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    SubjectRe: [PATCH 05/22] x86/srso: Fix SBPB enablement for mitigations=off
    On Sun, Aug 20, 2023 at 06:19:02PM -0700, Josh Poimboeuf wrote:
    > If the user has requested no mitigations with mitigations=off, use the
    > lighter-weight SBPB instead of IBPB for other mitigations.
    >
    > Note that even with mitigations=off, IBPB/SBPB may still be used for
    > Spectre v2 user <-> user protection. Whether that makes sense is a
    > question for another day.

    Well, with my user hat on, off means off.

    IINM, spectre_v2_parse_cmdline() will give SPECTRE_V2_CMD_NONE to
    spectre_v2_select_mitigation() when mitigations=off.

    spectre_v2_user_select_mitigation() will use the
    spectre_v2_select_mitigation()'s result, which turn into
    SPECTRE_V2_USER_CMD_NONE and then not enable *BPB either.

    So even if we set x86_pred_cmd to SBPB here, it won't do anything
    because X86_FEATURE_USE_IBPB won't be set and
    indirect_branch_prediction_barrier() will be a NOP.

    IOW, I think we should separate the check:

    if (cpu_mitigations_off())
    return;

    at the beginning of srso_select_mitigation() so that it is crystal
    clear. Maybe even slap a comment over it.

    Thx.

    --
    Regards/Gruss,
    Boris.

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

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