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    SubjectRe: [PATCH v2 1/5] perf/x86/intel/lbr: use setup_clear_cpu_cap instead of clear_cpu_cap
    On Tue, Sep 20, 2022 at 11:20:47AM +0300, Maxim Levitsky wrote:
    > If I understand that correctly, the difference between clear_cpu_cap and setup_clear_cpu_cap
    > is that setup_clear_cpu_cap should be called early when only the boot cpu is running and it
    >
    > 1. works on 'boot_cpu_data' which represents the boot cpu.
    > 2. sets a bit in 'cpu_caps_cleared' which are later applied to all CPUs, including these that are hotplugged.

    Yes.

    > On the other hand the clear_cpu_cap just affects the given 'struct cpuinfo_x86'.

    Yes.

    > Call of 'clear_cpu_cap(&boot_cpu_data, X86_FEATURE_ARCH_LBR)' is weird since it still affects 'boot_cpu_data'
    > but doesn't affect 'cpu_caps_cleared'

    Yes.

    > I assumed that this was a mistake and the intention was to disable the feature on all CPUs.

    peterz says yes.

    > I need this patch because in the next patch, I change the clear_cpu_cap such as it detects being
    > called on boot_cpu_data and in this case also clears bits in 'cpu_caps_cleared', thus
    > while this patch does introduce a functional change, the next patch doesn't since this is the only
    > place where clear_cpu_cap is called explicitly on 'boot_cpu_data'

    This is not needed - this patch doing setup_clear_cpu_cap() should suffice.

    But, there must be something you're fixing with this. Which is it? Some
    weird virt config?

    > I do now notice that initcalls are run after smp is initialized, which
    > means that this code doesn't really disable the CPUID feature on all
    > CPUs at all.

    Well, not exactly. There's do_pre_smp_calls() which is where the
    early_initcall() thing is run.

    So setup_clear_cpu_cap() will make sure that the feature bit is cleared
    when the APs come online.

    Do you have a virt configuration where you can test this case where the
    feature flag is clear on all CPUs when it fails?

    I.e., "arch_lbr" will disappear in /proc/cpuinfo completely.

    Thx.

    --
    Regards/Gruss,
    Boris.

    https://people.kernel.org/tglx/notes-about-netiquette

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