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SubjectRe: [PATCH 1/1] x86/acpi: acpi_is_processor_usable() dropping possible cpus
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On 3/27/23 14:57, Borislav Petkov wrote:
> 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?

Boris,

https://ueif.org/sites/default/files/resources/ACPI_6_3_May16.pdf
Section 5.2.12 MADT. Table 5-43 is the MADT Revision is numbered 5.
However, ACPI 6.x specs got a little "sloppy" with Revision, as this
is what I uncovered:

ACPI MADT Changes
6.0 3 Section 5.2.12
6.0a 4 Section 5.2.12
Adds ARM GIC structure types 0xB-0xF
6.2a 45 Section 5.2.12 <--- yep it says version 45!
6.2b 5 Section 5.2.12
GIC ITS last Reserved offset changed to 16 from 20 (typo)
6.3 5 Section 5.2.12
Adds Local APIC Flags Online Capable!
Adds GICC SPE Overflow Interrupt field
6.4 5 Section 5.2.12
Adds Multiprocessor Wakeup Structure type 0x10
6.5 5 Section 5.2.12

At any rate, Section 5.2.12.2 Processor Local APIC structure, has Table 5-47
which is the Local APIC Structure Flags, and this is where Online Capable
is introduced for the first time.


>
>> 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.
ok!

>
> Also, do
>
> $ git grep 'This patch' Documentation/process
>
> for more details.
ok, will do!

>
>> 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>
>> ---
>> 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
ok, will do!
>
>> + if (!acpi_support_online_capable || (lapic_flags & ACPI_MADT_ONLINE_CAPABLE))
>> return true;
>>
>> return false;
>> --
>
> Otherwise, the change makes sense to me.
ok!

>
> Thx.
>

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