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

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Message-ID: <87zfztib46.ffs@tglx>
Date:   Sat, 04 Nov 2023 20:15:05 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andrew Cooper <andrew.cooper3@...rix.com>,
        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: Re: Notes on BAD_APICID, Was: [PATCH 0/3] x86/apic: Misc pruning

On Fri, Nov 03 2023 at 19:58, Andrew Cooper wrote:
> On 02/11/2023 12:26 pm, Andrew Cooper wrote:
> 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.

Sigh...