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Acknowledge exception handler in Linux

Michele Portolan
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  • Arm Employee Badge
    Note: This was originally posted on 2nd April 2012 at http://forums.arm.com

    I'm afraid that I'm not a Linux expert, but I have a theory.

    The standard return sequence for an Undef exception puts the pc back to the instruction which triggered the Undef.  This would lead to the instruction being re-executed.

    This may sounds odd, but fits the use case for Undefs pretty well.  The common scenarios are:
    * Instruction that the task will never be able to execute (e.g. due to priviledge restrictions) - kill task
    * VFP/NEON instruction when VFP/NEON disabled - turn on VFP/NEON

    So you usually either want to re-execute the instruction or kill the task.
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  • Arm Employee Badge
    Note: This was originally posted on 2nd April 2012 at http://forums.arm.com

    I'm afraid that I'm not a Linux expert, but I have a theory.

    The standard return sequence for an Undef exception puts the pc back to the instruction which triggered the Undef.  This would lead to the instruction being re-executed.

    This may sounds odd, but fits the use case for Undefs pretty well.  The common scenarios are:
    * Instruction that the task will never be able to execute (e.g. due to priviledge restrictions) - kill task
    * VFP/NEON instruction when VFP/NEON disabled - turn on VFP/NEON

    So you usually either want to re-execute the instruction or kill the task.
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