linux-kernel - Notes on BAD_APICID, Was: [PATCH 0/3] x86/apic: Misc pruning

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Date:   Fri, 3 Nov 2023 19:58:01 +0000
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Steve Wahl <steve.wahl@....com>,
        Justin Ernst <justin.ernst@....com>,
        Kyle Meyer <kyle.meyer@....com>,
        Dimitri Sivanich <dimitri.sivanich@....com>,
        Russ Anderson <russ.anderson@....com>,
        Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Rob Herring <robh@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Cc:     linux-kernel@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-hyperv@...r.kernel.org, linux-pci@...r.kernel.org
Subject: Notes on BAD_APICID, Was: [PATCH 0/3] x86/apic: Misc pruning

On 02/11/2023 12:26 pm, Andrew Cooper wrote:
> Seriously, this work started out trying to fix a buggy comment.  It
> escalated somewhat...  Perform some simple tidying.

Another dodgy construct spotted while doing this work is

#ifdef CONFIG_X86_32
 #define BAD_APICID 0xFFu
#else
 #define BAD_APICID 0xFFFFu
#endif

considering that both of those "bad" values are legal APIC IDs in an
x2APIC system.

The majority use is as a sentential (of varying types - int, u16
mostly), although the uses for NUM_APIC_CLUSTERS, and
safe_smp_processor_id() look suspect.

In particular, safe_smp_processor_id() *will* malfunction on some legal
CPUs, and needs to use -1 (32 bits wide) to spot the intended error case
of a bad xAPIC mapping.

However, it's use in amd_pmu_cpu_starting() from topology_die_id() looks
broken.  Partly because the error handling is (only) a WARN_ON_ONCE(),
and also because nb->nb_id's sentinel value is -1 of type int.

I suspect there's a lot of cleaning to be done here too.

~Andrew