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A HPET timer can be started when HPET is not yet enabled. This will not generate an interrupt to the guest, but causes problems when HPET is later enabled. A timer that is created and expires at least once before HPET is enabled will have an initialized comparator based on a hpet_offset of 0 (uninitialized). When HPET is enabled, hpet_set_timer() is called a second time, which modifies the timer expiry to a time based on the difference between current ticks (measured with the newly initialized hpet_offset) and the timer's comparator (which was generated before hpet_offset was initialized). This results in a long period of no HPET timer ticks. When this occurs with a CentOS 5.x guest, the guest may not receive timer interrupts during its narrow timer check window and panic on boot. Signed-off-by: Matt Lupfer <mlupfer@ddn.com> Acked-by: Michael S. Tsirkin <mst@redhat.com> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Read the documentation in qemu-doc.html or on http://wiki.qemu-project.org - QEMU team