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

[Eric DeVolder]

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@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
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.