It was found that Hyper-V 2016 on KVM in some configurations (q35 machine +
piix4-usb-uhci) hangs on boot. Root-cause was that one of Hyper-V
level-triggered interrupt handler performs EOI before fixing the cause of
the interrupt. This results in IOAPIC keep re-raising the level-triggered
interrupt after EOI because irq-line remains asserted.
Gory details: https://www.spinics.net/lists/kvm/msg184484.html
(the whole thread).
Turns out we were dealing with similar issues before; in-kernel IOAPIC
implementation has commit 184564efae4d ("kvm: ioapic: conditionally delay
irq delivery duringeoi broadcast") which describes a very similar issue.
Steal the idea from the above mentioned commit for IOAPIC implementation in
QEMU. SUCCESSIVE_IRQ_MAX_COUNT, delay and the comment are borrowed as well.
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20190402080215.10747-1-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
958a01dab8