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    Subject[PATCH 3/6] x86/entry, mce: Disallow #DB during #MC
    #MC is fragile as heck, don't tempt fate.

    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    ---
    arch/x86/kernel/cpu/mce/core.c | 12 ++++++++++++
    1 file changed, 12 insertions(+)

    --- a/arch/x86/kernel/cpu/mce/core.c
    +++ b/arch/x86/kernel/cpu/mce/core.c
    @@ -1943,22 +1943,34 @@ static __always_inline void exc_machine_
    /* MCE hit kernel mode */
    DEFINE_IDTENTRY_MCE(exc_machine_check)
    {
    + unsigned long dr7;
    +
    + local_db_save(&dr7);
    exc_machine_check_kernel(regs);
    + local_db_restore(dr7);
    }

    /* The user mode variant. */
    DEFINE_IDTENTRY_MCE_USER(exc_machine_check)
    {
    + unsigned long dr7;
    +
    + local_db_save(&dr7);
    exc_machine_check_user(regs);
    + local_db_restore(dr7);
    }
    #else
    /* 32bit unified entry point */
    DEFINE_IDTENTRY_MCE(exc_machine_check)
    {
    + unsigned long dr7;
    +
    + local_db_save(&dr7);
    if (user_mode(regs))
    exc_machine_check_user(regs);
    else
    exc_machine_check_kernel(regs);
    + local_db_restore(dr7);
    }
    #endif


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