How to know the interrupt ID which is signaled currently without acknowledging it

ICC_HPPIR1_EL1 will tell you the HPPI at the moment in time when you read it.  So, yes, you can use it to peek a the current HPPI.

The issue is that interrupts are asynchronous events.   The HPPI changes depending on what interrupts are currently Pending (and a bunch of SW controlled config).  Meaning that just because INTID X was the HPPI when you read ICC_HPPIR1_EL1 doesn't mean that a read of ICC_IAR1_EL1 won't return INTID Y.

If you want to control which interrupts are presented to which guest, the standard flow is something like:

• Take Physical IRQ to EL2
• Ack physical interrupt by reading ICC_IAR1_EL1
  • Physical interrupt goes from Pending --> Active (or Active & Pending if Level)
• Determine what to do with interrupt (handle in EL2, inject to current vPE, ...)
  • Assume we're injecting into current vPE
• Create new Virtual Interrupt in the List Registers
  • Virtual interrupt is now Pending, Physical Interrupt unchanged
• Return to Guest
• Take Virtual IRQ to EL1
• Ack virtual interrupt by reading ICV_IAR1_EL1
  • Virtual interrupt goes from Pending --> Active 
• Handle virtual interrupt
• Deactivate virtual interrupt by writing to ICV_EOIR or ICV_DIR
  • Virtual interrupt goes from Active --> Idle
  • If HW=1, physical interrupt goes from Active --> Idle