[Bug 1850378] Re: RISC-V unreliable IPIs

[Thomas Huth]

The QEMU project is currently considering to move its bug tracking to
another system. For this we need to know which bugs are still valid
and which could be closed already. Thus we are setting older bugs to
"Incomplete" now.

If you still think this bug report here is valid, then please switch
the state back to "New" within the next 60 days, otherwise this report
will be marked as "Expired". Or please mark it as "Fix Released" if
the problem has been solved with a newer version of QEMU already.

Thank you and sorry for the inconvenience.


** Changed in: qemu
       Status: New => Incomplete

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https://bugs.launchpad.net/bugs/1850378

Title:
  RISC-V unreliable IPIs

Status in QEMU:
  Incomplete

Bug description:
  I am working on a project with custom inter processor interrupts (IPIs) on 
the RISC-V virt machine.
  After upgrading from version 3.1.0 to 4.1.0 which fixes a related issue 
(https://github.com/riscv/riscv-qemu/issues/132) I am able to use the CPU 
hotplug feature.

  However, if I try to use IPIs for communication between two cores, the
  wfi instruction behaves strangely. Either it does not return, or it
  returns on timer interrupts, even though they are disabled. The code,
  I use on one core to wait for an interrupt is the following.

   csr_clear(sie, SIE_SEIE | SIE_STIE);
   do {
    wait_for_interrupt();
    sipval = csr_read(sip);
    sieval = csr_read(sie);
    scauseval = csr_read(scause) & 0xFF;
   /* only break if wfi returns for an software interrupt */
   } while ((sipval & sieval) == 0 && scauseval != 1);
   csr_set(sie, SIE_SEIE | SIE_STIE);

  Since the resulting sequence does not seem to be deterministic, my
  guess is, that it has something to do with the communication of qemu's
  threads for the different cores.

  Update:
  The exact same setup works fine in spike (the actual sim, not the qemu 
board), which might give a hint, that it is related to the interrupt controller 
implementation.

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