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

[Borislav Petkov]

On Mon, Mar 27, 2023 at 03:10:26PM -0400, 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.

I can't find where in the spec it says that MADT.revision 5 means that
bit is present?

I'm looking at:

aa06e20f1be6 ("x86/ACPI: Don't add CPUs that are not online capable")

Mario?

I see in the 6.3 spec it says:

"1948 Adds a “Hot-plug Capable” flag to the Local APIC and x2APIC structures in MADT"

and the MADT.revision is 5 and in the 6.2 spec the MADT revision is "45"
- 4.5 maybe?

But I don't see the connection between MADT.revision 5 and the presence
of the online capable bit.

Anyone got a better quote?

> 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

Avoid having "This patch" or "This commit" in the commit message. It is
tautologically useless.

Also, do

$ git grep 'This patch' Documentation/process

for more details.

> 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@xxxxxxxxxx>
> Suggested-by: Boris Ostrovsky <boris.ovstrosky@xxxxxxxxxx>
> Signed-off-by: Eric DeVolder <eric.devolder@xxxxxxxxxx>
> ---
>  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.
> +	 */

In all your text

s/cpu/CPU/g

> +	if (!acpi_support_online_capable || (lapic_flags & ACPI_MADT_ONLINE_CAPABLE))
>  		return true;
>  
>  	return false;
> -- 

Otherwise, the change makes sense to me.

Thx.

-- 
Regards/Gruss,
    Boris.

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