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Message-ID: <36462e78-8014-4415-bc47-86fbb46d028b@citrix.com>
Date:   Fri, 3 Nov 2023 19:58:01 +0000
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From:   Andrew Cooper <andrew.cooper3@citrix.com>
Subject: Notes on BAD_APICID, Was: [PATCH 0/3] x86/apic: Misc pruning
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To:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, Steve Wahl <steve.wahl@hpe.com>,
        Justin Ernst <justin.ernst@hpe.com>,
        Kyle Meyer <kyle.meyer@hpe.com>,
        Dimitri Sivanich <dimitri.sivanich@hpe.com>,
        Russ Anderson <russ.anderson@hpe.com>,
        Darren Hart <dvhart@infradead.org>,
        Andy Shevchenko <andy@infradead.org>,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Wei Liu <wei.liu@kernel.org>, Dexuan Cui <decui@microsoft.com>,
        Lorenzo Pieralisi <lpieralisi@kernel.org>,
        =?UTF-8?Q?Krzysztof_Wilczy=C5=84ski?= <kw@linux.com>,
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Cc:     linux-kernel@vger.kernel.org, platform-driver-x86@vger.kernel.org,
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References: <20231102-x86-apic-v1-0-bf049a2a0ed6@citrix.com>
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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