title: What happens to a Startup IPI sent to an Active AP that is not in a Wait-for-SIPI state
body: <p>In a previous <a href="https://stackoverflow.com/questions/48763883/apic-multi-core-startup-protocol-and-icr-starting-up-address">Stackoverflow answer</a> Margaret Bloom says:</p>
  
  <blockquote>
    <p><strong>Waking the APs</strong></p>
    
    <p>This is achieved by inssuing a INIT-SIPI-SIPI (ISS) sequence to the all the APs.</p>
    
    <p>The BSP that will send the ISS sequence using as destination the
    shorthand All excluding self, thereby targeting all the APs.</p>
    
    <p><strong>A SIPI (Startup Inter Processor Interrupt) is ignored by all the CPUs
    that are waked by the time they receive it, thus the second SIPI is
    ignored</strong> if the first one suffices to wake up the target processors. It
    is advised by Intel for compatibility reason.</p>
  </blockquote>
  
  <p>I've been doing multi processing code for years and my observation of hardware has been that on some processors it seems different than stated. I'm pretty sure I've observed Application Processors (AP) have their Instruction Pointer modified upon receipt of Startup IPI even when it was active (not in a Wait-for-Startup-IPI).</p>
  
  <p>Is there any Intel documentation that states what an AP will do upon a <strong>receipt</strong> of a Startup IPI when not in a Wait-for-Startup-IPI state, or documents the behaviour as undefined? I can't seem to find a <em>definitive</em> answer in the <a href="https://software.intel.com/en-us/articles/intel-sdm" rel="nofollow noreferrer">Intel Software Documentation Manuals</a> or the supplementary Intel document <a href="https://www.cs.cmu.edu/~410/doc/minimal_boot.pdf" rel="nofollow noreferrer">Minimal Boot Loader for Intel® Architecture</a>.</p>
  
  <p>Generally I write the initialization code to initialize and start an AP by assuming that the AP may get a SIPI and have its Instruction Pointer reset while in an <em>active</em> state (not in a Wait-for-Startup-IPI state).</p>
  
  <p>I'm trying to determine the accuracy of Margaret Bloom's statement that a second Startup IPI will be ignored by an AP that has been previously awoken.</p>