[Bug 1834051] Re: IRQ2 ignored under KVM when using IOAPIC

[Launchpad Bug Tracker <1834051@bugs.launchpad.net>]

[Expired for QEMU because there has been no activity for 60 days.]

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

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https://bugs.launchpad.net/bugs/1834051

Title:
  IRQ2 ignored under KVM when using IOAPIC

Status in QEMU:
  Expired

Bug description:
  When using KVM, and an OS that supports the IOAPIC, interrupts mapped
  on IRQ2 (for instance, routing an HPET timer on interrupt 2) will
  cause the interrupts to never be delivered. This is because QEmu, when
  setting up the KVM interrupt routes, will not set one up for IRQ2[0].
  When running without KVM, IRQ2 is identity-mapped to GSI2.

  My understanding is that IRQs should be identity mapped to their
  equivalent GSI unless a redirection entry is present in the MADT. This
  is supported by ACPI 6.2 spec[1], 5.2.12.5 Interrupt Source Override
  Structure, which claims: "It is assumed that the ISA interrupts will
  be identity-mapped into the first I/O APIC sources.".

  I stumbled across this while working on my own custom OS, got very
  confused why the HPET wasn't triggering any interruption - and even
  more confused why the behavior only happened in KVM and not in non-
  KVM.

  EDIT: Interestingly, the HPET only supports IRQ2 when using the
  default PIIX chipset, which, combined with this bug, makes it
  completely unusable.

  [0]:
  https://github.com/qemu/qemu/blob/37560c259d7a0d6aceb96e9d6903ee002f4e5e0c/hw/i386/kvm/ioapic.c#L40

  [1]: https://uefi.org/sites/default/files/resources/ACPI_6_2.pdf

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