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[Bug 1834051] Re: IRQ2 ignored under KVM when using IOAPIC
From: |
Launchpad Bug Tracker |
Subject: |
[Bug 1834051] Re: IRQ2 ignored under KVM when using IOAPIC |
Date: |
Tue, 22 Jun 2021 04:18:13 -0000 |
[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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