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Date:   Wed, 22 Nov 2023 22:18:18 +0000
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Subject: Re: [tip: x86/urgent] x86/acpi: Ignore invalid x2APIC entries
From:   John Sperbeck <jsperbeck@google.com>
To:     tip-bot2@linutronix.de
Cc:     linux-kernel@vger.kernel.org, linux-tip-commits@vger.kernel.org,
        peterz@infradead.org, rui.zhang@intel.com, tglx@linutronix.de,
        x86@kernel.org
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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.