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

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Message-ID: <878r5yp357.ffs@tglx>
Date:   Wed, 13 Dec 2023 15:51:00 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Zhang, Rui" <rui.zhang@...el.com>,
        "jsperbeck@...gle.com" <jsperbeck@...gle.com>,
        "tip-bot2@...utronix.de" <tip-bot2@...utronix.de>
Cc:     "andres@...razel.de" <andres@...razel.de>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-tip-commits@...r.kernel.org" 
        <linux-tip-commits@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [tip: x86/urgent] x86/acpi: Ignore invalid x2APIC entries

On Wed, Dec 13 2023 at 07:39, Zhang, Rui wrote:
> Yeah, I agree.
>
> I have posted a patch to do more strict check
> https://lore.kernel.org/all/20231210143925.38722-1-rui.zhang@intel.com/
> in case there are some weird cases that LAPIC fails to probe any
> enabled CPU and we also lose the X2APIC cpus.

The return value of acpi_register_lapic() is not really useful.

It returns an error if

  1) the number of registered CPUs reached the limit.
  2) the APIC entry is not enabled

#1: any further X2APIC CPU will be ignored

#2: the return value is bogus as the CPU is accounted for as disabled
    and will eventually lead to #1

    In fact even 'disabled' entries are valid as they can be brought
    in later (that's what "physical" hotplug uses)

The topology evaluation rework gets rid of this return value completely,
so I really don't want to add an dependency on it.

Thanks,

        tglx