Hi, all:

The patchset (https://lkml.org/lkml/2014/3/18/309) fixed migration of
Windows guests, but commit 0bc830b05c667218d703f2026ec866c49df974fc
(KVM: ioapic: clear IRR for edge-triggered interrupts at delivery)
introduced a bug (see
https://www.mail-archive.com/kvm@vger.kernel.org/msg109813.html).

>From the description "Unlike the old qemu-kvm, which really never did
that, with new QEMU it is for some reason
somewhat likely to migrate a VM with a nonzero IRR in the ioapic."

Why could new QEMU do that? I can not find any codes about the "some reason"..
As we know, once a irq is set in kvm's ioapic, the ioapic will send
that irq to lapic, this is an "atomic" operation.
Then, kvm will inject them in inject_pending_event(or set rvi in
apic-v case). QEMU will also save the pending irq when doing
migration.

I can not find a point which guest could lose a irq, but this scenario
really exists.

Any ideas?


Thanks,

Wincy
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