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    SubjectRe: [v4.17-rcx] Lost IBPB, IBRS_FW support for spectre_v2 mitigation.
    On Tue, May 1, 2018 at 5:59 AM Thomas Gleixner <tglx@linutronix.de> wrote:

    > Then I really have no idea how reverting the patch you pointed out would
    > fix it.

    So I do think that the original patch is buggy.

    What I think *may* be going on is:

    - first we do that

    get_cpu_cap(c);
    get_cpu_address_sizes(c);

    but at that point, CPU levels may be masked, and that 0x80000008 leaf
    isn't seen

    - then we do

    if (this_cpu->c_early_init)
    this_cpu->c_early_init(c);

    which calls early_init_intel(), which does that

    if (msr_clear_bit(MSR_IA32_MISC_ENABLE,
    MSR_IA32_MISC_ENABLE_LIMIT_CPUID_BIT) >
    0) {

    which now raises the cpuid_level.

    - then we do

    get_cpu_cap(c);

    again, because the cpuid level has been raised, and _now_ it used to get
    that 0x80000008 leaf information.

    But with the change, that second call to get_cpu_cap() didn't do anything,
    because the 0x80000008 leaf handling had been moved away.

    However, I agree that your patch to just do that CPUID_8000_0008_EBX in
    get_cpu_cap() should have fixed it, and it's possible that Jörg mis-tested
    it.

    Jörg, are you sure you didn't somehow get the wrong microcode? Because
    another way for those bits to be cleared again is if
    bad_spectre_microcode() triggers. That should show up in dmesg as "Intel
    Spectre v2 broken microcode detected" though.

    Linus

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