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    SubjectRe: [PATCH 6/7] x86/spec_ctrl: Add sysctl knobs to enable/disable SPEC_CTRL feature
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    On 01/05/2018 03:16 AM, David Woodhouse wrote:
    > On Thu, 2018-01-04 at 09:56 -0800, Tim Chen wrote:
    >>
    >> + mutex_lock(&spec_ctrl_mutex);
    >> +
    >> + if (enable == IBRS_DISABLED) {
    >> + /* disable IBRS usage */
    >> + set_ibrs_disabled();
    >> + if (spec_ctrl_ibrs & SPEC_CTRL_IBRS_SUPPORTED)
    >> + spec_ctrl_flush_all_cpus(MSR_IA32_SPEC_CTRL, SPEC_CTRL_FEATURE_DISABLE_IBRS);
    >> + } else if (enable == IBRS_ENABLED) {
    >> + /* enable IBRS usage in kernel */
    >> + clear_ibrs_disabled();
    >> + if (spec_ctrl_ibrs & SPEC_CTRL_IBRS_SUPPORTED)
    >> + set_ibrs_inuse();
    >> + else
    >> + /* Platform don't support IBRS */
    >> + enable = IBRS_DISABLED;
    >> + } else if (enable == IBRS_ENABLED_USER) {
    >> + /* enable IBRS usage in both userspace and kernel */
    >> + clear_ibrs_disabled();
    >> + /* don't change IBRS value once we set it to always on */
    >> + clear_ibrs_inuse();
    >> + if (spec_ctrl_ibrs & SPEC_CTRL_IBRS_SUPPORTED)
    >> + spec_ctrl_flush_all_cpus(MSR_IA32_SPEC_CTRL, SPEC_CTRL_FEATURE_ENABLE_IBRS);
    >> + else
    >> + /* Platform don't support IBRS */
    >> + enable = IBRS_DISABLED;
    >> + }
    >
    > This doesn't take the retpoline status into account. If we have
    > retpoline, we don't need IBRS in the kernel.
    >

    If retpoline is used, we don't enable IBRS automatically during feature detection.
    But if the admin is paranoid, he still has the choice to explicitly issue
    a command to enable IBRS here.

    Tim

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