Re: [PATCH] x86/acpi: Fix LAPIC/x2APIC parsing order

["Zhang, Rui" <rui.zhang@intel.com>]

On Tue, 2024-12-10 at 23:51 +0100, Thomas Gleixner wrote:
> On Tue, Oct 22 2024 at 08:17, Zhang Rui wrote:
> > On some systems, the same CPU (with the same APIC ID) is assigned a
> > different logical CPU id after commit ec9aedb2aa1a ("x86/acpi:
> > Ignore
> > invalid x2APIC entries").
> > 
> > This means that Linux enumerates the CPUs in a different order,
> > which
> > violates ACPI specification[1] that states:
> > 
> >   "OSPM should initialize processors in the order that they appear
> > in
> >    the MADT"
> > 
> > The problematic commit parses all LAPIC entries before any x2APIC
> > entries, aiming to ignore x2APIC entries with APIC ID < 255 when
> > valid
> > LAPIC entries exist. However, it disrupts the CPU enumeration order
> > on
> > systems where x2APIC entries precede LAPIC entries in the MADT.
> > 
> > Fix the problem by separately checking LAPIC entries before parsing
> > any
> > LAPIC or x2APIC entries.
> 
> I really had to stare at the change to understand how this fixes
> anything. What you want to say is:
> 
> Fix this problem by:
> 
>     1) Parsing LAPIC entries first without registering them in the
>        topology to evaluate whether valid LAPIC entries exist.
> 
>     2) Restoring the MADT in order parser which invokes either the
> LAPIC or
>        the X2APIC parser function depending on the entry type.
> 
>        The X2APIC parser still ignores entries < 0xff in case that #1
>        found valid LAPIC entries independent of their position in the
>        MADT table.
> 
Exactly. Thanks for the rewording.

-rui

> Other than that:
> 
> Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
> 
>