewed-by: Borislav Petkov (AMD) <bp@...en8.de>
> Reported-by: Leo Duran <leo.duran@....com>
> Signed-off-by: Kishon Vijay Abraham I <kvijayab@....com>

Acked-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

> ---
> Changes from v1:
> 1) Changed the ACPI spec version to 6.5 in the commit log
> 2) Changed the Fixes tag to point to commit aa06e20f1be6
> 3) Added "Reported-by: Leo Duran <leo.duran@....com>"
>  arch/x86/kernel/acpi/boot.c | 19 ++++++++++++++++---
>  1 file changed, 16 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kernel/acpi/boot.c b/arch/x86/kernel/acpi/boot.c
> index 907cc98b1938..518bda50068c 100644
> --- a/arch/x86/kernel/acpi/boot.c
> +++ b/arch/x86/kernel/acpi/boot.c
> @@ -188,6 +188,17 @@ static int acpi_register_lapic(int id, u32 acpiid, u8 enabled)
>         return cpu;
>  }
>
> +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))
> +               return true;
> +
> +       return false;
> +}
> +
>  static int __init
>  acpi_parse_x2apic(union acpi_subtable_headers *header, const unsigned long end)
>  {
> @@ -212,6 +223,10 @@ acpi_parse_x2apic(union acpi_subtable_headers *header, const unsigned long end)
>         if (apic_id == 0xffffffff)
>                 return 0;
>
> +       /* don't register processors that cannot be onlined */
> +       if (!acpi_is_processor_usable(processor->lapic_flags))
> +               return 0;
> +
>         /*
>          * We need to register disabled CPU as well to permit
>          * counting disabled CPUs. This allows us to size
> @@ -250,9 +265,7 @@ acpi_parse_lapic(union acpi_subtable_headers * header, const unsigned long end)
>                 return 0;
>
>         /* don't register processors that can not be onlined */
> -       if (acpi_support_online_capable &&
> -           !(processor->lapic_flags & ACPI_MADT_ENABLED) &&
> -           !(processor->lapic_flags & ACPI_MADT_ONLINE_CAPABLE))
> +       if (!acpi_is_processor_usable(processor->lapic_flags))
>                 return 0;
>
>         /*
> --
> 2.34.1
>