linux-kernel - Re: [tip: x86/urgent] x86/acpi: Ignore invalid x2APIC entries

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Date:   Wed, 22 Nov 2023 22:19:47 +0000
From:   John Sperbeck <jsperbeck@...gle.com>
To:     tip-bot2@...utronix.de
Cc:     linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
        peterz@...radead.org, rui.zhang@...el.com, tglx@...utronix.de,
        x86@...nel.org
Subject: Re: [tip: x86/urgent] x86/acpi: Ignore invalid x2APIC entries

I have a platform with both LOCAL_APIC and LOCAL_X2APIC entries for
each CPU.  However, the ids for the LOCAL_APIC entries are all
invalid ids of 255, so they have always been skipped in acpi_parse_lapic()
by this code from f3bf1dbe64b6 ("x86/acpi: Prevent LAPIC id 0xff from being
accounted"):

    /* Ignore invalid ID */
    if (processor->id == 0xff)
            return 0;

With the change in this thread, the return value of 0 means that the
'count' variable in acpi_parse_entries_array() is incremented.  The
positive return value means that 'has_lapic_cpus' is set, even though
no entries were actually matched.  Then, when the MADT is iterated
with acpi_parse_x2apic(), the x2apic entries with ids less than 255
are skipped and most of my CPUs aren't recognized.

I think the original version of this change was okay for this case in
https://lore.kernel.org/lkml/87pm4bp54z.ffs@tglx/T/

P.S. I could be convinced that the MADT for my platform is somewhat
ill-formed and that I'm relying on pre-existing behavior.  I'm not
well-versed enough in the topic to know for sure.