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    Subject[PATCH 07/19] x86, mpx: boot-time disable
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    From: Dave Hansen <dave.hansen@linux.intel.com>

    MPX has the _potential_ to cause some issues. Say part of your init
    system tried to protect one of its components from buffer overflows
    with MPX. If there were a false positive, it's possible that MPX
    could keep a system from booting.

    MPX could also potentially cause performance issues since it is
    present in hot paths like the unmap path.

    Allow it to be disabled at boot time.

    Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
    Reviewed-by: Thomas Gleixner <tglx@linutronix.de
    ---

    b/Documentation/kernel-parameters.txt | 4 ++++
    b/arch/x86/kernel/cpu/common.c | 16 ++++++++++++++++
    2 files changed, 20 insertions(+)

    diff -puN arch/x86/kernel/cpu/common.c~x86-mpx-disable-boot-time arch/x86/kernel/cpu/common.c
    --- a/arch/x86/kernel/cpu/common.c~x86-mpx-disable-boot-time 2015-05-27 09:32:17.073559819 -0700
    +++ b/arch/x86/kernel/cpu/common.c 2015-05-27 09:32:17.078560044 -0700
    @@ -144,6 +144,22 @@ DEFINE_PER_CPU_PAGE_ALIGNED(struct gdt_p
    } };
    EXPORT_PER_CPU_SYMBOL_GPL(gdt_page);

    +static int __init x86_mpx_setup(char *s)
    +{
    + /* require an exact match without trailing characters */
    + if (strlen(s))
    + return 0;
    +
    + /* do not emit a message if the feature is not present */
    + if (!boot_cpu_has(X86_FEATURE_MPX))
    + return 1;
    +
    + setup_clear_cpu_cap(X86_FEATURE_MPX);
    + pr_info("nompx: Intel Memory Protection Extensions (MPX) disabled\n");
    + return 1;
    +}
    +__setup("nompx", x86_mpx_setup);
    +
    #ifdef CONFIG_X86_32
    static int cachesize_override = -1;
    static int disable_x86_serial_nr = 1;
    diff -puN Documentation/kernel-parameters.txt~x86-mpx-disable-boot-time Documentation/kernel-parameters.txt
    --- a/Documentation/kernel-parameters.txt~x86-mpx-disable-boot-time 2015-05-27 09:32:17.075559909 -0700
    +++ b/Documentation/kernel-parameters.txt 2015-05-27 09:32:17.080560135 -0700
    @@ -937,6 +937,10 @@ bytes respectively. Such letter suffixes
    Enable debug messages at boot time. See
    Documentation/dynamic-debug-howto.txt for details.

    + nompx [X86] Disables Intel Memory Protection Extensions.
    + See Documentation/x86/intel_mpx.txt for more
    + information about the feature.
    +
    eagerfpu= [X86]
    on enable eager fpu restore
    off disable eager fpu restore
    _

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